Hydronic heating in slab insulation?

[goosegunner]

I know many people say don't pour a slab in cold climate without tubing. Not real sure it will suit for occasional use.

I am going to build a second garage this fall. I would like to have a small workshop that I could heat occasionally. What is the recommended insulation for under the slab if you are putting tubing in slab?

I live in south central Wisconsin.

gg

 

[bpirger]

You'll hear many different answers....I used 2" extruded polysytene everywhere (pink or blue board). I'm in NY, you are likely a little colder and longer season. Whatever you do, don't use that foil faced bubble wrap. Geez, I think those guys should go to jail.
And indeed, I'd never pour a floor without tubing. I poured a patio years ago...put tubing in it...just in case someday there was an idea of a sunroom/sunporch addition. I didn't put insulation under it though...and would pay dearly for it if it ever happened. But in a planned enclosed space....put in tubing. It takes a little time and a little cash....but you only get once chance. It's pretty simple to do yourself...pex is rugged stuff.

 

[goosegunner]

Could you do half of a building with a slab?

Meaning if the building is 32X60 could you have half heated work space and half unheated storage?

I know you can do it with forced air but what about slab movement?

gg

 

[goosegunner]

Another thing I have been told about heated slab is that it can take several hours to get heat.

If you want to go out and work for a few hours a few random times a week I would have to heat the building 24/7.

vs

Just kick on the air exchanger when I want to be out there,

Is that true?

gg

 

[wood thing]

there are different products out there, bubble wrap, insulated tarps or blankets, foam. i like blue board the best because of the cost. it works fine if your grade is good. if the grade is uneven or has big rocks in it, the tarps do a good job. you should also insulate the foundation down a couple feet. i band the perimeter of the slab with pex on 6 inch centers for maybe 3 runs and then spred out to the required spacing. pex is cheap, you cant go back later. i would do the whole floor instead of half. it would only take longer to warm up. it sounds like a nice building, i wish i had one that size. good luck !

 

[woodsmaster]

I put 2" under My slab. I've read of putting 2" around perimiter and 1" in the middle.

 

[heaterman]

In a perfect world this is how I'd do it.

If the slab is 4" thick I'd use 1-1/2" foam underneath and make sure to insulate the edges down at least 2' around the perimeter. Tube the whole thing because you never know what you are going to be doing 10 years from now.** If the building or a portion of it is going to be temporary use. I'd set up the in floor to maintain the indoor temp at 45-50* and then install a secondary form of heat (your choice) to raise the temp up quickly when you want or need to work in there.

** I can't tell you how many times I have heard customers lament the decision of not installing tubing in a concrete slab over the years. Don't think over much about it, just do it. I'd lay 90% odds that you will be glad you did.

 

[bpirger]

Indeed, it takes a very long time to heat up a slab. If you want to come home on Friday evening and work out in the shop, and the floor is 45, and you want it 60, with the size you are saying, I'd say there is a chance that by saturday morning it might be close to 60. By Sunday I'm pretty sure! It moves VERY SLOWLY when you have a huge mass. Good thing is of course it will stay above freezing for a week if the power went out that long (well, depending on the initial temp anyways). And you will find no more uniform, constant heat.

What I have done....I have 8" block walls with a couple of courses of 6" blocks to set my 2x6 bottom plates on. I will place insulation on the inside of the 6" block to act as a little thermal break from the slab to the blocks...and as expansion room.

I have placed #1 stone on the top of my grade/fill level with the top of the 8" block. Then my slab sits on the 8" block "lip" that is at the bottom of the 6" block. You really want to make sure to have good compaction and fill. I've only had my tubing in the house floor since 2001 and not a single problem. But you don't want the floor to settle at all. Where I go into/out of the slab with the pex, i.e. at the manifold, I run the pex in 1" electrical conduit. I believe this is what Wirsbo recommended at that time....not sure what they are saying now.

It is very easy to work with pex. Most people lash it (loosely, not tightly) to the wire welded mesh layed in the concrete. Ideally this mesh sits up on chairs or similar so it is not sitting at the very bottom of the slab. When I did the first pour, I had help, and I was nervous about everyone walking on the tube, etc. No problems there....it is tough stuff. Make sure no one is wearing spikes! LOL

Heaterman hit it right....if you want "instant" 20degree tmep change, you need another method. I just (am) building the attached 32x72 garage/shop, all with radiant. I'm hoping to keep it at 45-50 with the GARN. If I know I will be out there working on the weekend, especially working with polyurethane preparing siding or similar, I might turn it up to 55 or 60 on about Thursday. It will move slow....

 

[woodsmaster]

Could you do half of a building with a slab?

Meaning if the building is 32X60 could you have half heated work space and half unheated storage?

I know you can do it with forced air but what about slab movement?

gg

If you only want to heat half pour the half you want to heat then put 1" foam insulation as an expanshion joint between the slabs so heat don't radiate threw as fast.

 

[Chris Hoskin]

I like Heaterman's suggestion of keeping the space at 50 degrees with the radiant floor and using a Modine (or other) fast recovery type system to bring the room up to temp when you are planning to spend time there. A radiant slab alone will not do well for sometime use.

I agree that you should put tubing in the whole slab, but you could always make it two zones and not heat the one zone for now but have the ability in the future.

have a great weekend everyone,

Chris