Replies

1. 
   DCContrarian | Mar 17, 2022 05:52pm | #1

   At first blush this sound super-sketchy but I need more details.

   One of the common mistakes made in building a house is not having a clear distinction between the conditioned parts of the house -- the part that is heated and air conditioned -- and the unconditioned parts. Basically the transitions should be treated like exterior walls, which have weather barriers, air barriers, vapor barriers, and insulation.

   Normally, basements are considered part of the conditioned part of the house, and those barriers go in the walls and floor of the basement. It sounds like what the architect is wanting to do is to treat the basement as unconditioned. That's not impossible, but it's unusual. This is sometimes done with crawl spaces, but rarely with basements. It means you have to treat the basement like it was outdoors, so no plumbing running through it if you live in a climate where freezing weather is possible.

   You don't say what climate you're in. In a heating climate, having an insulated floor over an unconditioned space can be problematic. Exterior surfaces need to be able to dry if any moisture gets in, and some always will. Moisture will tend to move from a warm area toward a cold area, and from a damp area toward a dry area. In the proposed assembly you'll have moisture drive from the outside in on both sides of the floor and it won't ever dry. The batt insulation will get wet and never dry. In a cooling dominated climate where you run AC most of the year the vapor drive will be from the basement up and the floor should be able to dry as long as the flooring material allows vapor through -- so no tile.

   Exhaust fan is a terrible idea, you should be relying on natural ventilation in an unconditioned space. The exhaust fan will create negative pressure which will draw air from the conditioned space. During the heating season this air will be more humid than outdoor air, which will make your moisture problem worse. It will also make your heating bills higher. In most of the US, in the summer, the outside air will have a higher moisture content than the basement, so ventilating with outdoor air will increase the humidity level.

   This article gives a good overview of the issues: https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

2. Expert Member
   BILL WICHERS | Mar 18, 2022 10:04am | #2

   Your architect is referring to a sheet of polyethylene as a vapor "retarder". Polyethylene is a vapor BARRIER. Vapor retarders are slightly vapor permeable, and often selectively vapor permeable in the case of "smart" vapor retarders like MemBrain and Intello. Vapor barriers are not permeable at all, and completely block vapor migration. There is a pretty big difference here when you're trying to do things inside and mold is a concern.

   I would use 1/2" polyiso under the joists instead of polyethylene, since the polyiso is often easier to work with.

   Note that the usual way to deal with a damp basement is to address any bulk water problems (which is a fancy way to say "water leaks"), which is typically done on the exterior by adjusting the grade away from the home, and possibly adding or modifying rain gutters. If you still have problems, a dimple mat against the wall with an interior perimeter drain will carry away any water that makes it through the wall. You would next insulate and seal the basement walls with rigid foam insulation, air seal and insulate the rim joist areas (there are different ways to do this depending on your specific site), and possibly adding an insulating subfloor over the original basement floor.

   It's unusual to try to seal off the main home from the basement, so I agree with DC that some additional information regarding your specific circumstances would be helpful here to help to understand what is going on.

   Bill