# Insulate exterior concrete block



## jackpine (Sep 3, 2008)

Let me pass this by you all. I am going to build a deck along the side of our home. It will be attached along the rim joist. I want to insulate the block wall below the deck, approx. 4 courses of blocks to the ground. My idea is to glue 1 or two inch foam sheeting to the block, then cover this with some treated 1/2 inch plywood screwed through the foam into the cinder blocks. This plywood would serve to hold the foam tight against the block and to cover the foam sheeting from the weather. I would then lay some fabric or waterproof plastic down along the ground under the deck for drainage and put crushed rock on top of it. As far as finishing the plywood, I would either paint it or something. Does anything jump out to you as wrong with this picture?


----------



## AllanJ (Nov 24, 2007)

My gut feeling is to use just the foam. Glued on, it should not fall off.

I take it that the foam is not exposed to sun or rain. Then it won't deteriorate any faster without a plywood cover. The plywood is one additional item that can rot especially if it touches the ground. Also the concrete blocks are better without screw holes in them than with.


----------



## Windows on Wash (Aug 30, 2011)

Allan is correct.

If you are really worried about the foam coming off, you can rip down some battens out of PVC composites and provide additional holding power there.

The only reason to cover the foam would be to prevent the UV degradation.


----------



## jackpine (Sep 3, 2008)

AllanJ said:


> My gut feeling is to use just the foam. Glued on, it should not fall off.
> 
> I take it that the foam is not exposed to sun or rain. Then it won't deteriorate any faster without a plywood cover. The plywood is one additional item that can rot especially if it touches the ground. Also the concrete blocks are better without screw holes in them than with.


I agree with the concrete blocks being better off without being compromised, but I don't want that pink insulation board visible from the yard, under the deck which it would be if I only glued it in place. I thought of putting cultured stone on it instead of plywood, but that is a lot of expense and work for something that while visible, would be mostly hidden by the deck anyway. If there is another solution, such as just the insulation board then backfill with soil, I could do that. Just not sure which way would be best. I do know that plywood is much cheaper than a dump truck load of fill.


----------



## cleveman (Dec 17, 2011)

I have found that mice like to burrow into the polystyrene and will ruin it in short order.

If you are able to bury it, then do so.

If it must be left exposed, try to minimize the area. Then, I have had luck with the cheesy fake stone or "hand layed" brick pattern heavy heavy vinyl siding. Add a 2" nailer to your sill plate which will extend out to the thickness of your polystyrene. Attach the vinyl siding to the sill plate. If this sticks out proud of your wall, then you can flash it with some aluminum up under your current siding.

I've never seen the stuff left exposed in sunlight or not with a happy ending.


----------



## jackpine (Sep 3, 2008)

cleveman said:


> I have found that mice like to burrow into the polystyrene and will ruin it in short order.
> 
> If you are able to bury it, then do so.
> 
> ...


I like the vinyl siding idea, flashed out from the house from under the deck. Will it attache to the foam ok without something solid to nail it into? I have never put it up before.


----------



## cleveman (Dec 17, 2011)

The way I did it was to use a 2x10 PT bottom plate which extended out past the poured concrete basement walls by 2". So the xps was flush with the bottom plate. I nailed the heavy vinyl fake stone siding to the sill plate and let it hang down over the xps. I think the siding was 18" or 2' high. I then backfilled maybe 6"- 1' to hold the siding in place and had someone come with a brake and put some aluminum flashing over the nailing area of the siding.

So I was thinking if you don't have the sill plate sticking out, then you could just attach a 1 1/2" x 2" piece to the existing sill plate. Then you can flash over this as necessary.

The heavy vinyl is the only product I liked which you could cover the xps with and have below grade.

I got a few compliments about the nice stonework on the foundation walls!


----------

