Do I need water vapor barriers on a this SIP and stud double wall?
We are building a house near St. Paul MN and we have a double wall consisting of 8″ SIP panels with Neopor GPS on the outside (R40) and a 2×4 wall assembly with studs on 16 inch centers that is 2 1/2 inches away from the SIP panels with dense pack cellulose (overall 6 inches of dense pack cellulose) (R20) for an overall insulated wall of R60. We will have a vapor permeable membrane on the outside wall. The design temperature for St. Paul, MN is -11 degrees F. The record low temperature is -34 degrees. My question is in regards to moisture management. If I have the inside of the house at 74 degrees maximum temperature and 40% relative humidity, do I need to be concerned about condensation buildup in cellulose insulation of the inner walls? Do I need a vapor retarder somewhere within this wall sandwich? If so, what is recommended and where should it go? The walls are going up now so if we need it we need to add it soon.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You have what is called a Hybrid Wall Assembly, meaning the wall consists of both continuous exterior insulation (provided by the structural insulated panel or SIP) and cavity insulation. If a wall is able to be designed the so that a specific ratio of exterior insulation to cavity insulation is met, you can move the interior vapor retarder requirement to a class III vapor retarder, which is simply painted drywall. That exterior to cavity insulation ratio for climate zone 6 (Minneapolis/St Paul) is 35% exterior to 65% cavity. Your ratio is the opposite with about 65% exterior and 35% interior. The potential for condensation will be inside the foam of the SIP, you do not need a class I or II vapor retarder (usually polyethylene sheeting in Minnesota). You can read more about insulation ratios here: https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
40% relative is on the high end for what is recommended in cold climates. You won't have to worry about condensation in the wall, but the window glass might become wet during the colder temps of winter. A good triple pane window will help.
Thanks Randy. We have ZOLA ZNC windows which are triple pane. Our preliminary HERS score without solar is 26. Very well insulated and very tight house. Nice to know there is a simple solution to the potential problem of condensation within the exterior wall.
I don't think any vapor will have the potential of condensing within the cellulose fiber ...
What you have to be concerned about is the first condensing surface potentially being the inside facing OSB surface of the SIP panel
That is when Randy's 35% (rigid exterior) to 65% (fibrous interior) ratio kicks in ...
I would make sure the inside seams of the SIP panel are taped per manufacturers recommendations
Thanks. We have decided to over engineer a bit and add Intello Plus on the inside walls. Not only should it address any concerns with condensation it will help with the air tightness. According to ZOLA we will probably have to start adjusting the RH down when it gets below zero to avoid condensation on the windows anyway. We will see if we can keep it above RH 30 on really cold days. We will have the SIPS taped according to manufacturer recommendations. Thanks for your insightful and valuable comments. We will have a tour of the house while still in construction in late August. Please send me an email if you wish to join and I can give further details.