Hi all,

I searched the old forum topics hoping this was already answered, but didn't find anything.  Could someone please tell me if it's OK to apply spray foam directly to the inside of wood siding? We are thinking of building a 2x6 wall, cover the outside with the siding (no sheathing or house wrap), spray closed cell 2 lb insulation in the cavity from the inside, and then put on drywall on the inside.

The spray foam guys, (soy-based) says this should work just fine as it will prevent any moisture for getting behind the wall, seal all the cracks and hold the siding nicely in place.  The foam is an air barrier, moisture and vapour barrier. There is some risk it might cause the siding to bulge, but the foam expansion is only 30%, and the cavity is open, so they think expansion will go towards the cavity, not the siding.

The store that sells the siding thinks this should work well too, again because it will seal any moisture off from the back of the siding. However, it would nullify the warranty since this isn't what the makers of the siding suggest.

The makers of the siding, CanExel, (I called) says I can't do this because the back needs to breath in case water gets back there, and that I should have sheathing, house wrap and furring. I couldn't get them to understand that moisture won't get back there in the first place if the foam was sprayed on it. We would have to carefully caulk the outside, and repaint religiously, but we would do that anyway.

Since spray foam is sprayed onto OSB sheathing, we think it should be fine on the wood siding.  The foam helps to solidify the wall, and the siding, nailed on every stud, serves the same function as the OSB in strengthening the wall.

Our house is a one story, 1500 sq ft. house.

Are we asking for trouble?  Is this something that would work?  Any suggestions?

Brian

Posted By blue on 23 Apr 2010 11:07 AM
Hi all,

I searched the old forum topics hoping this was already answered, but didn't find anything.  Could someone please tell me if it's OK to apply spray foam directly to the inside of wood siding? We are thinking of building a 2x6 wall, cover the outside with the siding (no sheathing or house wrap), spray closed cell 2 lb insulation in the cavity from the inside, and then put on drywall on the inside.

The spray foam guys, (soy-based) says this should work just fine as it will prevent any moisture for getting behind the wall, seal all the cracks and hold the siding nicely in place.  The foam is an air barrier, moisture and vapour barrier. There is some risk it might cause the siding to bulge, but the foam expansion is only 30%, and the cavity is open, so they think expansion will go towards the cavity, not the siding.

The store that sells the siding thinks this should work well too, again because it will seal any moisture off from the back of the siding. However, it would nullify the warranty since this isn't what the makers of the siding suggest.

The makers of the siding, CanExel, (I called) says I can't do this because the back needs to breath in case water gets back there, and that I should have sheathing, house wrap and furring. I couldn't get them to understand that moisture won't get back there in the first place if the foam was sprayed on it. We would have to carefully caulk the outside, and repaint religiously, but we would do that anyway.

Since spray foam is sprayed onto OSB sheathing, we think it should be fine on the wood siding.  The foam helps to solidify the wall, and the siding, nailed on every stud, serves the same function as the OSB in strengthening the wall.

Our house is a one story, 1500 sq ft. house.

Are we asking for trouble?  Is this something that would work?  Any suggestions?

Brian

Trapping rain moisture behind the siding will result in early failure of the siding.  Siding needs at least some back-ventilation to be able to dry.  Wind driven rain, and even vapor drives of sun on dew or rain-wetted siding sends the moisture inward, and if blocked by a highly water repellent surface like foam it'll stagnate there in/against the wood, with nowhere to go- the siding will rot from the interior side first, but rot it will. (Unless you live in a very arid place, that is.)  If you used vapor-retardent paint & caulk it would only make the moisture trapping worse, not better. The siding manufacturer has it right, the foam guy and the guy at the store are all wet on this one.

When sprayed on OSB sheathing, the OSB has housewrap or felt on the exterior working as a drain-plane to let gravity do it's work on liquid water, and at least micro-gap behind the siding for vapor-drives give the moisture somewhere to go.  If a rainscreen gap is provided for back ventilation of the siding it works even better- the siding stays drier, and less liquid finds it's way to the drain plane for even less vapor permeation into the OSB.  IIRC building codes in Canada now require 10mm of rainscreen under wood siding.

If you went with siding materials unaffected by moisture it might work, but you may still run into issues at the stud edges.

Assuming the wall is properly framed/braced against racking forces and doesn't need structural sheathing, putting something thin (such as housewrap) held in place with strips of 3/8" or thicker furring at every stud as a matrix to spray the closed-cell foam onto might work. The furring would then be providing the back-ventiation/rainscreen for the siding, and everybody is happy.  If the foam is intended to provide the racking-force structure, I hope this has been properly engineered.  Thin steel X or corner bracing may be required to keep the thing from falling down in a high wind otherwise.

Dana1,

Well said.  Easily understood.

Dana1,

Thanks for the clear and decisive answer. We will follow your advice and put OSB sheathing and a house wrap.
The extra cost doesn't justify the omission if it means our siding rots off.

Best,
Brian

Posted By blue on 23 Apr 2010 09:16 PM
Dana1,

Thanks for the clear and decisive answer. We will follow your advice and put OSB sheathing and a house wrap.
The extra cost doesn't justify the omission if it means our siding rots off.

Best,
Brian

if you're going to use sheathing of some sort, it may be worth looking into using foam board instead of osb.  We're in the middle of a building project right now & that's what we did.  For wind bracing, some areas required structural sheathing.  In those areas we used 1/2" OSB.  For the rest, we used 1" foam board.  The areas with 1/2" OSB got an additional 1/2" of foam board to keep it all on the same plane.  It gives us an extra R3-R6 with a complete thermal break.  The total cost was about $200 more than using OSB for the entire structure.  It's a very cost effective option for some extra insulation.

Depending on budget, there is also a product out there called structural insulated sheathing - it comes in 1" or 1/2" and is used in place of OSB, but offers (if I remember correctly) R6/inch.  In our case it was significantly more expensive though.

The Dow Styrofoam SIS insulating structural system is R5.5 @ 1" thick, which would indeed form a good thermal break on a 2x6" stud, basically doubling the R-value of that thermal short-circuite. Whether it's sufficiently structural for this building would have to be assessed. (Probably is, at 1-story, even more so with the additional structure of closed-cell foam as cavity fill to add rigidity.) Trading an inch of closed-cell in the cavity-fill for something like SIS would result in a higher clear-wall R-value due to the thermal break at the studs, if it comes down to a budget issue. It doesn't hurt to use their online bracing calculator to check.

But doing the corners with half-inch OSB and the rest with 1" XPS would probably work as well. Steel bracing (no OSB) and non-structural foam over the whole thing, is also a fairly cost-effective solution. The CanExel siding will stiffen it up a lot too, if mounted on furring long-screwed through foam sheathing into the studs rather than using less-structural rainscreen methods.