Correct way to detail crawl space walls
I have a 100 year old Townhome in Alexandria, VA. One of the first things I did when I moved in was to put down a 20mil plastic membrane and a dehumidifier in the dirt crawl space. The membrane terminates a couple of inch from the top of the brick wall and attaches with mechanical fasteners. Fast forward a couple of years and I have been reading that I should also insulate the walls (not the floor joist, which is what I currently have.)
My thought was to attach XPS to the exterior foundation walls which would insulate and act as a water barrier but unlike all the articles I see my walls are not smooth (it’s 100 year old brick!).
Q?
1) Is the above XPS idea workable? I’m guessing not.
2) If not is my only other option to use closed cell spray foam on the inside walls? My concern in doing this is that the clearance in the crawl space is only about 8inch (floor to bottom of floor joist) in about 2/3 of the area. Kind of hard to pull those spray foam tanks around in.
3) In either case do I need to cut back the plastic membrane so that it is not going all the way up the wall? In other words am I creating a moisture trap between the membrane and the XPS or spray foam?
Thanks Mark
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Replies
Mark,
The easiest (and probably best) way to insulate an uneven masonry wall is with spray polyurethane foam.
If the crawl space access is limited, it is always possible to dig out some dirt and lower the floor. But that's a lot of work.
If the bricks are somewhat uneven, but not terribly uneven, you might be able to make rigid foam work. However, rigid foam will only work if you can find a way to seal the perimeter of each sheet of rigid foam. The best way to do this would be with canned spray foam. Then you would need to mechanically fasten the rigid foam to the wall and let the spray foam cure.
If you decide to insulate the walls with rigid foam, you should leave the 20 mil plastic membrane in place. If you decide to use spray foam, it probably makes sense to remove the plastic membrane first.
More information here: Building an Unvented Crawl Space.
Martin, Thanks for taking the time to answer a question on a topic that I'm sure you feel has been talked to death. Just a quick follow-up for clarity. You say "with rigid foam, you should leave the 20 mil plastic membrane in place". Can I really just leave the membrane in place and add ridged foam to the interior side. (assuming I get the air sealing right)? I guess I was thinking that the foam needed to be between the foundation and the membrane to prevent condensation.
Many thanks.
Mark,
If you install your rigid foam correctly -- that is, in an airtight manner -- there won't be any condensation against the poly, because the indoor air will never be in contact with the poly.
If this detail bothers you, though, just remove the poly from the wall. Either way will work.
For those that find themselves with a similar question I also found this pervious Q/A useful.
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/27785/insulating-exterior-stone-foundation