Slab edge insulation retro-fit

Jack

I've run into this a couple times recently where folks have radiant slabs and have not insulated under the slab or slab edge or both. In one case recently it led to the abandonment of the radiant as costs were $600-800/month, and that is here in the CA foothills with a very much more moderate climate than many of you work. With the landscaping and attached patios, etc, adding the slab edge insulation was just to much to do in the owners eyes. I just had a fellow ask me today if he could dig around the footing and foam it? He had insulated under the slab. I would lean towards 2' rigid that is flashed in, but I'm not an insulation expert. How do you handle these situations?

Zman

The edge insulation can be a tough one. Any time you try to get other trades to do anything different from the way they were taught it is a problem. The area of exposed concrete on the outside of the building is a tough one to detail. I think the best detail for most jobs is to insulate the inside of the concrete wall. I like this detail.

Jamie Hall

That detail is the way to do it -- if you are doing it when you are pouring the slab and footings. Otherwise... not so much.

If you are dealing with an existing installation, the best approach -- which isn't all that good -- is to dig down to the footing and use the 2 inch rigid all the way up to the top of the wall. It should be adhered somehow to the wall -- some adhesives work better than others, and make sure whatever you use doesn't attack the foam! Then backfill on the outside with at least 6 inches of free draining gravel and, if possible, a footing drain to daylight somewhere. Then carry on. That works, but not as well as the detail will.

hot_rod

With exterior foam the flashing detail is the challenge.

When I did slab on grade metal buildings we had the metal supplier break a 2" wide Z flashing with the metal building package.

The flashing went up under the siding and about 8" down the front of the foam. It also served as the rat guard, which closes off the bottom corrugations of the metal siding

An exposed edge of foam will not hold up well. UV, bugs, lawn mowers and trimmers, etc.

You need to buy foam adhesive, the typical "Liquid Nail" construction adhesives will eat right through.

Solid_Fuel_Man

I've done 2" foam retrofit ramset nailed on. Then 1/2 PT plywood with tapcons painted on all sides prior to install. After the groundwork was done, job went quite quickly and has held up well. Backfield with crushed stone so water retention wouldn't be an issue.

BobC

Silicone adhesives work well with foam sheets, liquid nails just melts it..

Bob