best way to insulate concrete slab when using hot water heating

michael j. bonk_2

IM going to install hot water heat system in a concrete slab
need to know the best way to insulate the slab from heat loss to the earth. The slab is 46 feet by 24 feet. I will be install 4 - 350 feet runs in slab. What would recomend for best insulation under and around slab?

Using 50 gal. hot water tank for heat

Thank you,
Michael J. Bonk

Weezbo

... Plenty

Constantin

Just a homeowner...

...IIRC northern climate guidelines I have come across indicate at least 2 inches within the first four feet of the slab edge, and down to 1 inch in the middle. Some people go up to 4 inches, the further North you live, the quicker insulation will pay for itself in the winter months.

Naturally, you may also elect to turn part of the ground under the house into part of your heat mass. This is achieved by only insulating the periphery (and hence avoiding edge losses), while leaving the center bare. There, the earth will interact with the slab to either store or release heat, depending on the temperature of the slab relative to that of the ground and air.

... as an amusing aside, Dan mentioned how the lack of insulation under the radiant floor slabs in Levittown ensured that come spring the flowers around the houses there always bloomed quicker than elsewhere on Long Island... because they were being heated from below.

From the reading I did, I would recommend extruded polystyrene (XPS) due to its low water absorption and pretty good R-value (5/inch). I'm planning on using 2 inches everywhere below the slab as well as outside the foundation wall. If you live in a termite/insect-prone area, ask for borated XPS, that way no insects will attempt to make their home in it. Cheers!

Mark Wolff

underslab insulation

Under slab should be 2" polystyrene (pink or blue foam is fine) giving you R-10. The slab perimeter when possible should have 1" foam insulation and the outside of your footers should be 2" foam insulation.

Some people skip the insulation (especially the slab perimeter, but when you do a heat loss calculation with and without it, you'll see drastic btu/hr demand differences. Over the life of your home (even 3-5 years) you will pay much less overall if you insulate well.

Unknown

slab insulation

90% of your loss is in the first 4 feet.

In Sweden they insulate their slabs on a horizontal pitch 4 feet away from the wall. This keeps the frost from gettin to the slab and allows a heat mass to build.

I suggest 4 feet of Verticle 2" board insulation (made for below grade use) around the entire perimeter. Then replace your vapor barrier with Slab Shield or InsulTarp reflective insulation. I prefer closed cell foam/alum/foam myself.

Steve

Rojoho_2

Do you know the PSI

rating for Insul-Tarp? Thermolow-E claims 70 lb. psi/R-3 for its Slab Shield blanket. Does Insul-Tarp have a special tape to join tarps?