Airspace between basement wall insulation and foundation
Hi all,
CZ5, Eastern MA
First off let me say that I know leaving an airspace between the interior insulation and the foundation wall is a bad idea. Furthermore my question is related to fiberglass cavity insulation in basements, which I know is another bad idea.
Explanation: In my area virtually every homeowner, contractor, and home builder finishing a basement is using paper faced fiberglass insulation and wood stud walls. This is allowed by code and the cheapest option so I don’t see this practice changing . I would say the contractors on half of these projects intentionally leave the airspace with the reasoning “this will prevent mold”. Even worse I have heard of local code officials requiring the airspace even though the building code is silent on it.
My question is this…Is there any solid documentation that is available from a reputable source to explain the convection loop created in this air space and how it can increase the chance of mold over no air space. Again, I cant stop builders from using fiberglass insulation below grade but maybe I can convince them to stop leaving an airspace since it wont cost anything extra or cause them to use unfamiliar products.
Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Hey Jon.
I have not seen any "documentation" for or against an air space between the fiberglass and the foundation wall. I'll be interested to hear from others with experience with basements insulated like this.
Here are some random thoughts:
Do these houses have perimeter drains in the basements and sump pumps? It is possible that the space may be left for leaks in the foundation wall to drip to the perimeter drain. In those cases, you wouldn't want insulation tight to the wall.
Also, I guess the air space keeps the fiberglass away from one source of moisture, the concrete wall. For the air space to promote drying though, it would have to be ventilated, and that is not the case. And for a wall like this to be effective, it would have to be airtight on the interior, with no leaks between the conditioned space and the air space behind the stud wall.
Better than trying to convince people to push the wall and the insulation up against the foundation wall would be to try to get them to use a layer of rigid foam insulation between the foundation wall and the stud wall. This would offer an opportunity for air sealing, more, continuous R-value, and some vapor control.
Brian,
Thanks for the response. The houses in question are all different ages and types so its not possible to say whether there are french drains or sump pumps installed. I have read a lot negative opinions and articles regarding leaving the air space but I wanted to see there were actual building science related facts dealing with this method. The argument I continually see is the the air space can create a convection loop behind the insulation moving the warmer moist air from the bottom up to the top of the wall (where it would be colder in the winter) causing condensation to collect behind the insulation. However, I do agree installing the fiberglass directly against the concrete does't sound like a great Idea either but I'm not sure how easily fiberglass can absorb moisture from the foundation wall.
I do suggest using continuous rigid foam but it doesn't seem to be implemented. Maybe I need to work on my sales pitch...