Staple-up PEX questions

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Idahoan208

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Hey All,
Spent some time reading through some threads but am still a bit confused about the best way to staple-up PEX for in floor heating still.

My situation is the following: I moved a log cabin onto a full foundation. The cabin has framed 2x4 walls on the interior (much of which are spaced a bit off the log wall but in some areas they are directly to the log) which I plan on spray foaming. The cabin is getting all new triple glaze windows and extra foam on the roof, which will end up a total of 10.5" rigid, all on the exterior of the home. Home footprint is 700 sq ft, and its something like "1.5 stories" with the second story being very low and half open to the downstairs (loft situation with a cathedral ceiling on about 1/3 of the downstairs.) The basement has 2" rigid under the slab with PEX embedded, 2" rigid against the interior wall, and 2x4 framed walls in front which will be insulated with fiberglass. I left the rigid foam down a few inches from the top of the poured concrete walls and plan on spray foaming from the top of rigid to the subfloor, encasing the rimjoist and silplates in foam.

What I'm looking for:
The basement is a separate apartment completely with no interior access to the main level and is split into two zones with the plan being in-floor as the main heat source and several cove heaters as backup


Main level:
I plan to have several heat sources at the main level including a wood stove, in-floor, and several cove heaters as backup. I realize this is overkill but the thought process is this: I live in a place with long cold winters and have lived in cold, uncomfortable living situations for too long. Wood heat is important to my partner a number of reasons, but we travel a reasonable amount and need something we can set to low to keep the pipes from freezing while we're gone. I have experienced in-floor heat in a rental long ago (just in the kitchen and bathroom) and it was so wonderful feeling I have wanted it ever since. So the goal of in-floor in this project is to provide a steady temperature to temper fluctuations of the wood stove (and also to be able to set it and leave, or turn it up and take a break from burning wood if desired) and warm feeling (so I'd ideally like to feel the warmth on the floor) and not necessarily as a primary and sole heat source. The second story is very open and there will be a fan in the cathedral ceiling to move air around.

Here are the issues/questions: the floor assembly on the main level is very thick - there is 3/4 skip sheathing with about 1/2" gaps, felt paper, 3/4" ply, then 3/4" pine. I'm concerned about heat transferring up with how thick this is. Because the basement will be a separate apartment, I planned on insulating the full subfloor cavity for sound reduction also. The floor joists are all super wonky so I planned on dropping the ceiling a bit anyways to make sure the drywall is flat (built to 9' to be able to drop a bit). I had an HVAC guy come in and say he thought this staple-up would work fine, but my questions are:

1. I have read mixed things about air gap between insulation and the PEX. Do I need an airgap?
2. heat plates - do I need "track" instead of the aluminum one to transfer heat properly with this thickness subfloor? I'm also unclear about how to space these plates - do they need to be continuous on the PEX or gaps? I am planning on running 2 runs per bay, so a line every 8" or so of 1/2".
3. Would there be a problem with spray foaming over the PEX line? I am considering using a "flash and batt" or spray foaming it entirely since I will already have a spray foam installer there doing the rim joist and I am concerned with sound transmission to the basement so I'd like to firm everything up.
4. Since I will need to drop the ceiling a bit anyways for drywall, would it be worth dropping it with 1" rigid and then drywalling over? Is this a "the more insulation underneath the PEX the more efficient" type situation, or is it more about the direct contact with the subfloor being more important. From reading a bunch of threads here, it seems that the contact is very important, but intuitively I would think that if you have a full 2x6 cavity of insulation below, wouldn't the heat pretty much be trapped and transferring to the material on top of it?

Appreciate any help!
 

John Gayewski

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Think of radiant heat as light. Don't think of it as air. The closer you are to the source the more concentrated it is.

The plates are meant to keep a hotter surface spread out farther. The space is meant to reflect the heat to spread it out.
I would say using a space and a plate would be spreading the heat load farther than is needed. So one or the other.

Your floor being so thick I would go with plates although your limited by temperature so they will only be so so at effectiveness, but likley more effective than a gap.

No matter what you do need to trap the radiation in the joist space so you do need effective insulation.

So the takeaway. The plates are to prevent cold spaces between the tubes. An "air gap" is meant for the same purpose.
 
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