Poured radian floor advice

Daniel Moore

I’m planning redoing a small (100 sq’) room that was built on top of a slab. There is currently 3.25” available on top of the slab.

Will there be enough room for insulation & Gypsum concrete on top? If so how thick should I make each? My guess is, on top of the slab is:

Existing Slab
6 mil vapor barrier on slab
1” rigid polystyrene insulation
5/8" rewire w/ radiant tubing on top
1” gypsum concrete
1/2" Thin wooden or tile flooring

1/2 the room will be covered with a wooden floor (3/8” Bruce: http://www.armstrong.com/resbrucewoodna/prod_detail.jsp?itemId=51964&lineId=191) and the other half in tile.)

Where can I get gypsum concrete and polystyrene insulation, do they carry these at home depot? Do they go by another name? Is there a difference between thinset cement and gypsum? How thin can I make them?

Troy_3

Gypsum concrete

Gypsum concrete over polystyrene insulation needs to be min. of 1-1/2" thick . Polystyrene gets glued down over existing slab. Whatever thickness you can fit. More is better. Make sure all seems get taped. If the gypsum gets under the insulation it lifts it and makes a mess. Been there.

Daniel Moore

Gypsum concrete

Troy,

Thanks for the gr8 advice about securing the insulation before the pour! What a potential mess! Do you know where I can purchase the Gypsum concrete (I tried a few building supply houses in NJ – with no luck) USG makes gypsum specifically for radiant heat but only supply their installation channel.

Also, I am looking for 1” rigid polystyrene- Dow’s makes styrofoam but it comes 2” thickness minimum.

Any ideas? Thanks again!
Daniel

S Ebels

Gyp crete

Is not a self install product. It is mixed on site and pumped into the home with some very heavy duty machinery. It resembles more of a slurry type material than cement. You won't find it at HD or most Redi-Mix plants. Try Hacker Industries on Google or Dogpile and see if they list a dealer in your location.

Dow makes blueboard all the way down to 1/2". You might want to investigate high compression foam if your pour is going to be really thin.

Cheeze-Tech

Insultarp

Have you thought of using insultarp (tarp made of bubble wrap and a silver facing) . It is supposed to insulate as well as 1" styro board. I am considering trying it out in my basement (existing slab).

Any of you seasoned vets at radiant had much experience with using the stuff?

Jason Horner

personally I would not put insultarp (or similar) under a thin gypcrete pour - too much flex in my opinion. I would not hesitate to use it under a 4" slab though.

YMMV

Unknown

I would hesitate, if you don't have any real R value on your perimeter or edges. They need it if the slab is heated, and insultarp and the other "reflective" products don't give it.

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Daniel Moore

The thinnest radiant subfloor...

Thank you all for the gr8 advice! I think I’ve sorted out the problem of the thinnest possible insulated radiant subfloor! See attached diagram. The only 2 minor questions remaining: 1. should a reflective foil be used on top of the insulation or under the sleepers for better insulation, 2. would heat transfer plates be recommended instead of sand around the radiant tubing?

a. Barrier: Apply 6 mil Polyethylene vapor barrier on slab

b. Attach 2 1/2" sleepers (build up 2x4 to 2 1/2x4” nominal) to the slab 6” on center (typically 6, 9 or 12” OC).

c. Place strips of 2” thick rigid polystyrene insulation (eg. Dow’s Blue styrofoam Highload 40) between the sleepers.

d. Place radiant tubing on top of the insulation between sleepers.

e. Fill remaining cavity around tubing with sand (would heat transfer plates be any better?)

f. Lay 1/2" blueboard nailed to the sleepers, be careful not to penetrate tubes.

g. Install the 12” tile on top of the blueboard

Unknown

heat transfer plates would be a lot better than sand and reflective foil would be a waste of time.

However, wouldn't just doing a 1-1/2" overpour over 1/2" XPS, then doing tile or floating floor over it be the thinnest?

Whatever you do, do NOT do sand!!!!!!

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Daniel Moore

Really no sand...

What a quick response! Thanks!

> However, wouldn't just doing a 1-1/2"

> overpour over 1/2" XPS

True that's thin- my prior statement is withdrawn, however sleepers provide a good thin alternative in this situation, where the house is in Jersey so ground heat loss is a factor, and at least 2" of insulation would be required under the Gyp. Also, avoiding the slurry pour in the 100sq' space reduces the opportunities of delays waiting for another contractor, and the required drying time- USG says "Adequate ventilation must be provided to ensure uniform drying of the installed gypsum floor underlayment, which typically occurs within 10 to 14 days at 1-1/2-inch thickness."

> Whatever you do, do NOT do

> sand!!!!!!

Thanks I hear you- I guess the sand is messy?? It was recommended at: http://www.cozyheat.net/in-floor-radiant-heating.phtml "sand acts as a heat mass to hold heat in the floor and the room" - oh it's cheap as well

Unknown

yes, and it's also crappy at conducting heat and it's near impossible to get good, predictable performance out of it.

Plates will kick its **** all the way. Use a Pex-Al-Pex to reduce your tubing expansion concerns though.

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Cheeze-Tech

One concern

The only concern I would have with the sleeper set up (and I admit it is not based on experience, just a concern) is the possiblility of water getting under your floor and creating a mold situation. My house sits up on a hill, but there are some rare occasions where we do get a little water in the basement. A cement slab on slab would leave no room for mold.

cruizer

Suggestion

The first thing I noticed from your diagram was that you have the blueboard on top of your tubing. This is a big no no. Basically what your doing is forcing the heat downward. You want the path of least resistance between your tubing and the finished floor. So therefore you want the insulation UNDERNEITH the tubing. Do you have access to the joists beneith the subfloor? If so, use your insulation there. Then above the subfloor, place your sleepers just wide enough to put in the heat transmission plates, then place the tubing in the plates. Since you're going with a tile application, probably your best bet is to place concrete board above the sleepers, then tile over the concrete board.