vapour barrier in basement – yes or no
alrightythen
| Posted in General Discussion on
I’ve read the code and it talks about high moisture pressure and low pressure….. dosen’t say put it up in basement or not to. I understand the fundamentals of vapour barriers, and yet in some regards it’s a mystery. when I start to hear about houses being too tight and builders slashing the vapour barrier after inspection. yet others going strictly by the code simply because when done by the code it’s covered by NHW.
anyway….I’m getting carried away, I simply want to know what the right thing to do. put vapour barrier up in basement or not. the crib walls and joist cavaties already have it. any info you require ask and I will provide as far as the building details go etc…
Replies
According to http://www.buildingscience.com:
- yes to VB on the floor.
- no to VB on the walls (condensation on walls should be able to dry to interior). Vapor retarder (up to 3/4" XPS or 3" EPS if I remember right) is okay.
The Building Science.com metyhod of extruded polystyrene against the concrete and then an interior framed wall with airtight drywall is the best way to do a basement wall. Be careful with all the airsealing details. The guy that devised this system, a Canadian, is a principal at Building Science Corp.
The other technique that you see people using is known as the CMHC wall and will work but again airsealing details need to be executed near perfection and you have to use dry framing lumber. You can find it in a book called "Keeping the Heat In" at http://www.nrcan.ca .
The DoE report that BS co-authored "Read This...." recommends EPS over XPS because of its higher permeability. Ideally one should use the borate-treated EPS, of course good luck finding it around here.There is some contradiction in the BS documents at that web site about XPS. They clearly state that XPS becomes vapor impermeable beyond 3/4", EPS I believe 3" thick for vapor impermeability. Yet in an earlier version of their basement insulation article they are advocating 1" to 2" of XPS on basement walls. The "Read This" report says less than 1" for walls and no more than 3/4" for floors for permeability (sounds like 3/4" to me). But their Designs That Work reports are back advocating 1.5" for basement walls (and Dow and Owings-Corning are selling 1.5" basement insulation boards).If I could find a local supplier, I would go with borate-treated EPS for basement insulation. Of course the only EPS panels I can find (at Lowes) are faced with plastic!