Insulating Under Slab for Radiant Heat - Confused

[kwright]

OK, I'm confused...

I'm working though the details for my garage build (28' x 36'), which includes radiant heat.

Both the concrete and heating contractors my general contractor has contacted have suggested (independently) using a "bubble wrap" foil-coated insulation under the slab. They've both indicated this is pretty much current-practice and they've been using it successfully the last few years in their radiant heat installs. Note that both these contractors are established and well regarded in my area (Michigan).

What I'm confused by is I've read on the Internet that bubble wrap insulation is not recommended, but rather, blue or pink sheet foam is the preferred way to go. However, I've *also* read that modern bubble wrap/foil covered insulation/vapor barrier IS the recommend way to go (this was on on different sites of course).

In fact Radiant Floor Company (http://www.radiantcompany.com/), mentioned here in other posts, specifically suggests NOT to use a bubble wrap type under slab insulation.

Here's a direct quote from their web site:

"For the record, Radiant Floor Company does not sell under-slab insulation of any kind. Our opinion is based on customer feedback and our own experience. We recommend Extruded Polystyrene."

Can anyone shed some light on this apparent contradiction?

Thanks...

 

[Franz©]

If I just tell you the contractors are wrong, and the Radiant company is half right, you'll insist on more information, so I may as well type my fingers off.
First, discount all contractor reputations, and visit their jobsites yourself while work is in proggress. The first thing you'll notice is there will generally be a bunch of 18 to 23 year old lackeys doing the actual work, and a supervisor with a good idea of what he can get away with. Contractors are experts at sliding around rules and bullshytting the public, till they get caught on TV. All Contractors want maximum profit, and will cheat an unknowing customer blind. It's their religeon.

Bubblewrap is bubblewrap, and it damn sure ain't insulation in Florida, let alone Michigan. Pink foam board and or blue foam board are insulation. If you're planning to dump the hovel on a new owner within a few years, go with the cheap bubblewrap, pocket the money you should have spent, and get out of town. Actually, you can save the cost of the bubblewrap because it will effectively be flat poly film before the concrete drys, so just use poly.

As far as foil faced bubblewrap, again, a complete joke unless you are planning to block solar radiation. The foil will disintigrate on the concrete side in weeks, and honestly, unless you are in or near Hell Michigan, there ain't a whole lot of radiation coming up from the Earth.

Basic Physics tells us HEAT rises. When applied to the floor, that means the majority of heat given off by the PEX will rise, and minimum heat will go into the ground below the slab. Properly installed PEX ain't at the bottom of the pour for good reason.

There is one well proven way to insulate a slab, and it's the only way you want to go in Michigan.
First, consider how frost operates. The freeze begins where the grass leaves the ground, and travels down and horizontally. Your slab will be a hot island in the middle of an ocean of frozen ground. Heat gives off to cold, that's the way things work, and Mother Nature won't be changing the rules soon.

You want to put the least possible number of BTUs into the slab you can to keep the floor hot. If you have extra fuel money, send it to me, I can use it.
Since heat rises, and your slab is sitting in the middle of a sea of cold in winter, you want to minimize the heat transferr from the slab to the ground.
Two things come into play. First, you minimize ground contact between the slab and the ground below with a layer of foam, thickness determined by location and temperature differential. Second, you want to prevent the frozen ground from coming into contact with the edges of the slab.
Since frost travels down as well as across the earth, and frozen dirt exerts a lot of force on the edge of the slab, you install a verticle foam board that is also thick enough to absorb the force generated by the frost. Michigan = cold winter + lots of frost, so you need to continue the vertical down to the bottom of the normal frost depth.
Finally, you install an apron running a few feet out from the slab. The apron prevents the frost from continuing under the slab in those 10 year winters and heaving the entire slab.

Now, the builder and the heating contractor will tell you they never saw such a system, or it ain't necessary, and they may be telling the truth. This system has been standard in Alaska for years, and it pays for itself in 5 to 7 years in the lower 48.

If the picture attaches it should explain the system.

 

[kwright]

Thank you for the detailed info as well as the diagram. What you've shared is in the ball park of what I was originally thinking - though a bit more elaborate.

I need to think this thing though some more...

 

[z28toz06]

I dont think the bubblewrap would pop and go flat when you pour concrete on it. On the other hand it does break down much faster than the blueboard. I could have used either and went with 2" blueboard. As far as the heat rising that is true however i do believe that the heat would travel down in to the floor as it is radiant. I originally thought that I would be alright without blue board underneath, but was told otherwise by several pros. I do think that the outer perimeter walls are a very important loss place and used 2" around the perimeter also. .

 

[5wndwcpe]

Personally, I'd avoid the bubble-**** like the plague and stick with the 2" foam board. What I find most interesting is that the contractors you've talked to have used the bubble insulation "successfully." I mean, how do determine the success rate ? It's under concrete, so a visual is out. Futhermore, unless you build two identical buildings, in the same area, with the same heat load utilizing foam board in one and bubble wrap in the other, how could you determine ones performance against the other. I'm with FRANZ on this one.

 

[randydupree]

i'm going to use spray foam insulation under my next slab.

 

[jklingel]

Hi. I am a little late to join the party, but Franz is pretty well right on. My only comment is that "heat goes to cold", period. Not up, down, or sideways, necessarily. This new SlabShield stuff, etc, looks interesting, and may be a nice addition to blue foam. Its R-value is pretty low, but apparently (at least some of) it reflects radiant heat loss. My late 2 cents.