Polyurethane Foam sprayed on the OUTSIDE of a house

[eonibm]

In my quest to build the most energy efficient home possible, I am considering having about 1 to 1-1/2" of 2-lb (ie closed cell) polyurethane foam sprayed on the outside of the plywood sheathing of the next home I build (in addition to 4.5 inches on the inside to fill the 2x6 stud cavities, and 1" in the attic, in addition to blown in cellulose on top). I am doing this to avoid the thermal bridges created by studs and because of the extremely high performance of polyurethane spray foam, which seals the house unlike any other product other than maybe ICFs (which I decided to use only in the basement foundation walls, as I find using ICFs above grade makes future renovations of the exterior envelope more difficult). I had considered building a second 2x4 stud wall on the inside (offset from the primary exterior 2x6 stud wall) and insulating the entire space with cellulose or fibreglass, but feel that polyurethane is a better solution (and I sure didn't want to spray more than 4.5 inches of polyurethane). Also, one gets an extremely tight and seamless fit, without having to tape of foam the joints like with rigid styrofoam on the outside. I can also avoid putting in house wrap or a vapour barrier in this way. (as long as I don't have an unknowledgable building inspector!).

The home will have brick veneer tied to the sheathing. Has anyone had any experience with using polyurethane foam on the exterior of a home with brick veneer? I'll be providing the customary 1 to 1.5" airspace between the foam and brick wall, but am wondering if there are any other details or tips that are important. For example, am I correct that the brick ties are installed (on the stud walls) pre-foaming? I can't see how you would install them after the foaming..

[Martin Holladay]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

Dear Eonibm,
The system you propose is fairly common in Canada, especially for retrofits. To speak with someone familiar with the details, you might want to speak with a technical rep at a spray foam manufacturer, for example:
François Lalande, technical rep
Demilec Canada Inc.
870 Curé Boivin
Boisbriand, Québec J7G 2A7
Canada
450-437-0123
866-437-0223
Fax: 450- 437-2338
www.demilec.com

François is familiar with these details.

[Muttly]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

If you are worried about bridging, then why not use sips.

[eonibm]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

Actually, for the reason I mentioned in my initial post regarding rigid styrofoam panels. I believe that a continuous layer of spray in place polyurethane is the only sure way to acheive a perfectly tight seal (bridging is one thing, air-tightness is another). Taping the joints, or foaming them, might be close, but nothing beats spraying the foam right on, on-site.

[m beezo]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

I have not seen lots of spray foam jobs but the one I did have done I was surprised at how irregular it was. I actually wondered if the insualtion was done right until I did some more reading. I mean you spray it on and it forms a hard skin then it gets shaved off. Or it is a bit shy of the stud face and is not at all smooth. I do not know how you would be able to get a smooth surface that you could install something over it.

Plus I am a bit leery about the lenght of nails you would be using. You are dirving a long nail thru a inch of foam. I do not like the way you can bend a nail back and forth when it is not set into something solid. If I was going to want to use foam on the outside I would vote for the foam panels which would be a consistent thickness to insure a flat surface to nail onto.

[Martin Holladay]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

M Beezo,
The usual method includes the installation of exterior strapping over the sheathing before the foam is sprayed. The best jobs have two layers of exterior strapping: first a horizontal layer of 1 1/2" thick strapping, followed by a vertical layer of 1 1/2" strapping. Spray the foam, then install the siding. In the Southwest, however, exterior wall foam is sometimes covered with a coating so it looks like adobe or rough stucco -- the bumpiness becomes part of the charm.

[CB Rob]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

Eon
check out this link for some good brick wall assemblys.
http://www.buildingscience.com/docum...ion/main_topic

I think the only problem with what you have in mind would be the cost. that closed cell foam is pretty expensive, I believe its between 2 and 3 bucks a board foot. You might be able to achieve your goals with sheet foams and open cell foam. I think you might be over doing it with your concerns on air tightness, you dont want any leaks or drafts and the moisture problems that come with them. But the tightness of open cell is , in my opinion, enough to prevent air transported moisture from getting in the walls. You might even be installing an HRV system that will be swapping your air on a regular basis with some heat loss as a result.
If you have not gotten it already, you should pick up the BUilders Guide to Cold Climates. Its got all the info you are looking for on building the best home for your climate. Good luck on your quest.
Rob

[Ted S.]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

I am very familiar with this installation but it is typically for commercial projects.

Demilec is one manufacturer, the other is BASF and the product is Walltite.

http://www.basf.com/urethanechemical...tite/index.htm

The detail is now used down here in the states with the new energy codes such as the one we have in MA. Specifically Chapter 13 of the MA Building Code - which does not apply to low rise wood frame. I know it was running at $3.75 psf for 2 1/2" SPF not long ago. The brick ties were first installed on the sheathing and then the SPF applied around the tie using the tie as a screed for thickness of the foam. I have only seen it applied over densglass and CMU but I would think it could go over plywood.

For residential construction I would think you would be better off with a sheet of XPS outboard of the sheathing and an icynene or biobase in the stud cavity.

Or maybe you shouldn't chicken out and just go up with the ICF for crying out loud. Heck I did it, couple diamond blades and you can pop a window in later no problem.

[eonibm]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

Thanks for all of the comments. I think after reviewing everything and also realizing since I am going to be spraying polyurethane foam insulation on the entire inside of the house that I can suffer not having it on the outside. On the outside I think I will go with Dow extruded polystyrene shiplapped sheets with the seams either foamed (and the excess foam scrapped off) or taped. My main problem with the polyurethane on the outside is that I've seen this stuff sprayed and it is very difficult to get an even surface. (However, I may reconsider if a supplier can show me an installation with an almost even 1" surface. I put in a call to Demilec and am waiting to hear back). Also, I am wary of the need for a 1" airspace to allow the brick to breathe and any excess moisture to run down the inside and the polyurethane could get in the way if the surface is not even. To further complete the interior seal I think I will caulk any sandwiched studs to avoid any air leaks through those cracks (albeit they would be small and there will be polystyrene on the outside anyway).

[Sweep8]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

If you are worried about air sealing the exterior face of the sheathing consider using STO GoldCoat liquid acrylic coating with mesh reinforcing at the sheathing joints and wrapped into the window openings. Then put enough rigid insulation over it so that the dew point occurs within the exterior insulation and you don't need an interior vapor barrier. Anyone can apply GoldCoat with a brush, roller or sprayer.

Brick ties would be attached to the sheathing face.

[eonibm]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

I was going to use Blueskin wrapped into the window openings. Why would STO GoldCoat be better. Also, why does having the dewpoint in the exterior insulation mean I don't need an interior vapour barrier?

Also, with respect to the brick ties being attached to the sheathing, do you mean you attach them through the rigid polystyrene but to the plywood sheathing?

By the way, Dow does not have a single drawing on their site (but they seem to have about 50 drawings of everything else) detailing a cross-section of a wall with brick veneer, brick ties, airspace, their rigid insulation (Cladmate, Weathermate, etc) attached to plywood sheathing. I phoned them and they looked everywhere and then told me all they had was shown on the site. Amazing considering it's quite a common detail.

[CB Rob]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

Water vapor in your home, and walls is only a problem when it condenses in to liquid water. suppose your house is 70 degrees with 30% humidity, and outside it might be around 10 degrees. If you had no exterior foam, and no vapor barrier the air/vapor from inside will hit 100% humidity when it contacts the cold plywood. It will condense on the plywood just like the frost you might get on a single pane window. If you use a vapor barrier, the moisture from inside can not get through and condense on the plywood. If you use enough foam on the outside to keep the plywood warm the humidity at the plywood will not reach 100% and water will not condense on it.

While a material like drywall, or open cell foam may stop air from blowing through, water vapor is able to work its way through. kind of like goretex.
Adding foam to the outside to keep the plywood warm is generally preffered over useing a vapor barrier. There is also some middle ground here also, by warming up the plywood a little, and slowing the vapor a little (vapor retarder).
www.buildingscience.com is the source for tons of info on this stuff.
Also, a brick cladding baking in the sun after a rain can have a huge vapor drive inwards that may need to be addressed. (exterior foam can be part of the solution for this also).

[Ted S.]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

Henry/Bakor Blueskin is a great product, I am a fan of it over "other well market brands" . Carisle is another good line. If you go with Blueskin, try the Auqprime primer, you'll be glad you did. You can get it out of your hair with water. The Blueskin is an impermeable, self healing sheet memebrane, top of the line...the Sto Gold and Dryvit Backstop (all the major EIFS Manuf's have a similar product) are a vapor permeable, non-self healing fluid applied membrane. Also top of the line...but there is a difference between the 2 classes of membranes. You need to understand what is happening with your wall before selecting a membrane. Download the WUFI and run some models for you dewpoint if you really want to explore what you are doing. They call it a hygrothermal analysis now, "dewpoint" is old science.

For the Dow XPS insulation shown in an assembly, go to this web site:

http://www.pacerepresentatives.com/

go over to the left image and click on wall assemblies, everything you need is right there. They also rep a posi-tie brick tie which is driven through the XPS into a stud (if you can find it) which would be key for your ship lapped XPS as you don't have to cut and foam around the tie. It has a rummer gromment that seals as you drive it. Again though, you are into the commercial means and methods.

As for this statment "Also, a brick cladding baking in the sun after a rain can have a huge vapor drive inwards that may need to be addressed. " I total agree with that and the use of foam as part of the soultion.

[CB Rob]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

a couple months ago Ted told me about the WUFI software. The display of water and heat transfer, along with all the other data was the ultimate learning tool for me. Eon, download the free version and have some fun trying different assemblys. Thanks for telling me about the WUFI, Ted.
Rob

[eonibm]

Re: Polyurethane Foam sprayed on the OUTSIDE of a house

That www.pacerepresentatives.com website is a gem, but has given rise to even more questions.

They reiterate that my own CMHC here in Canada advises not to place any insulation inside the stud cavity and only insulate out side the studs (news to me - everyone puts insulation in stud walls). Pace modifies this recommendation, under their 'Best Practice Brick and Block' under 'Brick, Metal Stud Backup System C' and specifies that a maximum of 1.5" of DOW polyurethane spray insulation (R6.5) be placed on the inside of a stud cavity. I emailed them to get some clarification on this and they said that while they agree with the CMHC's recommendation, they recommend 1.5" of SPF as it stiffens and seals the wall. The further clarify that only 1.5" is used in order to keep the dew point outside the stud cavity and that if you filled the cavity with more insulation the dew point would potentially fall on condensation surfaces in the stud cavity.

This wall has brick on the outside, 2 inches of Dow Cavitymate rigid polystyrene, 1.5 inches of SPF, and other components providing small portions of the total R-value (Dens Armor, vapour barrier, Dens Glass) for a total R-value of 23.1. It seems I can't increase the SPF insulation without violating the CMHC guideline.

I understand all of the dewpoint/wall concepts, but now have more questions:

1. Am I to understand now that the common practice of filling wall stud cavities with 2-4, or more, inches of spray foam insulation (in which case the SPF acts as a vapour and air barrier) and not putting any rigid polystyrene on the outside of the house is wrong?
2. Is it really worse for the wall assembly to put more SPF in the stud cavity This seems counter intuitive to what I would have thought (but I do understand the science behind it).
3. Am I really to believe there should be no insulation in the stud cavities of walls? (99.999% of all homes are built with insulation in stud walls!)

Can anyone shed any light on all of this?