I have been out of school for a couple years, and I am doing a 1000 sq ft low budget guest house for my sister. I was upfront with her about having little experience with residential design, and she was fine with that.
It is located in Tucson and the roof is low-slope. I am using TJIs for the roof structure. So my question is: Should I slope the roof surface with a sprayed foam or tapered insulation. Or should I slope the joists.
I would like to fasten the ceiling gyp directly to the joists, so I am definitely leaning towards a tapered insulation solution. But any opinions would be very welcome.
- first cost: tapered insulation is typically significantly more expensive than a uniform-thickness rigid polyiso because of the time/coordination-intensive installation process. by comparison, the difference in cost associated with coordination of different joist bearing elevations in a sloped structure solution is typically less.
- future cost: in a future re-roof - while you SHOULD be able to salvage most of the insulation - it's not unlikely that it will be compressed, mold- or water-damaged (even if only slightly), or that the roofer will damage it while removing the old roof. all of which may mean expensive replacement if it's tapered insulation. if the slope is in the structure, as much of a *uniform* thickness insulation as possible could be reused and supplemented easily with new where necessary.
why use tji's for a low-budget residential project? they're a pain to work with in the field. just use pre-fab wood trusses; they're cheaper, you can run utilities through them, you can box out for hvac easily, and you can have a tapered truss.
What is the finish of the roof? The component that ultimately needs to be sloped is the waterproofing layer of your roofing assembly (so that water is shed away from the structure).
That means in a tapered insulation detail, the waterproofing will sit ON TOP of your insulation. This is known as INVERTED ROOFING system. Thus your assembly is: flat joists, sheathing, tapered insulation, waterproofing (and then maybe ballast if you are using *protected roofing waterproofing)
*protected roofing is less expensive, but has poor UV resistance, and needs to be protected either by ballast, pavers, or similar.
if you slope the joists, your options become much bigger. You can even do a green roof assembly (waterproofing needs to sit below the insulation).
In my opinion I would never use sloped insulation unless I had to. As Steven mentions, insulation tends to compress. I'd hate to lose the minimum roofing slope because of insulation sag.
So it sounds like tapered insulation is not the way to go. And it's either pre-fab wood trusses or TJIs with 'rippers'. I also read about tapered TJIs. Anyone have experience with those?
My main concern with pre-fab trusses are that I assume they would be deeper for the same span compared to TJIs. I mention that because I want to keep the guest house overall height to be as little as possible, so it doesn't compete for visual dominance with the main house.
hmm. a little misleading title. if you slope the joists, you will slope the roof.;.)
but besides the trivia, i agree with steven and rusty about fragility of insulation sloped roof. don't go with soft solution on the roof. slope the joists of your choice. watch for the bird baths. 1/4" to a foot okay but more the better. everything buckles a little in the heat and deforms eventually!
From a construction quality control standpoint, it seems as though sloping the joists would also be a much more consistent way to get the desired slope in the roof. Thus, reducing the 'bird baths' ;)
Also, you need to consider if uplift resistance is a design consideration. Does Tucson ever get crazy twister storms? If so, insulation panels should be mechanically fastened (as opposed to adhered). Factory Mutual (a major insurance institute that understands risk management) would not allow for certain types of roof assemblies depending on local weather conditions.
Judging by the flat roof requirement, I assume this is some variety of Southwestern adobe-styled property? You can hide the sloped roof behind a small unadorned cornice. This is often done on adobe buildings to give the impression of a true flat roof.
With all that being said, you could always use an insulate concrete form for a flat roof considering Tuscon doesn't get heavy snow or heavy rain.
"Rusty why insulation over waterproofing for green roof?"
I was oversimplifying, of course. You can have a green roof with either insulation being on top or below waterproofing. You can even not use insulation altogether (certain climates only). Almost all green roof systems (hydrotech, etc..) show insulation on top of waterproofing because: major concern in green roofs is aggressive roots puncturing the root barrier and eventually waterproofing. Insulation acts as an excellent root barrier.
if you put insulation below the waterproofing, you will need to beef up your root barrier above the waterproofing anyways, which often involves putting in an additional insulation layer. Which makes for a redundant assembly.
Well, I looked into it a little more and it seems that prefabricated trusses might be the best way to go for this application. And they are not as deep as I thought, as they start out at about 1' deep.
So my next question is about the sizing of these in the working drawings. I can find a rule of thumb depth sizing (span/18), would that be good enough until the contractor sources these? I imagine the lumber yard could do the sizing in about 2 mins.
So my next question is about the sizing of these in the working drawings.
Depends on the spacing of trusses. Live and dead loads (refer to local codes). Sounds to me you are not doing anything more complex than what's needed for a flat roof, fully-detached, residential parking garage. Consult a local professional (ha!) like the builder. Draw them at 1'-0" deep if you'd like, but don't dimension them. <depth as required> tag will suffice. Then talk to the builder.
Make sure you price them under unit-price cash allowance in Div01.
i wondered if it was a technical climate based thing or something. we just did a greenroof here in tokyo with insulatation under the waterproofing but this is a mild climate. we put in a thcik root guard over the waterproofing as well. the installers/manufacturers seemed to think that was sufficient (it was only like 3 mm thick) so am interested in the idea that insulation is also a part of the system. you usually do that?
apologies btw to the OP, for the non-pertinent question
As far as WP membrane above or below the Insulation, I think it can go either way, no? However, depending on the type of insulation used, protection board may be required between the membrane and the insulation. Considerations: When the insulation is under the insulation it is protected from extremes in hot and cold, which is a more critical consideration in an extreme climate, but it might prove cheaper to build a flat roof structure and simple taper the insulation (probably not the case in this post's building). One must still consider the location of the vapor barrier in the roof sandwich depending on climate. Be careful not to trap moisture between membrane and vapor barrier, I have even seen this happen in designs developed by roofing consultants themselves.
jump, green roofs have a history of failure. Newer ones are based on mistakes learned in the past.
Is your green roof intensive or extensive? Or both? There is a huge difference between sod and bush roots. What was used for waterproofing? Thermoplastics? Different materials have a different puncture resistance.
Benefit of placing insulation under the waterproofing is financial in nature. You can use a cheaper insulation material if the insulation will not be immersed in water.
Ideally your green roof came as a package and has a single source responsibility for all its components. Installer and manufacturer should also have shown a long history of successfully installing/making green roofs in your region.
Otherwise make sure you are up to date with insurance payments :)
What kind of roof spans are we talking about in a 1,000 SF house?
Tucson seldom of ever has snow....maybe a inch of fluff once in 10 years. I think for cost purposes you would be ahead to use convential lumber. That is unless your going to be doing some sort of living roof structure. A 2X12 can span a pretty far distance in Arizona.
Most likely the bigger concern would be shear walls, if your looking for an open floor plan and well placed exterior glass.
Remember one of the great things about Tucson is to take advantage of the outdoor space, intergrating it into the interior space.
Being a guest house you should also give it privacy from the main house.
@ rusty, we used this system. click on the top image if interested. sorry it is japanese.
in our case it is a wooden structure. you can see actual construction on our blog if interested (scroll down a bit to get to the green roof bits). the decks are sloped. i don't think i would ever taper the insulation to get a slope on a flat roof myself. not sure why i feel so strongly about it but in general i prefer to have gravity as my backup in case seals and plastics and all the other bits of technology fail. as far as the technical issues go we relied on tajima roofing. it cost a bit more but they had certified crews to install the system which meant we had a warranty in addition to insurance.
we are working on another small house in the city now and will likely use the same system (the govt is supporting green roofs to reduce heat island effect so its worth doing now). i will look into the insulation placement more closely this time. it will be interesting to hear what the manufacturers have to say on the topic. thanks for bringing it up.
rustystuds: I am talking about residential roof assemblies in that I am talking about what the OP described, which I assume will end up being a plywood sub-roof, and consideration will need to be made to keep the insulation dry. Vapor should not be able to migrate into the insulation to condense. I am not referring to an air barrier or building paper product such as Tyvek, but a vapor retarder. If the roof structure is exposed (which would seem to be the driving reason to put the insulation on top of the sheathing in the first place, and not in the joist cavity), then there is no opportunity to put in a vapor barrier behind ceiling GWB. Thoughts?
Also, I am curious about what you mentioned about air barriers being archaic. How would one detail a masonry cavity wall with gypsum sheathing? Just seal the joints of Densglas-type sheathing? I also assume the air-barrier is functionally separate from the vapor barrier. Is this also archaic? I'd be very interested in you thoughts.
The japanese detail from manufacturer's site is confusing as heck (even as just a graphic. where's ANY insulation?). Using google-translate helped, but not that much.
Your blog photos were helpful. Green roof components come off as a hybrid system. By that I mean, it looks like a built in roof planter. One green roof I spec'd was just a series of interlocking plant boxes that just sat on single-ply TPO waterproofing over insulation. Your roof is kind of like that, except the planter box is built on site. Plants that could grow in that little soil are probably wimpy, so the problem of roots digging up your entire roof is probably minimal. Heavy duty green roofs require durable details. Your just isn't one.
If Tajima roofing provides a good warranty, then you are probably safe.
The project is adorable. I think (to be fair) that green roof should be called green-roofish because it's so tiny :)
-you should use ventilation in order to remove condensation. In Tucson, this will almost be a non issue. If the ceiling finish is GDW, consider the space above as vented.
-we are talking about using rigid insulation in this case. Rigid insulation is inorganic. Some condensation on it will not lead to creation of black mold. You can also stick the insulation on top of the waterproofing, in which case your insulation will be rated for water submersion. If you were filling the ceiling cavity with lose batts, then you would have a completely different issue. Which bring me to next topic:
-Try never ever ever to use lose batts for exterior insulation. The system just sucks. Getting a continuous thermal rating around your building becomes impossible. It's also too porous for a building with performance in mind.
"How would one detail a masonry cavity wall with gypsum sheathing?"
from inside moving out:
-gdw (inside finish)
-cold formed metal framing (or wood)
-exterior sheathing (like Densglass) try avoiding organic materials like plywood or paper faced gypsum.
-waterproofing (can be peel and stick, or spray on, or trowelled on, you got plenty of options)
-Thermal insulation (rigid, extruded typ. expanded in some cases)
-air space
-Brick veneer (brick ties to be installed into the wall before waterproofing gets put in, brick vents at flashing)
really simple detail.
note that waterproofing is both an air and vapor barrier. Dewpoint will almost always occur inside of the insulation, which is on the cold side of our waterproofing. Good!
Now...
If you decide to also put batts inside the wall cavity, our clean little detail completely falls to shit. Now you have to worry about condensation happening in the cavity, which necessitates replacing the all-in-one waterproofing with separate air and vapor barriers. Messy as hell, and it leads to leaky buildings.
Owens Corning successfully lobbied for inclusion of cavity batt insulation into mandatory use in residential building codes. Which is a major pain in the ass, but avoidable with effort.
There is also spray foam (SPF) for cavity use, and some people swear by it. I'm not a fan.
disclaimer: I'm mostly a spec writer these days. I find most commercial systems to be logical and efficient. I find residential construction methods to often be ass-backwards in almost everything they do.
rusty: Thanks for the info. I guess I just figured wrapping the whole building in a bone-fide WP membrane would be cost/labor prohibitive. Is this partly affected by the evolution of membrane technology: peel and stick and cold fluid-applied? I can appreciate the rationality of this assembly, although, how might it be modified in a cooling climate such as Tuscon? I have seen buildings going up around NYC with the joints of the Densglass sealed with some kind of trowel-on material and then followed up with rigid insulation. Is there a fiberglass reinforced sheathing out there that is also a waterproof barrier? That would be cool.
Also, does this detail dovetail with super-tight envelopes such as are required by Passive house and the like? I would think yes.
And what does GDW stand for? Gypsum ____ Wallboard?
Lastly, and on the subject of the OP: if there is a finished ceiling, other considerations aside, wouldn't it make the most sense to put the insulation below the roof sheathing, assuming it could be SPF or rigid, such that any roof assembly is above the WP membrane?
I like this conversation, and I hope it hasn't strayed from the original subject too far.
"Lastly, and on the subject of the OP: if there is a finished ceiling, other considerations aside, wouldn't it make the most sense to put the insulation below the roof sheathing, assuming it could be SPF or rigid, such that any roof assembly is above the WP membrane?
I like this conversation, and I hope it hasn't strayed from the original subject too far."
I am planning on the composition to be:
1.Modified bitumen built up roof or single ply membrane.
2.Plywood sheathing
3.Probably pre-fab tapered truss
4.GWB
@ rusty, yes just a big flowerbox basically. for a three story wooden building in earthquake land the soil alone (300mm deep) even this small greenish roof was causing problems with beam thickness. we were building within 15mm of the height limit and didn't want to mess with ceiling height so that is the result. the structure is already pretty intense (check out rest of blog especially the beginning to see how massive the structure is - it's ridiculous! and its just a regular little project, nothing special at all)
insulation in japan is not a priority. it is kind of strange, but such is the culture. slowly changing now.
i've done big green roofs for schools back when i was working for someone else, but with concrete construction, in which case different detail. insulation usually is under the waterproofing. the roofing guys don't care to much about it, typically. it is a bit of a gap frankly.
your description of the wall assembly is also very interesting. cheers for that.
btw, in the graphic the insulation is over the concrete deck, t=35mm. typically the concrete slab will be sloped 1/100 and insulation (rigid) is constant thickness.
loac, that's the standard way to do it, minus the tapered truss. I think you could save some money without.
go do it said it: trusses = cheaper. Use pitched wedges, that's typically all we ever did, straight from our estimator's/ former contractor's mouth. So he knows the price involved. Slope to all fascia for a flat gutter profile. I have never used tapered insul. in my life.
But, unless the house is entirely open, I am also wondering why you're designing for clear span. And I'm not sure why you wouldn't just use batt in the sandwich instead of rigid, unless your required R-value is in the 40's, something I kind of doubt in AZ. Rusty, what rigid insul. are you using outside the sheathing of your wall assembly? Most places around here are moving to R-19/21. Even with polyiso, that would be 3-1/2" of rigid, more or less, giving you a 13" wall, requiring extension jambs all around, wouldn't it? If so, sounds expensive when you add it all up.
"Not all insulation types are compatible with all roof covering types. For instance, expanded and extruded polystyrenes should not be in direct contact with thermoplastic PVC single-ply membranes. When in direct contact, plasticizer migration occurs, causing the PVC membrane to become brittle. Modified bitumen membranes, as well as conventional built-up membranes, are not recommended for direct application over foam plastic insulation boards. Cover board insulation is recommended and required by modified bitumen and built-up roof covering manufacturers when utilizing foam plastic insulations. Wood fiber, perlite, and gypsum board are the most common cover board insulations in use today."
"Concentrated roof traffic can be as hard on a roof system as any other factor. It is not uncommon to see collapsed roof insulation where traffic (particularly wheel and equipment loads) is heavy. Compressive strength should be evaluated closely based on the anticipated traffic loads. Oftentimes, a combination of insulations is used to best address multiple selection criteria. As an example, polyisocyanurate is often used as the base layer of insulation due to its high R-value and code approvals, in combination with a gypsum cover board, which enhances the system's compressive strength and ability to withstand traffic loads and hail events. Selecting two layers of insulation with staggered joints is also advantageous since the system strength is enhanced and thermal shorts are reduced."
beakay:"Rusty, what rigid insul. are you using outside the sheathing of your wall assembly? Most places around here are moving to R-19/21. Even with polyiso, that would be 3-1/2" of rigid, more or less, giving you a 13" wall, requiring extension jambs all around, wouldn't it?"
With polyiso you can get R-20+ in under 3", but your point is valid. As far as need for extension jabs goes, keep in mind that your windows and doors will sit slightly forward in this detail in order to eliminate thermal bridging around all your openings.
If you used batts, you would have needed at least 6" (for R-20) of it in the cavity. With no batts, you metal/wood framing can be substantially thinner. Your overall wall thickness can actually go down.
loac, I just meant a typical truss joist, instead of tapered. I would also look at whether you could do away with trusses entirely and find some bearing for standard lumber. I don't think a 20x20 (assuming) guest house necessarily needs the trusses if you have anywhere to post down. Of course, you may need the TJ for MEP, I don't know.
Well may be the posts here are older one's. But in my point of view this is a very valuable information for me. As I am in the same situation I am searching for a right solution. I got it here. Thanks for everyone who shared information.
Slope the roof or slope the joists?
I have been out of school for a couple years, and I am doing a 1000 sq ft low budget guest house for my sister. I was upfront with her about having little experience with residential design, and she was fine with that.
It is located in Tucson and the roof is low-slope. I am using TJIs for the roof structure. So my question is: Should I slope the roof surface with a sprayed foam or tapered insulation. Or should I slope the joists.
I would like to fasten the ceiling gyp directly to the joists, so I am definitely leaning towards a tapered insulation solution. But any opinions would be very welcome.
all work fine BUT:
- first cost: tapered insulation is typically significantly more expensive than a uniform-thickness rigid polyiso because of the time/coordination-intensive installation process. by comparison, the difference in cost associated with coordination of different joist bearing elevations in a sloped structure solution is typically less.
- future cost: in a future re-roof - while you SHOULD be able to salvage most of the insulation - it's not unlikely that it will be compressed, mold- or water-damaged (even if only slightly), or that the roofer will damage it while removing the old roof. all of which may mean expensive replacement if it's tapered insulation. if the slope is in the structure, as much of a *uniform* thickness insulation as possible could be reused and supplemented easily with new where necessary.
why use tji's for a low-budget residential project? they're a pain to work with in the field. just use pre-fab wood trusses; they're cheaper, you can run utilities through them, you can box out for hvac easily, and you can have a tapered truss.
What is the finish of the roof? The component that ultimately needs to be sloped is the waterproofing layer of your roofing assembly (so that water is shed away from the structure).
That means in a tapered insulation detail, the waterproofing will sit ON TOP of your insulation. This is known as INVERTED ROOFING system. Thus your assembly is: flat joists, sheathing, tapered insulation, waterproofing (and then maybe ballast if you are using *protected roofing waterproofing)
*protected roofing is less expensive, but has poor UV resistance, and needs to be protected either by ballast, pavers, or similar.
if you slope the joists, your options become much bigger. You can even do a green roof assembly (waterproofing needs to sit below the insulation).
In my opinion I would never use sloped insulation unless I had to. As Steven mentions, insulation tends to compress. I'd hate to lose the minimum roofing slope because of insulation sag.
el jeffe is right engineered trusses is what i always prefer.
but if you must use TJI's the lumber yard can cut some "rippers" to be nailed on top of the joist to provide a slope. usually 1/4" to the foot.
rippers are just giant wedges
Wow, thanks for all the great replies!
So it sounds like tapered insulation is not the way to go. And it's either pre-fab wood trusses or TJIs with 'rippers'. I also read about tapered TJIs. Anyone have experience with those?
My main concern with pre-fab trusses are that I assume they would be deeper for the same span compared to TJIs. I mention that because I want to keep the guest house overall height to be as little as possible, so it doesn't compete for visual dominance with the main house.
THIS is the kind of good old-fashioned building-related thread that's so much fun to read on Archinect.
hmm. a little misleading title. if you slope the joists, you will slope the roof.;.)
but besides the trivia, i agree with steven and rusty about fragility of insulation sloped roof. don't go with soft solution on the roof. slope the joists of your choice. watch for the bird baths. 1/4" to a foot okay but more the better. everything buckles a little in the heat and deforms eventually!
From a construction quality control standpoint, it seems as though sloping the joists would also be a much more consistent way to get the desired slope in the roof. Thus, reducing the 'bird baths' ;)
Also, you need to consider if uplift resistance is a design consideration. Does Tucson ever get crazy twister storms? If so, insulation panels should be mechanically fastened (as opposed to adhered). Factory Mutual (a major insurance institute that understands risk management) would not allow for certain types of roof assemblies depending on local weather conditions.
Judging by the flat roof requirement, I assume this is some variety of Southwestern adobe-styled property? You can hide the sloped roof behind a small unadorned cornice. This is often done on adobe buildings to give the impression of a true flat roof.
With all that being said, you could always use an insulate concrete form for a flat roof considering Tuscon doesn't get heavy snow or heavy rain.
slope the joists and call it a day....
Ditto for the sloped joists. That is how we do it too.
Rusty why insulation over waterproofing for green roof?
I was oversimplifying, of course. You can have a green roof with either insulation being on top or below waterproofing. You can even not use insulation altogether (certain climates only). Almost all green roof systems (hydrotech, etc..) show insulation on top of waterproofing because: major concern in green roofs is aggressive roots puncturing the root barrier and eventually waterproofing. Insulation acts as an excellent root barrier.
if you put insulation below the waterproofing, you will need to beef up your root barrier above the waterproofing anyways, which often involves putting in an additional insulation layer. Which makes for a redundant assembly.
Well, I looked into it a little more and it seems that prefabricated trusses might be the best way to go for this application. And they are not as deep as I thought, as they start out at about 1' deep.
So my next question is about the sizing of these in the working drawings. I can find a rule of thumb depth sizing (span/18), would that be good enough until the contractor sources these? I imagine the lumber yard could do the sizing in about 2 mins.
Thanks again. I have already learned a lot.
Depends on the spacing of trusses. Live and dead loads (refer to local codes). Sounds to me you are not doing anything more complex than what's needed for a flat roof, fully-detached, residential parking garage. Consult a local professional (ha!) like the builder. Draw them at 1'-0" deep if you'd like, but don't dimension them. <depth as required> tag will suffice. Then talk to the builder.
Make sure you price them under unit-price cash allowance in Div01.
Kidding about last part.
Lump-sum is the way to go :)
thanks rusty.
i wondered if it was a technical climate based thing or something. we just did a greenroof here in tokyo with insulatation under the waterproofing but this is a mild climate. we put in a thcik root guard over the waterproofing as well. the installers/manufacturers seemed to think that was sufficient (it was only like 3 mm thick) so am interested in the idea that insulation is also a part of the system. you usually do that?
apologies btw to the OP, for the non-pertinent question
As far as WP membrane above or below the Insulation, I think it can go either way, no? However, depending on the type of insulation used, protection board may be required between the membrane and the insulation. Considerations: When the insulation is under the insulation it is protected from extremes in hot and cold, which is a more critical consideration in an extreme climate, but it might prove cheaper to build a flat roof structure and simple taper the insulation (probably not the case in this post's building). One must still consider the location of the vapor barrier in the roof sandwich depending on climate. Be careful not to trap moisture between membrane and vapor barrier, I have even seen this happen in designs developed by roofing consultants themselves.
jump, green roofs have a history of failure. Newer ones are based on mistakes learned in the past.
Is your green roof intensive or extensive? Or both? There is a huge difference between sod and bush roots. What was used for waterproofing? Thermoplastics? Different materials have a different puncture resistance.
Benefit of placing insulation under the waterproofing is financial in nature. You can use a cheaper insulation material if the insulation will not be immersed in water.
Ideally your green roof came as a package and has a single source responsibility for all its components. Installer and manufacturer should also have shown a long history of successfully installing/making green roofs in your region.
Otherwise make sure you are up to date with insurance payments :)
What kind of roof spans are we talking about in a 1,000 SF house?
Tucson seldom of ever has snow....maybe a inch of fluff once in 10 years. I think for cost purposes you would be ahead to use convential lumber. That is unless your going to be doing some sort of living roof structure. A 2X12 can span a pretty far distance in Arizona.
Most likely the bigger concern would be shear walls, if your looking for an open floor plan and well placed exterior glass.
Remember one of the great things about Tucson is to take advantage of the outdoor space, intergrating it into the interior space.
Being a guest house you should also give it privacy from the main house.
hbrain "Be careful not to trap moisture between membrane and vapor barrier"
Are you talking about residential roof assemblies? Commercial roofing systems don't need no stinking Tyvek wraps.
In fact the whole air/vapor barriers-as-separate-products system is archaic and should be left to the realm of subburban subdivision contractors.
"What kind of roof spans are we talking about in a 1,000 SF house?"
Just under 20'.
I think I'll be fine on shear walls. There aren't a ton of windows.
@ rusty, we used this system. click on the top image if interested. sorry it is japanese.
in our case it is a wooden structure. you can see actual construction on our blog if interested (scroll down a bit to get to the green roof bits). the decks are sloped. i don't think i would ever taper the insulation to get a slope on a flat roof myself. not sure why i feel so strongly about it but in general i prefer to have gravity as my backup in case seals and plastics and all the other bits of technology fail. as far as the technical issues go we relied on tajima roofing. it cost a bit more but they had certified crews to install the system which meant we had a warranty in addition to insurance.
we are working on another small house in the city now and will likely use the same system (the govt is supporting green roofs to reduce heat island effect so its worth doing now). i will look into the insulation placement more closely this time. it will be interesting to hear what the manufacturers have to say on the topic. thanks for bringing it up.
rustystuds: I am talking about residential roof assemblies in that I am talking about what the OP described, which I assume will end up being a plywood sub-roof, and consideration will need to be made to keep the insulation dry. Vapor should not be able to migrate into the insulation to condense. I am not referring to an air barrier or building paper product such as Tyvek, but a vapor retarder. If the roof structure is exposed (which would seem to be the driving reason to put the insulation on top of the sheathing in the first place, and not in the joist cavity), then there is no opportunity to put in a vapor barrier behind ceiling GWB. Thoughts?
Also, I am curious about what you mentioned about air barriers being archaic. How would one detail a masonry cavity wall with gypsum sheathing? Just seal the joints of Densglas-type sheathing? I also assume the air-barrier is functionally separate from the vapor barrier. Is this also archaic? I'd be very interested in you thoughts.
Jump, that project looks great!
The japanese detail from manufacturer's site is confusing as heck (even as just a graphic. where's ANY insulation?). Using google-translate helped, but not that much.
Your blog photos were helpful. Green roof components come off as a hybrid system. By that I mean, it looks like a built in roof planter. One green roof I spec'd was just a series of interlocking plant boxes that just sat on single-ply TPO waterproofing over insulation. Your roof is kind of like that, except the planter box is built on site. Plants that could grow in that little soil are probably wimpy, so the problem of roots digging up your entire roof is probably minimal. Heavy duty green roofs require durable details. Your just isn't one.
If Tajima roofing provides a good warranty, then you are probably safe.
The project is adorable. I think (to be fair) that green roof should be called green-roofish because it's so tiny :)
hbrain, here are some random thoughts:
-you should use ventilation in order to remove condensation. In Tucson, this will almost be a non issue. If the ceiling finish is GDW, consider the space above as vented.
-we are talking about using rigid insulation in this case. Rigid insulation is inorganic. Some condensation on it will not lead to creation of black mold. You can also stick the insulation on top of the waterproofing, in which case your insulation will be rated for water submersion. If you were filling the ceiling cavity with lose batts, then you would have a completely different issue. Which bring me to next topic:
-Try never ever ever to use lose batts for exterior insulation. The system just sucks. Getting a continuous thermal rating around your building becomes impossible. It's also too porous for a building with performance in mind.
"How would one detail a masonry cavity wall with gypsum sheathing?"
from inside moving out:
-gdw (inside finish)
-cold formed metal framing (or wood)
-exterior sheathing (like Densglass) try avoiding organic materials like plywood or paper faced gypsum.
-waterproofing (can be peel and stick, or spray on, or trowelled on, you got plenty of options)
-Thermal insulation (rigid, extruded typ. expanded in some cases)
-air space
-Brick veneer (brick ties to be installed into the wall before waterproofing gets put in, brick vents at flashing)
really simple detail.
note that waterproofing is both an air and vapor barrier. Dewpoint will almost always occur inside of the insulation, which is on the cold side of our waterproofing. Good!
Now...
If you decide to also put batts inside the wall cavity, our clean little detail completely falls to shit. Now you have to worry about condensation happening in the cavity, which necessitates replacing the all-in-one waterproofing with separate air and vapor barriers. Messy as hell, and it leads to leaky buildings.
Owens Corning successfully lobbied for inclusion of cavity batt insulation into mandatory use in residential building codes. Which is a major pain in the ass, but avoidable with effort.
There is also spray foam (SPF) for cavity use, and some people swear by it. I'm not a fan.
disclaimer: I'm mostly a spec writer these days. I find most commercial systems to be logical and efficient. I find residential construction methods to often be ass-backwards in almost everything they do.
Sorry for long post.
questions of loads, spans, design of building????is it a box are the bearing walls in the building? cuz if there are why not just use 2x 's?
rusty: Thanks for the info. I guess I just figured wrapping the whole building in a bone-fide WP membrane would be cost/labor prohibitive. Is this partly affected by the evolution of membrane technology: peel and stick and cold fluid-applied? I can appreciate the rationality of this assembly, although, how might it be modified in a cooling climate such as Tuscon? I have seen buildings going up around NYC with the joints of the Densglass sealed with some kind of trowel-on material and then followed up with rigid insulation. Is there a fiberglass reinforced sheathing out there that is also a waterproof barrier? That would be cool.
Also, does this detail dovetail with super-tight envelopes such as are required by Passive house and the like? I would think yes.
And what does GDW stand for? Gypsum ____ Wallboard?
Lastly, and on the subject of the OP: if there is a finished ceiling, other considerations aside, wouldn't it make the most sense to put the insulation below the roof sheathing, assuming it could be SPF or rigid, such that any roof assembly is above the WP membrane?
I like this conversation, and I hope it hasn't strayed from the original subject too far.
"Lastly, and on the subject of the OP: if there is a finished ceiling, other considerations aside, wouldn't it make the most sense to put the insulation below the roof sheathing, assuming it could be SPF or rigid, such that any roof assembly is above the WP membrane?
I like this conversation, and I hope it hasn't strayed from the original subject too far."
I am planning on the composition to be:
1.Modified bitumen built up roof or single ply membrane.
2.Plywood sheathing
3.Probably pre-fab tapered truss
4.GWB
"I am planning on the composition to be:
1.Modified bitumen built up roof or single ply membrane.
2.Plywood sheathing
3.Probably pre-fab tapered truss
4.GWB"
I forgot to mention batt insulation between the trusses.
@ rusty, yes just a big flowerbox basically. for a three story wooden building in earthquake land the soil alone (300mm deep) even this small greenish roof was causing problems with beam thickness. we were building within 15mm of the height limit and didn't want to mess with ceiling height so that is the result. the structure is already pretty intense (check out rest of blog especially the beginning to see how massive the structure is - it's ridiculous! and its just a regular little project, nothing special at all)
insulation in japan is not a priority. it is kind of strange, but such is the culture. slowly changing now.
i've done big green roofs for schools back when i was working for someone else, but with concrete construction, in which case different detail. insulation usually is under the waterproofing. the roofing guys don't care to much about it, typically. it is a bit of a gap frankly.
your description of the wall assembly is also very interesting. cheers for that.
btw, in the graphic the insulation is over the concrete deck, t=35mm. typically the concrete slab will be sloped 1/100 and insulation (rigid) is constant thickness.
loac, that's the standard way to do it, minus the tapered truss. I think you could save some money without.
go do it said it: trusses = cheaper. Use pitched wedges, that's typically all we ever did, straight from our estimator's/ former contractor's mouth. So he knows the price involved. Slope to all fascia for a flat gutter profile. I have never used tapered insul. in my life.
But, unless the house is entirely open, I am also wondering why you're designing for clear span. And I'm not sure why you wouldn't just use batt in the sandwich instead of rigid, unless your required R-value is in the 40's, something I kind of doubt in AZ. Rusty, what rigid insul. are you using outside the sheathing of your wall assembly? Most places around here are moving to R-19/21. Even with polyiso, that would be 3-1/2" of rigid, more or less, giving you a 13" wall, requiring extension jambs all around, wouldn't it? If so, sounds expensive when you add it all up.
Also... span chart:
http://www.trimjoist.com/spans.html
Here's an article I stumbled across to give you more to think about:
http://www.buildings.com/ArticleDetails/tabid/3321/ArticleID/5646/Default.aspx
"Not all insulation types are compatible with all roof covering types. For instance, expanded and extruded polystyrenes should not be in direct contact with thermoplastic PVC single-ply membranes. When in direct contact, plasticizer migration occurs, causing the PVC membrane to become brittle. Modified bitumen membranes, as well as conventional built-up membranes, are not recommended for direct application over foam plastic insulation boards. Cover board insulation is recommended and required by modified bitumen and built-up roof covering manufacturers when utilizing foam plastic insulations. Wood fiber, perlite, and gypsum board are the most common cover board insulations in use today."
"Concentrated roof traffic can be as hard on a roof system as any other factor. It is not uncommon to see collapsed roof insulation where traffic (particularly wheel and equipment loads) is heavy. Compressive strength should be evaluated closely based on the anticipated traffic loads. Oftentimes, a combination of insulations is used to best address multiple selection criteria. As an example, polyisocyanurate is often used as the base layer of insulation due to its high R-value and code approvals, in combination with a gypsum cover board, which enhances the system's compressive strength and ability to withstand traffic loads and hail events. Selecting two layers of insulation with staggered joints is also advantageous since the system strength is enhanced and thermal shorts are reduced."
beakay:"Rusty, what rigid insul. are you using outside the sheathing of your wall assembly? Most places around here are moving to R-19/21. Even with polyiso, that would be 3-1/2" of rigid, more or less, giving you a 13" wall, requiring extension jambs all around, wouldn't it?"
With polyiso you can get R-20+ in under 3", but your point is valid. As far as need for extension jabs goes, keep in mind that your windows and doors will sit slightly forward in this detail in order to eliminate thermal bridging around all your openings.
If you used batts, you would have needed at least 6" (for R-20) of it in the cavity. With no batts, you metal/wood framing can be substantially thinner. Your overall wall thickness can actually go down.
beekay: Thanks for taking the time to share you knowledge, but I am a little confused at this part:
"loac, that's the standard way to do it, minus the tapered truss. I think you could save some money without.
go do it said it: trusses = cheaper. Use pitched wedges, that's typically all we ever did"
You said "minus the tapered truss" then you said "trusses = cheaper".
I need a detail. My architectural training made me useless without visuals.
loac, I just meant a typical truss joist, instead of tapered. I would also look at whether you could do away with trusses entirely and find some bearing for standard lumber. I don't think a 20x20 (assuming) guest house necessarily needs the trusses if you have anywhere to post down. Of course, you may need the TJ for MEP, I don't know.
Well may be the posts here are older one's. But in my point of view this is a very valuable information for me. As I am in the same situation I am searching for a right solution. I got it here. Thanks for everyone who shared information.
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