Rigid Foam on Cinderblock

ajamison

I'm going to putting up some 1.5" Thermax rigid insulation on my crawl space and unfinished basement walls. I plan on using adhesive, but am also going to put a few Tapcon screws with large washers in each sheet to hold it in place while the adhesive dries. How long of a screw should I use - how far do I need it to penetrate the block wall?

jogr

PL 300 foam adhesive would hold it pretty well without the tapcons. I put ribbons of PL300 from floor to ceiling 16" oc, push the board in place firmly, slap it and slide it a little. I had a couple spots where the wall wasn't real flat I just placed one small piece of plywood scrap flat against the foam and a 2x4 leaning against it to hold it till it set up. Where the wall was flat it held fine with no assistance.

The directions did say you could pull it off after installing and let the adhesive set a few minutes and then reinstall it to make it hold better. I didn't need to do that.

Houston_Bldr

You'll probably need a 2 1/2" tapcon to get an 1" penetration. But I think you're going to really regret it once you start predrilling through that block. Even a heavy duty hammer drill will wear you out quick.

I'd go with 2" concrete nails and washers. As long as you're pretty accurate with a hammer, you only need to drive the nail about 1/2" and it should go quite a bit faster.

Even better than that would be tape to hold the sheets flat while the glue dries. But it's effectiveness really depends on how uneven the block wall is.

High Gear

Just glue the heck out of it , buy the large caulk gun as the large tubes are lots cheaper per ounce.

Just keep pressing it tight with a board ect until it takes hold.

I'd be leery of penetrations in cinder block.

Master Brian

I just did a lot of this to insulate my basement as well and the glue works great. I think you are just wasting time and money if you drill and tapcon. It takes about 1 tube of glue per 4x8 sheet of rigid foam.

mcwolfe6

I just did the same thing in my basement and didn't need to use any tapcons. I just applied the PL300 to the rigid insulation board and pressed it into place and it held to the cinderblock wall without any problems. good luck.

Red Squirrel

I want to do this with my basement too, then later on add studs against the foam, and fiberglass insulation. Do you need to put some kind of sealer over the cinder blocks before? Does the foam act as a vapor barrior or would I still add one over the studs before I put the drywall?

Master Brian

My understanding was the foam acts as the vapor barrior. I used blue foam, never can remember the type, then I built a stud wall. My wall doesn't sit right against the foam, just because I had to bring it out a bit to go around things such as sewer line, etc... I then used fiberglass batting and then the drywall. I'm actually using the green board, just as an added layer of protection. Again my understanding is you want the wall in the basement to dry to the inside.

Just adding the foam as well as using can spray foam to insulate the tops of the blocks really made a difference in my basement. The fiberglass batting was just icing on the cake, but I'm not finished yet either!

Hope this helps.

ajamison

A couple of things I forgot to add. The Thermax is foil faced - do you think that would make it "slipperier" than blue board and would require some mechanical fastening? Also, I am only doing the above grade portion of the basement walls, so I can't really wedge it between the basement floor and floor joists above - do you still think the just PL300 would hold it while it dries?

Master Brian

I haven't tried the foil faced, but if it was the blue board, yes the glue would hold. At least it did on the stuff I put up.

Can I ask why you are only doing the above grade portion? The reason I ask is because my block walls are not filled with concrete and so they are basically hollow. To me that is important because the R value of the blocks is very poor and the cold air could still get in the channels and since cold air sinks, I would think you would still get cool air leaking in through the walls.

ajamison

Well in the full height portion, I have bricks walls (very old ones), so I am just doing the above grade portion so that the lower half can still dry to the inside. My understanding with Thermax is that since it is foil faced and fire rated, it is not as permeable as blue board (or pink). The block walls are only in my crawl space portion and those will completely covered.