Ok to insulate the top of hollow concrete brick foundation wall? - DIY Chatroom

creamaster · 10-28-2009, 11:16 AM

Looking at my basement at the rim joist area, specifically the top of the foundation wall that is hollow concrete brick, is it ok to cover the top and therefore seal off the openings of the brick with semi permeable rigid foam insulation. I am hoping to provide an air barrier,not a vapor barrier, at this point and slow down the transmission of air laden with water vapor into my basement. Looking at the Building Sciene website they do show pics where the top of the foundation has foam yet it doesn't say if it is hollow or solid concrete foundation wall. My concern is of coarse the plate that sits on the foundation wall and would moisture accumulate on it if I seal off the opening even though it is a permeable material?

Thanks for any input

Maintenance 6 · 10-28-2009, 02:14 PM

By concrete brick, I am assuming you mean hollow concrete block. Your plate should be isolated or protected from any moisture wicking up from the block. The plate should either be treated lumber, depending on the age of the home, or should have a sill sealer between the wood and masonry. Sometimes this was tar paper or a thin closed cell foam sheet. The plate is protected on the outside by the exterior finish of the home which should drop below the level of the plate. Masonry, particularly concrete block, is very porous and will allow moisture to easily pass through it. Adding a rigid foam board on top of the open cores will help to slow any air migration through the cores and into your basement area. It will not stop moisture transmission through the masonry.

creamaster · 10-28-2009, 03:22 PM

Sorry I did mean hollow concrete block. the hosue was built in 1964, ranch home, and there is a capillary break between the plate and block, some type of material, likely the tar paper like you said. I dont intend to stop the transmission of vapor through the block, I know this is impossible, yet the Building Science site said that water vapor carried by air transmission, like a crack or hole has the tendancy to carry more moisture than by means of diffusion like moving through the block. So I thought that by blocking the holes at the top of the wall I may slow the transmission of moisture into my basement by means of the rigid foam acting like an air barrier, not a vapor barrier. I just dont want to do anything to harm the plate.

The BS site also mentions using a mastic material to seal the foam and make an airtight seal. Is this mastic simply a builders caulk like Quad OSI or what are some examples of a mastic?

Maintenance 6 · 10-28-2009, 10:43 PM

Sealing off the top of the block surely won't hurt the plate. I think I would use some expanding foam in the cores.