Using R-foil under concrete for radiant floor heating

[Choptopjimmy]

Has anyone heard good or bad about using R-foil under the concrete slab when installing a radiant heat system? Also, could anyone tell me what the risers are called that you put under the wire mesh to suspend the wire into the middle of the slab? Any help is appreciated. Thanks, Jim

 

[Mr onetwo]

Bad, bad idea! Use 2 layers of 1.5" blueboard styrofoam with the joints staggered and taped.Believe me, it's worth the little extra money.Pay extra attention to your sides and edges too.

 

[Choptopjimmy]

Why is it a bad idea? Please explain. Jim

 

[spy604]

The foil is absolutly worthless in conduction. Think about it this way, would you rather hold a cup of boiling hot coffee with a foil cup or a styrofoam cup?

 

[Mr onetwo]

Spy604 is right.Also the foil might get crushed and damaged during the pour and you would never know it.....not the right application for this stuff.The things you were asking about are called "chairs"....you should think about rebar....mesh is completely useless.

 

[Highbeam]

The foil is absolutly worthless in conduction. Think about it this way, would you rather hold a cup of boiling hot coffee with a foil cup or a styrofoam cup?

Great job with the comparison. Perfect analogy.

Insulation works by trapping air. Since that foil and any attached blanket of fuzz will be crushed by the concrete, it is not functional as an insulator.

 

[Wangstang]

Also, could anyone tell me what the risers are called that you put under the wire mesh to suspend the wire into the middle of the slab? Any help is appreciated. Thanks, Jim

Rebar Chair

There are a few different types, some prefer the plastic expoxy coated, some prefer the dobies/concrete trapazoid with the wire tie. Depending on how you do your concrete, if the walls are going to go up before or after you pour and how well the pour area can be accessed, you may find that you have to put the chairs in as you pour as you may have to wheelbarrow the concrete into a far corner if you don't want to pay for a pumper truck and all.

It's clear that the minds here think foil is a bad idea. I would be curious to know their throughts on using the bubble foil insulation material that some people use under metal roofing as a vapor barrier layer. I'm suggesting that you'd lay it out in place of a heavy mil plastic sheet as it's a heavy plastic material itself with a foil coating on the side that's intended to reflect heat. Put the foil side up to reflect some of the heat in the concrete back up and the air bubles should provide some small level of insulation from the ground. Thougts? ...Would it pop like a roll of packing bubbles when you drove over it?

Wes

 

[stingry]

Rebar Chair

There are a few different types, some prefer the plastic expoxy coated, some prefer the dobies/concrete trapazoid with the wire tie. Depending on how you do your concrete, if the walls are going to go up before or after you pour and how well the pour area can be accessed, you may find that you have to put the chairs in as you pour as you may have to wheelbarrow the concrete into a far corner if you don't want to pay for a pumper truck and all.

It's clear that the minds here think foil is a bad idea. I would be curious to know their throughts on using the bubble foil insulation material that some people use under metal roofing as a vapor barrier layer. I'm suggesting that you'd lay it out in place of a heavy mil plastic sheet as it's a heavy plastic material itself with a foil coating on the side that's intended to reflect heat. Put the foil side up to reflect some of the heat in the concrete back up and the air bubles should provide some small level of insulation from the ground. Thougts? ...Would it pop like a roll of packing bubbles when you drove over it?

Wes

Heat is not lost to the ground by radiation, it is transferred by conduction.

Cheers
Steve

 

[rburke65]

Have you done an Internet search for radiant heat and look at pictures or read the posted information on the different sites? The only folks recommending foil is the foil producers. There is no benefits to using foil.

 

[Flexia]

i can see maybe the possibility of putting foil on top of the foam insulation to give just a tiny bit of reflection back into the slab. But prob won't do anything worth the money on the foil

 

[TheEquineFencer]

I have F/F/F , ( Foil/Foam/Foil) under my floor, You can use F/B/F too, I did my own and it was what was recomminded. I researched the heck out of everything before I did it. Insulation4Less.com is where I get mine from. You can get it in 4 x 125 foot long rolls. It's suggested to put 2 inch foam insulation around the edges of the flooring, but I didn't. I also put it in the walls and ceiling. My G/F is an engineer and she didn't think it would work as well as the 'galss stuff in the walls and ceiling. After the tornado came through and wrecked my shop and her horse barn, she had it put in the barn too. It's reflective and rejects heat. You need top make sure when you use it to give an air gap and a place for the hot air to go when used above floor. My shop enclosed area is 40 x 60 x16 tall, post steel contruction, it got to 100* outside last summer and was 2-3 PM before it reached that inside with the doors closed. It's NEVER been hotter inside than outside with the F/F/F insulation installed.

 

[rsa]

TheEquineFencer, you found just about the only sensible use of the stuff as building insulation: a metal building.

From Radiant Barriers: A Solution in Search of a Problem,

Radiant barriers make sense for uninsulated barns

The effect of a radiant barrier on a building assembly's R-value may be significant or insignificant, depending on whether the assembly is well insulated or poorly insulated. Radiant barriers do not significantly benefit well-insulated assemblies.

For example, consider drywall installed on a SIP wall. If the SIP has an R-value of R-30, the emissivity of the drywall hardly matters. Since the drywall is at room temperature, it's at thermal equilibrium with the other objects in the room, so radiant heat transfer isn’t a significant heat-transfer mechanism for people or objects in the room. (Radiant heat transfer only becomes significant when a radiating surface is at a significantly higher temperature than surfaces in the room or air space which it faces.)

A poorly insulated assembly, however, will benefit from a radiant barrier. For example, consider an uninsulated barn with galvanized steel panels for roofing and siding. On a hot sunny day, the steel panels are warmer than the interior of the barn, so they radiate a lot of heat. If a radiant barrier is installed on the interior of the wall, it cuts down on the transfer of radiant heat from the steel panels to the interior surfaces.

Because building codes require the walls and ceilings of new homes to be insulated, there isn’t any need to install a radiant barrier in a well-designed home.

 

[BadgerBoilerMN]

You are getting a little mixed up here.

Bubble foil is the one of the most successful building material hoaxes of the century. Shiny, cheap and easy to install, it is just too good to be true; seriously. Canada's own David Bean, through his web site Healthy Heating, has done an excellent job of exposing the false advertising the bubble-foil people have perpetrated on a ill-informed public.

http://www.healthyheating.com/Page 55/Page_55_o_bldg_sys.htm

The long and short of it, is that aluminum foil is an excellent conductor and reflector of heat. Unfortunately, you have to pick one. If a radiant slab is in full, direct contact, with foil or some type of "insulating blanket" it will conduct heat directly through the foil or blanket, at what ever the "real" R-value will allow. If the insulation can be compressed or has very little R-value to start with e.g. bubble-wrap/foil, it will make a poor insulator.

Here in Minneapolis, and in our designs around most of N.America, rigid extruded polystyrene (XPS) or expanded polystyrene (EPS) is specified in a compressive strength appropriate to the combined load of concrete and anticipated live weight.

http://www.finehomebuilding.com/item/4822/which-rigid-insulation-should-i-choose

Aluminum is a very good conductor of heat and would likely improve the performance of the slab in terms of thermal equilibrium. However, if you waste time suspending the tube (on chairs) an extended radiant surface below the finished floor is of no value, much like bubble-foil itself.

As indicated, if you need a radiant "barrier" a foil product makes sense, but an air-gap is essential, lest you make a conductor instead of barrier.

We have designed many radiant floors with no insulation in the field of the slab, in mild climates the ground temperature may be high enough to make slab insulation all but useless. This is where a bubble-foil/placebo works best and perpetuates the "industry's" ridiculous claims.

No serious radiant floor heating designer ever specified bubble-wrap or blanket to insulate below a concrete slab. If you want a vapor retarder use 10 mill lapped a half foot. Lapping XPS is another time waster, since the space between panels can be taped, if you must, and no advantage is gained in doubling your labor.

 

[TheEquineFencer]

As long as you understand how it works, you can use it in a lot of places. You can use it in a regular house in the attic. Most attics have the roof uninsulated, with the insulation on top of the grid above the ceiling. If you install this stuff directly accross the A-frame of the roof frame and give it a place for the heat that it traps to be moved to the outside, the airspace above the ceiling will be a LOT cooler. A 1/2 -1 inch gap is all you need from what I remmeber between the heated surface and the insulation, then a way to move that air outside. It also makes a building airtight. You can also use it around duct work too. My brother-in-law bought one of those cheap prebuilt buildings like they sell at Lowes. He stapled it to the rafters, then nailed the 1/4 inch plywood over it to finish the ceiling. he made the peak into what looks like an "A" with the top cut off, and ducted the airspace above it to the outsdie with an exhaust fan. He put the stuff in the walls the same way and drilled a hole at the top so the trapped heat could get to the roof area. To be honest, you'd think you were in a regular well insulated building an it cost a bunch less to insulate. It sits in direct sunlight.

 

[TheEquineFencer]

You are getting a little mixed up here.

Bubble foil is the one of the most successful building material hoaxes of the century. Shiny, cheap and easy, it is just too good to be true. Canada's own David Bean, through his web site Healthy Heating, has done an excellent job of exposing the false advertising the bubble-foil people have perpetrated on a ill-informed public.

All I can say to this, if used as designed, it works great. My shop prior to instralling it has a roof temp, shot with a IR heat gun of around 141*F, Linda's horse barn was the same. After installing it, on a 100*F day I shot the roof temp, 101*F inside my shop, hers was still around 141*F. After the tornado came through and removed the roofs for us, we installed it as it was built. You can now stand inside her horse barn when it's hot and stand the temp, before you could not. Linda's a retired enginner, when you can install a product that to start with she did't think would work and it does you know you did something right. Impress a woman and an enginner at the same time, I think I got it right. To tell me this stuff is a hoax, when I can now work inside my shop in the middle of the day and before I could not, it works; if used properly.

 

[Highbeam]

This thread is about underslab insulation where foil is a hoax.

 

[HoosierBuddy]

I agree with Badger.

Using FBF or any other foil and bubble product under a slab is pretty much the same as using no insulation under the slab. And while that may work in mild climes, it is not optimal.

Phil