radiant heat in slab

[itm]

looking for input on radiant heat, eps or xps foam board ?
vapor barrier above or under the foam?
thanks

 

[ADSR]

I used EPS and VB above it. Code wanted it above it, i would have put it below the foam.

 

[larry4406]

Itm - welcome to the forum with your 2nd post.

Try the search function or use google with garage journal in the search string. Radiant heat has/is extensively addressed.

 

[gasaxeman]

Used 2" high density pink foam.
VB below foam

 

[tfinniii]

Put a good vapor barrier under foam ,if inspector wants it on top use cheap stuff there.

 

[rburke65]

Do you have a specific question?

 

[Toomanytools?]

Sounds like his question is specific. You could do a search on Google. I have always seen compacted base , foam XPS has better ratings than EPS not sure on price difference probably small, vapor barrier 6 mil, then wire mesh or rebar then pex. tied to mesh. But have also seen pex directly on foam then wire on top. Seems everyone has their own way, check codes for your area.
http://www.greenbuildingadvisor.com/blogs/dept/musings/polyethylene-under-concrete-slabs

 

[BadgerBoilerMN]

You can get XPS or EPS in high density, usually 25 psi. Greeny's like the expanding agent in EPS since it does not release CO2.

If your slab is more than 4" you may want to tie PEX to wire. There is no benefit in raising PEX in a 4" slab or leaving the wire on the bottom.

When we tube a garage we put down the vapor retarder first or not a all, the 2" of XPS stapled directly, PEX tubing and 5000 psi fiber concrete.

For snow melting driveways we add 6-6 x 10-10 wire over the PEX, which brings it into the functional middle of the slab.

 

[Highbeam]

Price difference between EPS and XPS is HUGE! Both are available in 25 psi. I used 25 psi EPS and saved big money. 6 mil VB under it.

 

[BadgerBoilerMN]

It depends on the area you are in I think.

 

[theoldwizard1]

If your slab is more than 4" you may want to tie PEX to wire. There is no benefit in raising PEX in a 4" slab or leaving the wire on the bottom.

Nothing personal Badger, but I did not think concrete was your forte. Most of the installs I have seen have the wire raised up on little "stools".

Can any one else confirm what Badger has said or post a link to a reference ?

 

[ADSR]

Price difference between EPS and XPS is HUGE! Both are available in 25 psi. I used 25 psi EPS and saved big money. 6 mil VB under it.

Agreed. The price diff is HUGE!

EPS was 14 bucks for a 4x8 sheet.

XPS was over 60 bucks a sheet.

It's a no brainer.

 

[theoldwizard1]

I am no expert, but somewhere I heard that if you do a double layer of foam board with the appropriate tape, and stagger the seams, that you do not need a vapor barrier.

Can any one back me up on this statement ?

 

[ADSR]

Nothing personal Badger, but I did not think concrete was your forte. Most of the installs I have seen have the wire raised up on little "stools".

Can any one else confirm what Badger has said or post a link to a reference ?

Some do, some don't. I've had some inspectors wanting it on chairs, and then the next guy wants it on the ground.

 

[BadgerBoilerMN]

There is no question that reinforcing steel belongs in the middle of the slab for crack control and lower--but not on the bottom--to increase load capacity a feature rarely needed in non-commercial slabs.

As for radiant heating; most use wire in order to have something to tie the PEX to. Since we do it for a living we can't afford this painfully slow process, which lends nothing to efficiency or response time in the typical residential, light commercial application. We staple it to 2" of XPS--about a dollar per sq.ft. in Minneapolis and put wire over it to control cracks and make my concrete guy feel better cutting 1" control joints in my snow melting slabs.

http://www.forconstructionpros.com/article/10116892/reinforcement-for-crack-width-control

2" XPS has a perm of .07 making it an effective vapor "retarder", good enough for my office. I used 6 mil under the shop area to prove there is no appreciable difference in the quality of concrete or the migration of appreciable moisture from ground to conditioned space via the slab.

We have done many basements and can't tell the difference, even in our passive radon measurements under a PE's supervision.

 

[theoldwizard1]

There is no question that reinforcing steel belongs in the middle of the slab for crack control
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As for radiant heating; most use wire in order to have something to tie the PEX to
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We staple it to 2" of XPS--about a dollar per sq.ft. in Minneapolis and put wire over it to control cracks ...

Sounds logical !

2" XPS has a perm of .07 making it an effective vapor "retarder", good enough for my office.

I assume this was 1 layer ?


Slightly off topic. Have you seen any data on how much "heat" is transferred through 2" of foam to the ground below ? To say it a different way, if the ground temperature beneath the PEX and foam is 55° before the heating season, how much warmer does it get during the heating season ?

 

[BadgerBoilerMN]

Not much.

http://www.jlconline.com/formwork/super-insulated-slab-foundations.aspx

We use 2" of XPS here in Minneapolis. In many applications, like my garage, 1" would be sufficient but 2" will stand up to construction traffic better.

Two layers will work but adds to labor, which adds nothing to the end product, we won't do it.

 

[rburke65]

I stand corrected.....VP below.