Can electric floor heating be installed under wooden subfloor in the joist space?
antonvk
| Posted in General Questions on
I need to heat a bathroom with an estimated heat loss of 6,000 BTU/h. I am not ready to replace the tile floor at this stage, but I have access from the crawl space to the joist space under the wooden subfloor. I would like to use electric heating source due to the small size, but it looks like people install heating wire only under the tile, and never under a wooden subfloor, where PEX tubing is typically used.
If anybody has tried using heated wire (or anything similar) under the subfloor, how was your experience?
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Hydronic heat is commonly installed under the subfloor. Electrical fixtures have to be installed in accordance with the manufacturer's instructions. One product I know that the manufacturer says can be installed from below is Step Warmfloor.
Installation video is here: https://www.youtube.com/watch?v=9Sq2kUnoQBM
Most other systems are 18" wide and can't be trimmed for width, which would be a problem with 16" joists.
They recommend having more insulation below than above the heat element.
Why not just get a small electric wall heater, baseboard or ceiling heater and/or light combo.
I'm planning on installing this mat by suntouch for spot heating in bedrooms and near windows. I'm using it mainly so I can use it under nailed down hardwood on my main floor. https://www.homedepot.com/p/SunTouch-Floor-Warming-4-ft-x-16-in-Underfloor-Radiant-Floor-Heating-Mat-1200416U2R/100607080 .
That's pretty pricey.
I was happy with this:
https://www.amazon.com/gp/product/B07MZ5F5NQ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
I had it put on a smart switch and turned on for about 4 hours a day on days where it was very cold. But - after insulating the basement it doesn't help enough to justify the time it is on.
Also - you won't get much value out of it unless you put at least R10 below it.
Jay Thomas,
Late getting back on this: the one I linked was specifically for putting under a subfloor and indeed is more expensive. It appears the only difference is they add an (assumed) aluminum sheet. That does pose the question on whether that’s required.
I doubt it. Heat is quite even when I use it... on it camera too. Just make sure to insulate below it.
>"I need to heat a bathroom with an estimated heat loss of 6,000 BTU/h."
What is that, an uninsulated outhouse located in Whitehorse Yukon?
Seriously, that's a huge heat load for a normal sized bathroom in an insulated house. How was that load estimated?
Dana, I haven’t punched any numbers out yet, but I am wondering if you accounted for the heat generated from the aerobic digestion :-)
Hi Dana, this is a rough guesstimate - bathroom space / house space x furnace BTUs/h x furnace efficiency. The bathroom is 14 x 14 x 10 feet, with 3 out of 4 walls being exterior walls, crawl space underneath and attic above. 2 windows 3 x 3 feet.
On a day with average winter temperature, the heat loss will likely be 2-3k BTUs/h (different online calculators give different numbers in this range).
Electric wire can produce this amount of heat, the question is can it be done from under plywood subfloor?
The thing about electric is if you supply the juice, it will supply the heat. This is in contrast to a hydronic system where it's possible for the water to return uncooled and no heat has been released.
First issue is whether that heat going where you want it -- up into the floor rather than down or sideways. Second issue is whether enough heat will escape, or will the wire or the area around it get dangerously hot. Third issue is whether at that heat load the floor will be comfortable and not too hot to stand on.
Insulation should solve the first issue, and it's something you can assess with your hands with the system running and add more insulation if you're not satisfied.
The third issue should be solvable. You're asking the floor to provide 30 BTU/sf. The generally accepted number for max heat out of heated floor is 35 BTU/sf. So you're near the top of the range, but within it.
The second one I don't know how to address. I guess you could estimate the r-value of the floor and calculate what the temperature of the bottom of the floor would have to be to give 30 BTU/SF/hr at a floor temperature of 100F. Then see if the manufacturer's literature has a maximum safe operating temperature. I image it has some sort of thermostatic safety, but you don't want to be tripping it because you'll end up with a cold floor.