Hi we are building a new sfr in washington state and spray foamed our lid and walls (2×8 with staggered studs) with urethane based corbond.
To save money we thought we would just have the insulation contractor spray the amount nessissary for code and we would fill the rest of the cavities with batts or cellulose.
Then we were wondering if water would condense on the bottom side of the corbond if we put insulation below it.
Also if we used cellulose in the walls and contained it with poly if hiving two vapor barriors would leave the wood currently exposed in the cavity with no place to dry to. It is dry now but I have always heard that wood rots because it exceeds its capability to dry not because it just gets wet.
Does any one have any reccomendations or warnings?
Replies
If you have closed-cell foam (or a similar impervious insulation product) on the outside and fiberglass (or a similar non-impervious insulation) on the inside, there needs to be enough insulation value in the foam to assure that the dewpoint of the inside air will never (well, rarely) be reached in the fiberglass.
That is, as you cool air it's relative humidity goes from "room" conditions (say 35%) to saturation (100%). The temperature at which the air achieves saturation is the dewpoint. You need enough insulation value in the foam (and relatively less insulation value in the fiberglass) to keep the temp at the foam inner surface above this point of saturation.
For instance, for inside winter conditions of 70F and 35% humidity the dewpoint is 40F. If half of your insulation value is in the foam and half in the fiberglass, the dewpoint will be reached (using "meatball" calculations) when the outside temp is about 10F (since 40F is halfway between 10F and 70F). (Note that generally significant window condensation will occur before this point, leading to a reduction in inside humidity.)
In this situation, since the foam is such a good vapor barrier, having an interior vapor barrier would not be useful and may be harmful.
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