Radiant Tubing Beneath Vapor Barrier

chuckml

We are looking at using Rockwool products for underslab insulation. The comfort board 110. They recommend placing the vapor barrier between the insulation and concrete to protect the insulation from moisture during concrete placement. That's well and good, but we'd also like to staple down tubing, and this would poke many holes in the vapor barrier, rendering it useless.

What are the reasons to NOT staple the tubing to the Rockwool comfort board and then place the vapor barrier over that, followed by concrete?

The only thing I can conceive is the potential for condensation on the topside of the vapor barrier ... but is that an actual possibility if the concrete mass is in the conditioned space? Or is it possible that the location of the radiant tubing under the vapor barrier now causes us to define the conditioned space to the underside insulation assembly? 🤔

I want to know if the proposed assembly of soil, stone, insulation, radiant tubing, vapor barrier, and concrete poses any moisture risk to the interior space or floor.

hot_rod

I wonder that you might get air pockets around the tube with the barrier over the top?

Does that product hold foam staples well?

Seems it is not really a waterproof product, whereas the xps foam board products are, and pass as the vapor barrier if you tape the seams.

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psb75

I don't think this is the correct insulation product for gound contact. I don't believe there is a good way to secure the tubing to that kind of insulation. This also keeps your tubing at the very bottom of the slab and not very much "in it."

Jamie Hall

As @psb75 noted, that will put your tubing at the bottom of the slab — which is the worst possible place for it. You will have reinforcing in the slab — at least I hope you will — and it would be much better to tie the tubing to the reinforcing a few inches off the bottom of the slab.

hot_rod

Here is an example of how you can insulate, reinforce and raise the tube into the pour. Fiber in the pour is better than rebar at the bottom of the pour, IMO

The tube 2" below the surface is about right. The 250 number on foamboard indicates 25 psi/sq ft.

Plenty for residential slabs.

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Jamie Hall

Fiber in the concrete is very strongly recommended. Most suppliers can do that. Rebar may — or may not — be needed, depending on the loads. If it's to be a garage, I'd use both. A living room? Maybe just the fiber.

hot_rod

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With rebar you will need chairs to keep it up into the slab at least 1” off the insulation. With foam you need strip type chairs so they don’t push into the foam.

So in a 4” slab with a cross grid of 1/2” bar and 5/8” od tube, either the tube or the rebar ends up in the correct location. Since you want adequate coverage over the tube, 2”, the bar ends up sitting on the foam, of little value down there.

Use 6x6” #10 wire mesh and a fiber mix for the best of both, skip the bar unless it is an engineered design requiring it. If so that would be a 5 or. 6” slab to accommodate all the components

chuckml

@hot_rod thanks for all the thoughts.

The comfort board does hold the staples well. We created a sample section of it to test. I was concerned about the lower efficiency due to air pockets around the tubing, caused by the vapor barrier. Less than ideal for sure.

I attached the Rockwool detail showing their Slab on Grade recommendation. It is rated for that, but evidently not well adaptive to stapling tubing.

Do you have any thoughts on the tubing under the vapor barrier causing condensation within (and ultimately on the surface of) the slab?

We will likely do the 6x6 wwf on top of the vapor barrier, just from a heating efficiency standpoint.

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Jamie Hall

In answer to the direct question, placing the tubing under the vapour barrier under the slab should not cause condensation in either the slab or the vapour barrier.

Now. As several of us have said, and I will repeat here, the correct location for the tubing is within the slab, somewhere in the middle third of the thickness of the slab. It really doesn't matter how you get it there, or how you hold it in place during the pour — there are many ways to do that — but that is the only correct place.

Any other location will kill your heating efficiency and, with your proposed location, the responsiveness of the heating system to demand changes.

What is your reasoning to avoid the correct location?

hot_rod

Tube at the bottom will cost 12-15 degree hotter supply and, or it will have a lower output. And a slower warm up

Why not the tube elevated on the mesh? You only get one chance to get it right😯

GroundUp

VB should be between the ground and the insulation to prevent moisture from the ground from getting at anything else. Concrete will dry upward, so having the VB above the insulation really doesn't serve a purpose- especially since Rockwool isn't permeable. In a case like this, I think your best bet (assuming you're stuck on the Rockwool product) would be to drop the VB on the bottom, then your insulation, then tie your tubing to a mesh panel (NOT the roll mesh, something like a "cattle panel") suspended mid-slab with chairs.

psb75

The Rockwool Comfort Board 110 IS permeable according to its specs on website.

GroundUp

https://forum.heatinghelp.com/discussion/comment/1808428#Comment_1808428

Then we DEFINITELY don't want it in contact with the ground.

psb75

There seems to be a generally 'wonky' approach to this design and not much feedback from the original poster/designer. What say, chuckml?

chuckml

Thanks for all the comments and thoughts.

@jamie hall - Appreciate the direct answer regarding the vapor barrier and any potential for condensation on the top side if the tubing is underneath.

I have spoken with the Rockwool design team, and the performance of the rockwool depends on its ability to drain freely which is why they use the vapor barrier between the concrete and the insulation. They maintain that it prevents moisture from the wet concrete from becoming trapped in the insulation (unable to exit the bottom) which would degrade its performance. It is my opinion that eventually water will evaporate out, but they only recommend installation according to their details.

The performance due to the radiant tubing placement within the concrete is a separate issue, in my opinion. That is the optimization of a system rather than a building envelope health and safety issue (which mold would be).

@hot_rod, thanks for the details on the performance concessions by putting the tube at the bottom. IF the VB must be on top of the insulation AND the homeowner wants to staple it down, then I see the only option as putting it below the VB at the bottom of the concrete to avoid perforating the whole VB.

As we all know, it really comes down to educating the homeowner (who also happens to be the GC, and framing is his specialty) and giving them the scenarios with implications.

@psb75 - agreed. 'wonky' sums it up. But sometimes exploring the 'wonky' reveals options. Maybe bad options, but still options! 🤔

Jamie Hall

Frankly, I wouldn't use rock wool as insulation under a slab. But I wasn't going to address that question. You will have vapour saturation problems with it, wherever you place the vapour barrier — you can't avoid it — and its performance will be much worse than expected.

I'd never use anything except closed cell foam board in that application. If the grade is done with any care at all — I'd use a foot or so of free draining sand, with foundation drains all around to keep the water out — and graded smooth, you'd have no problems with the foam board.

hot_rod

vapor barriers do change the dynamics of the concrete cure. All the waters of convience will need to come out the top.

Some concrete finishers will use cure additive's when they know there is plastic under the pour

On one pour, a surly concrete finish took a rake to the vapor barrier and perforated it

Im not seeing a lot of advantage to the insulation you are considering? How does it ever dry out?