Should I put foam under the entire garage floor?

foam it all

I advocate putting at least 1" extruded styrofoam and heavy poly film under ALL slabs here on the humid east side of the Rockies, even if they won't have radiant tubing installed in them. It won't keep them warmer in the winter in an unheated building but during the summer they won't be damp from condensed humidity in the air. Leaving things on the floor won't corrode them on the bottom and they don't get mildewed in a couple days.

If you're putting tubes in the concrete, not insulating under there is guaranteed to cost more money in heating than the foam in very few years. This is all just my own opinion but lots of others on this forum share it. Here's a good article on the subject. You can skip the formulas and just read the text.

http://www.pmmag.com/Articles/Column/BNP_GUID_9-5-2006_A_10000000000000372298

I have never done a floor without full foam under the crete and I've never spoken with anyone who has done just the perimeter as you described.

I would foam the whole thing. The particular characteristic of radiant heat that makes me say that is that it travels 360* from its source. The only way to make radiant energy "rise" in a slab application is to ensure there is more R value below than above the tube.

I used to help build as big as 30 acre sized buildings, some were freezers that ran coolant through the floors.
It was always the same; 2" styrofoam three feet down around the exterior, and 2" styrofoam 3' wide
around the perimeter of any exterior walls.
That's in Rochester/Buffalo NY figuring a frost depth of 30".

Just jumping in from the free-standing forum:

Ditto on recommending foam on at least 75% of the base (some leave a grid core uninsulated to act as a heat sink -- that's more gospel than science I believe and depends on how you feel about it. Definetely insulate the perimeter though. I see way too many houses without the perimeter insulated and I can literally find grass growing around the slab in February -- and we're talking East Coast Canada winter where grass shouldn't be growing that time of year.

I'm wrong. Where there was cooling lines planted, the entire floor beneath it was insulated.

As a minimum 2" foam under the entire slab. And also either 1" or 2" foam as a thermal break between the slab and the frost wall, or your heat will migrate sideways out of the building. Rip it on a tablesaw to 4" or whatever thickness your slab is and attach it to the frost wall before you pour. We run it right to the snapped line and screed the slab off the foam.

foam it all, at least 2 inches thick. Also between the pad and the foundation.

I agree foam it all. I did the same on my last building.
BUT, Just to be the devils advocate, I have heard there have been buildings where foam was put down and concrete poured over it and later it was pulled up for some reason and the foam was gone. The heat from the concrete setting melted the foam. Has anyone else ever heard of this? If it is true kinda sucks on the investment.

I have heard there have been buildings where foam was put down and concrete poured over it and later it was pulled up for some reason and the foam was gone. The heat from the concrete setting melted the foam.

Click to expand...

Did you hear this from a salesman for one of the miracle foil roll insulation products?

I can believe that the foam might melt if the building burnt down and the concrete was being removed because it got so hot it disintegrated. Not the foam's fault.

But if you do your own concrete with foam under it be aware that the concrete might set faster than you're used to when not insulated. Because it will get a little warmer from the curing heat. Especially on a hot sunny day. Have that power trowel ready!

I was pretty much the first guy in my area to install a radiant floor, and took the advice of a website in which I bought the pex tubing. They even sent me a nice little book on how to "properly" install a radiant slab. I did leave the middle of the pour uninsulated and have regretted it ever since. The floor felt great, but I have abandoned it since it was using so much btu's, or at least I felt it was. I may try it again at some point to be 100% sure, but listen, Insulate the hell out of your slab man, you can never add more later.
Anyone I can find with this question I try and help so they dont make the mistakes I did.

I foamed under the living space but not the shop, and the entire perimeter. Most of the heat loss occurs at the perimeter. Use as much insulation as you can afford there, especially if the floor is heated. As you move to the middle of the building less and less insulation is needed unless you want to be able to change the floor temp fast.

If the soil has moving water under it you need to insulate below the slab.

As was said earlier def insulate around the perimeter as the heat will go sideways as well as down. I installed 2600 sqft of radiant and insulated with 2" of blue sm lap joint and every joint was sealed as well. I happened across some free rebar so I used this as my grid for the whole slab instead of mesh.

Something to keep in mind--- No loop should exceed 250 ft and around the perimeter should be 6-8 inches on center for a few rows where as the middle can be on 12" centers.

We love our radiant heat and I am glad I did the install myself.

Rob

Mid Michigan said:

I agree foam it all. I did the same on my last building.
BUT, Just to be the devils advocate, I have heard there have been buildings where foam was put down and concrete poured over it and later it was pulled up for some reason and the foam was gone. The heat from the concrete setting melted the foam. Has anyone else ever heard of this? If it is true kinda sucks on the investment.

Click to expand...

I would be seriously surprised, but it only takes one case of even partiall accuracy to get a tale like this going... Only two ways (besides the fire mentioned earlier) that I can think of this happenning...

1. Seriously overheating the slab - i.e. maximum boiler temperature water getting pumped through... I'd be surprised by this as I'd expect the building to be nearly uninhabitable with that much heat in the floor, and it would take a lot because even the low temp foams are good to a fairly high temperature.

2. Seriously mis-specifying the foam, or having the wrong foam installed - i.e. something that couldn't take the conditions under the slab...

I really doubt that the concrete setup could even come close to melting the foam, I mean concrete curing is mildly exothermic, but it isn't THAT hot of a reaction...

Gooserider

Just a point of interest.Here in Nova Scotia,as of Dec. 31,09,all concrete slabs within 1 metre of grade will be required to be insulated to a minimum of R-10.This would be 2" of extruded foam.The perimeter will also have to be insulated to the same level.In your case I would place foam beneath the whole slab and include a thermal break where it joins the exterior walls.I have never heard of the foam being affected by the concrete.I don't believe any heat generated during the curing process would be enough to damage the foam.It could have been a chemical additive to the concrete such as calcium or a retarder to slow down the cure.

Earl

We insulate the entire floor 100% of the time. Once everything has reached equilibrium with the system running you can "see" how even the floor temp is with an infrared temp meter. It's a thing of beauty.

I heard of the disappearing foam phenomenon also in a local case. I checked it out and the product that had been used under the clab was the white bead board stuff. Not suitable for the intended use. The failure was due to crushing not melting.

Yes,the expanded foam,(white bead board),is not suitable for use beneath a slab.The extruded board,(SM or similar),is the way to go.

Earl

yooper rich said:

Thanks to all for the info. It sure helps having you folks to call upon. I have already installed two inch foam along the walls to a depth of two feet below the slab. This weekend I will be putting foam on the entire floor.

Click to expand...

GREAT!! Time and $$$$'s Well Spent.

Don't forget a vapor barrier, like 6 mil polyethylene. Most of the solar and energy saving books from as far back as 30 years ago stated that moisture draws the heat from concrete or any storage medium and any heat lost to the earth is gone forever. Also a good reason for total insulation coverage. I can guarantee you that there will be moisture under the slab.

Don’t forget a vapor barrier, like 6 mil polyethylene.

Click to expand...

I second that. Not only will moisture help draw heat out of the floor but moisture will wick up through it. Think mildew on the bottom of things left on the floor.

The extruded foam is also a type 1 vapour barrier so you can poly if you want but it's not necessary.Of course you do have to tape the joints in the sheets of foam.I'm a belt and suspenders type so I used the 6mil poly along with the extruded foam insulation.

Earl

As a note, almost all of Nova Scotia is within inches of bedrock, which conducts heat amazingly well, so leaving the middle of a slab uninsulated is not an option for a heated space.

E.W. said:

The extruded foam is also a type 1 vapour barrier so you can poly if you want but it's not necessary.Of course you do have to tape the joints in the sheets of foam.I'm a belt and suspenders type so I used the 6mil poly along with the extruded foam insulation.

Earl

Click to expand...

Very true. Plastic is indeed like belt and suspenders.