Insulating Joist Space Rot Concern

Old 06-11-03, 11:52 AM
Visiting Guest
Posts: n/a
Insulating Joist Space Rot Concern

I am insulating the basement of a 1955 bungalow. The upper floor joists are directly imbedded in the concrete wall (2 x 10, ~ 3 inches imbedded).

The plan is to use 3.5 inch steel stud, 1 inch from wall, 6-mil poly for moisture barrier to grade with fiberglass stuffed into the wall cavity.

The concern is the joist space above the wall. I was planning to just insulate with fiberglass until I read on Jon Eaks site that this space is subject to wood rot if insulated and he does not recommend insulating this space if the joist is imbedded in concrete.

I plan on using 6-mil poly for the vapour barrier and I don't understand how water would come in contact with the wood to cause the rot.

Has anyone had any experience with the joists rotting in this type of construction?

I live in the Prairies in Canada, so I wouldn't call it a humid climate.
Old 06-11-03, 01:03 PM
Doug Aleshire's Avatar
Join Date: Feb 2002
Location: United States
Posts: 4,455
Upvotes: 0
Received 0 Upvotes on 0 Posts

Unfortuantely any wood that is in contact with concrete is subject to rot. Codees now ay that any wood in contact with concrete/block must be WOOD TREATED to avoid such damage as you are now seeing. Concrete is porous and as such will act like a wick, hold moisture and anything that can absorb it, like wood, insulation will get damaged. This method of construction is an old way, rarely seen but it is sure not a good thing.

What you mentioned about this area (rim joist) being subject to wood rot if insulated is correct if wood (the sill plate) is UNTREATED. Virtually all homes today have a wood treated sill plate and as such no rot will occur. As mentioned the Code now forbids untreated wood in contact with concrete/block.

Hope this helps!
Old 06-11-03, 01:15 PM
Visiting Guest
Posts: n/a
No Rot Observed

I don't see any rot in the joists. There is no sill plate as the joists lie right in the concrete.

From your post, I gather that you feel that I should leave this space uninsulated.

Bit of a drag since I have stuffed insulation in a lot of it already and this would appear to be an area of major heat loss.

Oh well...

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: