Another Vapor Barrier Question
I’ve read the recent postings regarding vapor barriers and didn’t find one that answered my question, so I’ll post it here.
The building is a 70 year old cabin in the San Francisco pacific coastal range, no freezing temps, humid/wet winters and warm/dry summers. The building has shiplap without vapor barrier in the walls. The original tarpaper VB between subfloor and joists has fallen away. 24″ crawlspace from grade to joist bottoms with crawlspace ventilation.
Due to being closed up during winter there are noticable moisture problems that developed over the years; rusty nails in trim, some nail stains in drywall. The building will become year-round occupancy soon with forced air heat.
I’ve added 6 mil poly on the ground and this helps lots. I am about to refloor with VCT, so the question is:
do include a VB between the subfloor and the VCT underlayment plywood?
I want to understand if moisture between the VB and subfloor would promote decay in the subfloor.
Thanks! Karl
Replies
Don't put a vapor barrier in your floor.
If you want additional vapor barrier protection, make sure that you crawl space vapor barrier is laid tight. Overlap the seams 6" to 12". Glue the VB to the crawl space walls so that it extends up the wall 6" or so above grade.
Also, keep in mind that a leading cause vapor moving through the structure is air leaks so when you install your underlayment, if there are gaps around the perimeter, you may want to block them some way (caulk?) before installing you base and/or shoe moldings, especially on the exterior walls where the wall's sole plate meets the subfloor (assuming standard platform framing). I'm guessing you will use floor leveling compound on your underlayment cracks too. Also, be sure that any plumbing or electrical penetrations through the subfloor are blocked with fire proof caulk or some other material.
Really though, my expertise is new construction some other people here may have better first hand experience with retrofit type work.
I want to understand if moisture between the VB and subfloor would promote decay in the subfloor.
No. I would not put the VB in between layers of the floor. You should insulate in between the floor joists and then apply VB to the bottom of the joists.
You get out of life what you put into it......minus taxes.
Marv
"You should insulate in between the floor joists and then apply VB to the bottom of the joists."
The building inspectors in my area (NC) require the VB on crawl space insulation to be twards the floor. This is in sync with everything I have read too. I think the idea is that you don't want to trap moisture in the floor assembly. Only possible exception is in the deep south where AC runs most of the time.
Matt
The building inspectors in my area (NC) require the VB on crawl space insulation to be twards the floor
By towards the floor do you mean between the joists and the sub floor?. I can live with that.You get out of life what you put into it......minus taxes.
Marv
See pic #1 and it's caption about 1/2 way down the page:
http://www.owenscorning.com/around/insulation/project/crawlunderfloor.asp
Matt
Makes sense to me.You get out of life what you put into it......minus taxes.
Marv
you don't mention insulating. if you intend to insulate, kill 2 birds with one stone & use a product such as corbond or icynene. http://www.corbond.com http://www.icynene.com no air movement through these products, greatly reducing if not eliminating moisture concerns above by trapping the dew point in the middle of a continuous mass of insulation with no air transmission.
He could also staple a layer of Tyvek to the under side of the joists,
which will stop a lot of air flow into partition walls etc. It won't trap
moisture but will trap air which will help insulate and stop the flow through the verticals into the attic. He should definitely vapor barrier the ground and seal correctly to all walls, piers, and overlaps. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home