I plan on building an ICF home (approx 4 ft walls for a crawlspace, on a footing and 9 ft main floor walls, with a stick roof (trying to keep costs reasonable). Can someone explain to me, the complete process AND idea behind insulating the underside of the roof. I assume you would not then insulate your interior ceiling (drywall or whatever you interior finish is). I'm from Winnipeg, Manitoba and we routinely get -30C (-22F) and colder in the winter nights. Vapor barrier and R40-R50 is the norm nowadays, above your interior roof. If not sealed and insulated properly, you're prone to condensation in the attic, from warm air rising into the attic. Sooo, I guess my question is, why is spraying the underside of the exterior sheathing with foam and essentially heating the attic, a better option than insulating the top of your interior wall and let the attic 'breathe' with soffits and vents.  Thanks in advance, Greg.

In my opinion closing the roof with spray foam allows you to better seal the roof, even though you wrap all the electrical boxes with poly and put acoustical seal around wires and pipes penetrating the top plate, it's never perfect.

You will not notice any additional cost heating the bit of space above the drywall, chances are your heating system is oversized anyways.

I have done several houses where we used a closed roof and the feel inside the house is different than one with an open attic.

Chris, what is the typical amount (2 - 3 inches), or is it much more (what kind of R-value do you typically shoot for)? I assume where your fascia normally is, that gets sealed with plywood and spray foamed as well, to make the attic tight. I guess it would be easiest to spray the attic, before the main floor ceiling drywall gets installed. I assume this means no vapor barrier on your main floor ceiling or any insulation either. You just let the heat rise into the attic? Sorry for all the questions, but I'm an ICF virgin and know even less about the insulated roof system. Also, if anyone has any pictures or a short video an of insulated roof, that would be great as well.

R-50 is I believe 8" of spray, cannot be done in one day as it must off gas and cool off, some people have tried to build it up in one day and poof the roof catches on fire...keep that in mind when talking to applicators, and check it out on youtube/google to see what I'm talking about.

We generally block the ends of the trusses with cardboard just as a backer for the spray foam. No vapour barrier required

Sledderbm, here's some pics of the spray foam I put on the underside of my roof sheathing. It's roughly 10" thick for an R value around 40. It's low density foam. High density foam wouldn't need to be so thick. At the wall I nailed OSB pieces on the outside edge of the top plate to close in the space between the plate and roof sheathing. This was sprayed in one pass. The soffit is vented in ususal manner. I'm in southwest Idaho where the design temp is 9F and winter temp can drop to -10F.

An additional benefit of spray foaming the sheathing is it makes it much nicer to use the attic for storage. I had the trusses designed for open space in the center and for storage live load. In my case, I use the attic space for the return air supply for the heating/cooling system.

Posted By sleddermb on 26 Oct 2012 04:51 PM
Chris, what is the typical amount (2 - 3 inches), or is it much more (what kind of R-value do you typically shoot for)? I assume where your fascia normally is, that gets sealed with plywood and spray foamed as well, to make the attic tight. I guess it would be easiest to spray the attic, before the main floor ceiling drywall gets installed. I assume this means no vapor barrier on your main floor ceiling or any insulation either. You just let the heat rise into the attic? Sorry for all the questions, but I'm an ICF virgin and know even less about the insulated roof system. Also, if anyone has any pictures or a short video an of insulated roof, that would be great as well.

sleddermb - if yours is a simple roof with around a 4/12 pitch, it might make sense to foam the underside of the sheathing. If on the other hand yours is a 12/12 complicated roof line, you have both a lot of square footage of redundant insulation as well as a lot of cubic feet of air to condition. Consider a couple inches of spray foam on the ceiling including the bottom chord of the trusses and then blowing cellulose insulation up to R50. It will be much more cost effective. Some like to put in a barrier (anything from osb to house wrap) on the underside of the trusses to isolate the drywall and or strength it to carry an additional load. Not necessary but might be helpful in certain situations. The advantage of spraying the sheathing is that it is fairly simple and quite easy to achieve a good seal and it limits the movement of the trusses due to thermal and moisture differences between the bottom chord and the balance of the truss.

Are you doing anything to accommodate expansive soils? i.e. pilings?

Oh man, this gets more confusing as time goes on Mechanical (horizontal air handler with electric backup heat - main heat and a/c will be an air source heat pump - I know, that's another thread altogether), will go in crawlspace and ducting ran in crawlspace as well. Does this change anything in regards to having a vented vs unvented attic. After doing some more reading yesterday, it appears that a good number of people that are insulating the underside of the roof have their mechanical up there as well.

1) Chris, by blocking the ends of the trusses, you mean you're sealing where air would 'normally' come up through soffits, correct, so there is no infiltration?

2) Dmaceld, thanks for the picts, but what is the purpose of putting in soffits, if you close off the wall-to-ceiling space or am I misunderstanding?

3) FBBP, The cottage wil be a mixed roof - probably 6/12 in the center (vaulted) with scissor trusses and then a standard 4/12 on both ends. Entire ceiling drywall, plus pine in vault area. By spraying a couple of incheds on the ceiling, are you referring to the underside of the roof and then blow in cellulose on top of the ceiling. As for the barrier, I assume using a vapor barrier (poly) above the drywall as used in standard construction. So now we have spray foam insulation on the underside of the roof decking, blow in cellulose with a vapor barrier. Would the soffits be sealed off as well, creating an 'envented attic'. In regards to the foundation, that's another thread as well. At the moment, I'm planning on a version of a FPSF - frost protected shallow foundation. (Insulated footing with ICF's ontop and probably pour an insulated pad in the crawlspace as well).

Sorry for all the questions guys, but it seems as there is nothing 'cut and dried', about this and that insulating thoughts change drastically, depending on the area you live as well. Also, how do you post a picture on here. I don't see any attachment tabs. I thought a plan of the cottage, may help for visualizing. Thanks, Greg

Not sure if this will be any comfort, or the opposite, but just for info.

This is the first house I ever built although I've done some remodeling in years past. I decided to do the entire design myself, with HVAC nephew helping out on heat/cool system. From inception of the floor plan to start of construction was about one year! And I'm retired! There are millions, well not quite but close, of issues to address and learn about. You're just getting started.

ENJOY the journey. It's great!!

Unfortunately retirement is about 15 years away. Plan on doing the majority of the work and design myself (with spec by engineer of course). Any regrets? Thanks for the moral boost :)

Posted By sleddermb on 27 Oct 2012 10:26 AM
3) FBBP, The cottage wil be a mixed roof - probably 6/12 in the center (vaulted) with scissor trusses and then a standard 4/12 on both ends. Entire ceiling drywall, plus pine in vault area. By spraying a couple of incheds on the ceiling, are you referring to the underside of the roof and then blow in cellulose on top of the ceiling. As for the barrier, I assume using a vapor barrier (poly) above the drywall as used in standard construction. So now we have spray foam insulation on the underside of the roof decking, blow in cellulose with a vapor barrier. Would the soffits be sealed off as well, creating an 'envented attic'. In regards to the foundation, that's another thread as well. At the moment, I'm planning on a version of a FPSF - frost protected shallow foundation. (Insulated footing with ICF's ontop and probably pour an insulated pad in the crawlspace as well).

Sorry for all the questions guys, but it seems as there is nothing 'cut and dried', about this and that insulating thoughts change drastically, depending on the area you live as well. Also, how do you post a picture on here. I don't see any attachment tabs. I thought a plan of the cottage, may help for visualizing. Thanks, Greg

Actually Greg - I was refering to sealing the drywall ceiling with foam and than appling cellulose over the foam. Gives you a good seal and high R at lower costs. However if you have a vaulted section and only 4/12 on the rest, foaming the underside of the roof sheathing is probably a good way to go. It would never make sense to separate the two insulation layers. Make sure you get a total seal so that any condensate happens on the foam and not between the foam and the roofing. If you are doing a truss roof, maybe have the truss co hold the truss heels back 2" from the outside, than you can apply 2" of sheet foam over the heels and notched around the tails to close your soffits.
The reason I mention the pilings is that there is a good chance you are in an area where the soils lift and fall due to water content. This is why you probably see piling rigs in your neighbourhood when a foundation is going in. I don't think FPSF will give you any protection from expansive soils.

Thanks for the reply. The soil is a real mix-mash including clay and throw in a huge rock shelf on the back 1/3 of the property. I'm really not going to know what's underneath, until I dig out the foundation. Probably dig some pilot holes in the spring and see what I hit. Again, this will be another thread in itself. I originally thought piles under the footing, but from what I've been reading, as long as you slope back fill properly and allow good drainage, people seem to be happy with the FPSF.

A vented attic is well proven and cost effective. I would add some type of air barrier between the drywall and the trusses.

One of the main benefits of insulating at the roof deck is where the HVAC duct work is in the attic. This keeps the HVAC inside the conditioned space. With your HVAC air handler and ducts in the crawlspace, this benefit of insulating the roof is lost and the cost of spraying foam at the roof line far exceeds the cost of a vented attic with blown cellulose above the ceiling (~5 times more for equivalent R values in my area). I would definitely consider the R49+ insulated ceiling / vented attic approach in this case.

If you go with the vented attic approach, look into raised heel trusses so that your full or nearly full depth of attic insulation extends all the way over the top of the walls. The raised heel especially helps with the scissor trusses by allowing sufficient room for the insulation at the wall.

To get the air sealing and vapor retardancy benefits of the spray foam, you might consider having ~2" of closed cell spray foam applied to the top of your ceiling boards, then covered with ~12" of blown cellulose. An added benefit of this approach is the strength provided by the foam lessening the possibility of ceiling board sag from the weight of the cellulose.

Also, I would consider putting your soffit vent baffles only in the truss bays that aren't scissor trusses to allow more insulation space in the vaulted areas.

Thanks Arkie, that was my original thought as well. A couple of inches of spray foam on the top of the drywall ceiling and top it up to R50, with blow in cellulose. Should you install a vapor barrier between the drywall and foam. Just curious, as obviously you can't apply any vapor retardent between the two. I'll probably change plans 10 times, before we build next year, but it's good to get different opinions. This is a great site and thanks to everyone for all the help and thoughts. Keep it coming :)

thats the way my buddys house is built here in winnipeg, spray foam over the drywall and then blown in on top of that.. i dont know if they used vapor barrier. i wonder if doing spray foam under the roof sheeting would over heat the shingles on a hot day, where as a vented attic would have some air movement

Posted By fallguy on 27 Oct 2012 09:16 PM
  i wonder if doing spray foam under the roof sheeting would over heat the shingles on a hot day, where as a vented attic would have some air movement

The roof shingles getting hot during a sunny summer day would have nothing to do with whether or not the roof has spray foam under the sheathing. The shingles themselves are responsible for either reflecting heat form the sun or absorbing it. Hence the reason why cool roofs stay cool, they reflect the suns solar heat, while dark asphalt shingles absorb it and amplify it.

The venting in an attic is there to help dry out any wood that gets wet. It's not really a means of cooling the roof surface from underneath the sheathing.

I wonder if anyone (roofing companies or otherwise) has done any studies on this. IE - shingle temperature and condition on a vented vs non-vented attic. I would love to try the unvented attic idea, but I don't think I want to be a guinea pig in the climate I live in (-30 winters). It sounds like a great idea in a moderate climate, slab construction, with your mechanical in the attic.

I reshingled my roof about 12 yrs ago. When they came out to do the work, he said even though my attic is vented, it wasn't enough. He said he was gonna add at least one more vent or 2. He said my roof would have lasted allot longer if it had been vented properly. He also said if you don't have your roof vented properly, it would cut the life outta of shingles by almost half. He had been in the business for over 25 yrs.

Posted By robinnc on 28 Oct 2012 10:13 PM
I reshingled my roof about 12 yrs ago. When they came out to do the work, he said even though my attic is vented, it wasn't enough. He said he was gonna add at least one more vent or 2. He said my roof would have lasted allot longer if it had been vented properly. He also said if you don't have your roof vented properly, it would cut the life outta of shingles by almost half. He had been in the business for over 25 yrs.

Posted By sleddermb on 28 Oct 2012 09:42 AM
I wonder if anyone (roofing companies or otherwise) has done any studies on this. IE - shingle temperature and condition on a vented vs non-vented attic. I would love to try the unvented attic idea, but I don't think I want to be a guinea pig in the climate I live in (-30 winters). It sounds like a great idea in a moderate climate, slab construction, with your mechanical in the attic.

Most roofers rely on anectodal "evidence" for these claims.

Regarding the first post, I don't doubt the roofer was trying to make an extra buck by selling and installing more roof vents. The same "scam" is done here in Phoenix. They try and sell you all these roof vents and electrical fans that push and circulate the air out of the attic. All with the "hope" that they will make your roofing material last longer and keep your home AC costs down. I call BS. We did a test with two identical homes, next door to each other, one home installed more attic venting and even had a fan that would cycle the air out of the attic. We shot thermal readings and the roof tile had identical readings on the exterior of both homes. Thermal reading were done inside of the attics and both had the same ambient air temperatures and the same roof sheating temperatures on the interior OSB roof sheating panels.

Now where the extra venting helped a little was at night, right before sunrise, the attic that had the electric fan showed a 5F degree cooler ambient attic space reading but the roof interior sheating were identical. The exterior roof tile was also identical in temps. What this told us was that the fan helped move the hot attic air out at night quicker than the attic without a fan. Yet it made no difference in roof tile temps because the roof tiles are both exposed to outside air. The 5F degree attic air temps made NO difference to help cool the underside of the tiles.

There is NO SCIENTIFIC proof showing the roof life is extended by adding electric attic vent fans or by adding more than needed cross ventilation inside of a cold attic. Attic venting has always been engineered as a means to dry out any wood that gets wet. At night when cooler temps arrive the attic vent fans help push the hot air out and bring in cooler outside air but this is all inconsequential on breezy days, as the attic vents on breezy nights will circulate the air out via natural breeze. No fan needed. Even with the 5F temp difference, it made no noticeable reduction in roof material temps, both outside or underside.

We did see a negative with the added roof venting put in by the roofer was that on really windy days, the wind would channel through the roof attic so much that it actually began to life the blown in insulation and push it around the attic area. During thunderstorms the high winds would actually cause the home with more venting to creak and pop the trusses more. We attributed this to there being more lift inside the attic, causing the wind to try and lift the roof trusses off of the top plates. These "added' vents were put in on the side of the gable walls.

Posted By robinnc on 28 Oct 2012 10:13 PM
I reshingled my roof about 12 yrs ago. When they came out to do the work, he said even though my attic is vented, it wasn't enough. He said he was gonna add at least one more vent or 2. He said my roof would have lasted allot longer if it had been vented properly. He also said if you don't have your roof vented properly, it would cut the life outta of shingles by almost half. He had been in the business for over 25 yrs.

Experience does not guarantee increased knowledge. Check out this quote from here.

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/view?searchterm=ROOF%20VENTING

Effect on Shingle Life

In general, shingles installed on unvented attic assemblies operate at a slightly higher temperature. This has impacts on the durability of roof assemblies. A 2 or 3 degree F. rise in average temperature is typical for asphalt shingles and a corresponding 10 degree F. rise in average temperature for sheathing (Parker & Sherwin, 1998; Rudd & Lstiburek, 1998; TenWode & Rose, 1999). All other things being equal, applying the Arrhenius equation (Cash et.al, 2005), a 10 percent reduction in useful service life should be expected. This is comparable to the effect of the installation of radiant barriers. What is more significant to note is that the color of shingles and roof orientation have a more profound effect on the durability of shingles than the choice of venting or not venting (Rose, 1991) – double or triple the effect of venting/non venting.

Light vs. dark is much more of a factor than is vented vs. unvented.