Insulation choices, California North Coast (mediterranean climate)
I’d been planning, in the current remodel, to insulate the walls with sheet foam (preferably polyisocyanurate for its higher R-values) sprayed in place with the spray-in-a-can. I am now getting very confused, though about vapor barriers. Polyiso and spray foam are both very low in permeance (at least what I can get around here; can’t get unfaced polyiso). That would have made the insulation the vapor barrier. However, because the walls on our old house are unsheathed, we’re sheathing with 1/2″ CDX on the inside of the framing, for shear strength (that is then covered with drywall). And the reading I’m doing indicates that plywood is relatively low in permeance. If I insulate with foam, am I creating a double vapor barrier that could lead to mold in the wall cavity?
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Insulation water vapor
You are in a hot, wet maritime area?
The sun is always going to be a problem.
You need to keep this in mind when deciding where to place your insulation and vapor barrier.
Keeping in mind that most heat will enter your home by conduction, via the roof, wall and floor timbers, the best place for your insulation is on the inside/room side of your walls, floors and ceilings.
This will give you an insulated box - rather like a Thermos flask or refrigerator.
If you keep in mind also that water vapor moves from warm/hot to cold, then you need to decide are you running air conditioning - where the movement of water vapor is inwards, towards the cold.
Or is it colder outside, where the movement of water vapor is outwards, to the sky. If there are thousands of microscopic holes in the sides of the walls that are not insulated - then any water vapor that gets into the walls can escape. In your area, while the sun will drive water vapor into a wall, the heat of the sun will dry the walls again, or the cold at night will draw the vapor out. As long as the walls can dry, one way of another you won't have a problem.
3 inch thick sheets of polystyrene are water proof and water vapor proof. But these need to be fitted carefully, to avoid any holes and cracks. The molecules of water vapor are far, far smaller than air, and they can move between the molecules that make up a plank of wood. Therefore, it makes sense to cover the polystyrene with water vapor proof sheets of plastic with all joins carefully taped. The plastic should be fitted on the warm side of the polystyrene sheets.
While I know that polyiso is useless at low temperatures, OK for your area, I do not know if it is open or closed cell and therefore is /is not water vapor proof. This is something you need to check.
Insulation
I live in NorCal in a marine climate (napa) and also am currently in the middle of an upgrade seemingly similar to yours. My house was built in 1940 approx 850 sq ft and has no exterior sheathing and wood lap siding. I had mold growing on the lower portion of my interior walls on my outside facing walls. Originally planned on pulling the siding and resheathing to modern times with OSB, Tyvek, a rainscreen, and try to reuse the wood siding. We decided we like our wood siding and dont want to replace it or damage it. Knowing the project could get pricey that way, we decided to pull the walls down on the inside and add Roxul insulation batts. I investigated other options, but for this project I knew there was fire breaks in the wall so blow-in was not really desired also most other insulation options dont like to get wet. The Roxul can get wet and resists mold. I am just going to remove the old builders paper attached to the siding and coat the backside of the wood siding with a primer. Also tighten up any gaps between the siding and the studs. Place the roxul batts and then cover with 5/8"- "greenboard"(mold and water resistant sheetrock). I just assume water will travel in the cavity whether its vapor or liquid. Vapor barriers from what I research have gotten over use and tend to create more problems than solve. Water vapor is always present in your house if you breath ,cook , boil water, run the sink, take a shower, etc. Latex paint is also considered and effective vapor barrier.
Are you placing anything between the plywood and sheetrock? I never thought to put the OSB on the inside...I sort of like that idea....I wanted to add strength as well which is why it was a hard choice.....
I found some good info about vapor barriers here:
http://www.buildingscience.com/search?SearchableText=wood+siding