High R-value and WRB
We have a home that was build in the 70’s. By our estimations there is probably R7 in the walls, OSB, building paper and shingles on top. We would like to take off the current siding and add additional insulation.
Our main concern is lower energy bills and comfort. Due to esthetics (windows already set) we cannot go higher than 1 inch (or so everybody says), so polyiso would make most sense in order to add the highest R-value. If we use it as a water drainage plane, the vapor barrier would have to go on the outside but wouldn’t that render its vapor retarder function useless?
For housewraps, most recommendations are of at least 3/8 inch drainage/ air flow space so putting it on the OSB then adding the rigid foam boards and leaving that drainage space seems ineffective since cold air will still get close to the OSB/ interior walls.
The house is located in zone 4, NY. Does polyiso with the foil side attached to the OSB, followed by housewrap (maybe ThermaWrap by Tyvek) and vinyl siding make sense for our climate? None of the contractors we talked to had this suggestion. A few of them also recommended the siding shaped foam (“insulated siding”), but again where would the water drainage plane be in that case and would there be any advantages to using it?
Any advice of how we can get a high R-value on the exterior and have good water management is welcomed. Thank you!
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Replies
L.S.,
Q. "Polyiso would make most sense in order to add the highest R-value. If we use it as a water drainage plane, the vapor barrier would have to go on the outside but wouldn’t that render its vapor retarder function useless?"
A. Every wall needs a water-resistant barrier (WRB). This is different from a vapor barrier, however. Most (but not all) WRBs are vapor permeable.
Most walls do not require a vapor barrier. In cold climates, however, a vapor retarder is useful. A vapor retarder is installed on the interior of a wall, not the exterior.
It is possible to use taped foil-faced polyisocyanurate foam as a WRB. It is also possible to use housewrap as a WRB. If you use housewrap as a WRB on a wall with OSB and polyiso, the housewrap can go under the polyiso or over the polyiso -- your choice.
Q. "For house wraps, most recommendations are of at least 3/8 inch drainage/ air flow space."
A. You are describing the functions of a rainscreen or drainage gap. The rainscreen or drainage gap can be adjacent to the WRB, but it doesn't have to be.
Q. "The house is located in zone 4, NY. Does polyiso with the foil side attached to the OSB, followed by house wrap (maybe ThermaWrap by Tyvek) and vinyl siding make sense for our climate?"
A. You could do it that way. I think it's better to put the housewrap on top of the OSB, followed by taped polyiso, followed by vertical rainscreen strapping, followed by siding.
Q. "A few of them also recommended the siding shaped foam ('insulated siding'), but again where would the water drainage plane be in that case and would there be any advantages to using it?"
A. Not all walls have a drainage gap. It's a good idea if you can create one, but it isn't essential. The advantage of using insulated siding is reducing the thickness of the foam + siding.
Thank you very much for the quick response! I had imagined a water barrier needs always a drainage gap adjacent to it which was complicating things!