extra foam boards on exterior sheathing
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we have new construction planned for coastal North Carolina, aiming for Energy Star rating. we plan to have concrete lapped siding on top of 5/8″ OSB, 2×6 framing and foam insulation (pro application) under drywall. Question – is adding 1/2″ blue foam board on outside of OSB before Hardie plank worth it or are we adding headaches? we would need a new nail gun at least b/c old one only delivers up to 2.5″ nails and we figure we ought to use face nailing instead of blind nailing (hurricane country).
Edited 8/31/2005 7:30 am ET by cat
Replies
Google up the building Science forum and study it all, starting with Joe's top ten...
Why oh why does everyone keep getting it backwards?
In a northern heating climate, the foam should be on the inside, under SR. In your southern cooling climate, the foam should be on the exterior side of the assembly.
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Piffin - I don't think I'm getting it wrong, I am hoping to put the foam board outside b/c I'm in the south. we were looking at insulation options for the interior wall cavities too though - extruded foam is an option, just as fiberglass batts are an option. but if I'm risking excess moisture and future condensation within the walls, I'll not use foam on booth sides, just the outside.trying to google building sciences forum - do you mean NIBS.org or "jlconline"? can't find a "Joe's top ten" there; can you be more specific?
http://www.buildingscience.com/Outside is right in your climate. I had the impression that you already had some inside too. It is often easier to place on the exterior, so even in cold climates, some guys do it there which is too bad.
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You are going to add more grief to the project than 1/2" of foam is worth, my opinion.
Joe H
I agree with Joe H--a lot of fooling around for very little value (1/2" of foam maybe gets you R-3?).
As Piff said, the foam usually goes on the outside in the south. Putting it inside and outside may create a situation where the dewpoint of the assembly gets reached inside the wall cavity, creating condensation, mold, and rot problems. Best to have the proposed assembly modeled by someone who knows what they're doing than to just start throwing extra insulation on.
Sometimes, what seems like a good idea can turn into a disaster.
Bob
You mentioned using a nail gun to apply siding to the OSB. You may want to consider furring out the OSB (use ceramic coated SCREWS) to provide an air space - minimizing mildew, etc; then nail your siding with SS nails to avoid staining.
A second look sez, this is a poor idea. You're going to use a nail gun to face nail hardi?
Every nail is going to pull the siding into the foam, so you are going to have waves down each piece.
Nailing into OSB isn't my idea of hurricane proof, although seeing what's on TV today looks like not much is really hurricane proof.
Joe H
If you are spraying in foam like icynene, I doubt you will have a problem passing the energy star. I talked to a few energy star reps and what they really are looking for is air leakage. They make sure you have the necessary R values for the insulation but even the fiberglass will pass that, but fiberglass is much tougher with air leakage, the icynene will seal everything up very well. As others have said, I think the foam may create a moisture problem and make it diffcult for flush nailing siding. Also, if you are concerned with hurricanes and high winds, having the foam rather than the sheathing directly behind the lap siding I think would also be a negative.
I'm not in hurricane country but from what I alway see with hurricanes and tornados, the siding is usually fine on the houses built to current codes but the roofs take a beating. Hardie sits very tight to the sheathing, if you stay with the 4" or 5" exposures and blind nail into the studs, it should be fine. The siding sits so tight that air cannot easily get behind there and create the up lift. What you see on tv now and in other disasters are the houses with larger roof overhangs create a lip for the wind to grab onto and remove the roof as one piece. I would work with an architect and avoid larger overhangs and any other type of design that traps the wind.
Third look, if you are spraying inside, how much are you putting there?
Couple inches oughta do it for your locale, but if you want more it's sure easier to just spray it.
Joe H