Zone 6 basement and ceiling vapor barriers
I have two general questions related to vapor barriers on the house I am currently constructing in North Dakota (Zone 6).
1. Aside from the top 18”, the basement is below grade with 2” XPS foam on the exterior. The interior walls are framed out. What approach should I take for insulating the interior of the walls? I would prefer to stay away from spray foam and, seeing as the walls are framed, rigid foam does not appear possible anymore.
Additionally, local code requires a vapor barrier. Will this be okay? Should I use a smart poly barrier or question the use of a barrier altogether with the local officials?
2. The home has cathedral ceilings using parallel chord trusses, creating a 2’ space within the rafters. The roof has soffit vents and a continuous ridge vent. I intend on using fiberglass bats to insulate and leave an air channel for ventilation. Should poly, smart poly, or neither be installed behind the drywall? I have not yet gotten ahold of the building inspector to confirm local code but would like to know what the best practice would be.
Thank you in advance!
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Replies
Jeff,
You don’t want air permeable insulation against the foundation. Martin outlines the whys here: https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall
Is the framing tight against the wall?
Martin also has an article on cathedral ceiling that outlines several options for your climate: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
Have you installed your roofing material? If not, would you be open to installing rigid foam on the exterior?
Steve,
I understand that fiberglass is not desirable in the basement but wasn’t sure if mine is an exception due to the exterior foam keeping it warmer. We left a gap, 0.25”-0.5” or so, between the studs and the walls. What are my options? Can I cut rigid foam to fit between the studs or do I need to spray foam? Most people around here use fiberglass but I know that isn’t good. And then there is the issue of code requiring a vapor barrier..
The roof is on, so rigid foam is not an option. At this point I want to determine my best option or if fiberglass bats with an air gap behind them will work. Also whether or not I should have a vapor barrier on the ceiling or not.
Jeff,
The two inches of is worth about R-8.4, but you really need R15 for the total wall. Maybe one of the experts (that’s not me) will weigh in with possible options.
Here is another article that may answer your ceiling question: https://www.greenbuildingadvisor.com/article/insulating-scissors-trusses
Did you use raised-heel trusses?
Steve - thanks again for the help.
I know I need more than just the 2” of foam, I’m just not sure if the foam exterior alleviates any of the concerns with using air permeable insulation. But probably not. I may just need to use a cut and cobble method of rigid foam.
Yes, the roof trusses have a raised heel of 2 feet. So there should be plenty of room for insulation and air movement.