Radiant under double subfloor?

MikeB_2

I want to install radiant floors in joist bays with a double 3/4 subfloor above. I plan to add 3/8" pre-finished hardwood on top of the 2 3/4 subfloors. Will the heat get through? I plan to use 1/2 pex, 8" on center with heat diffusion plates and foil inslulation to close the joist bay. Thanks!

Al Letellier_9

radiant

It depends....you need to do a heat loss to calculate how many BTU/sq. ft. you need to heat the space. Then you will know if you can get the job done with radiant. It's the only way to know for sure.

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MikeB_2

heatloss

heat loss calc was done and system was sized, but I don't think they realized it was a double subfloor. Anyone else run into this?

ALH_4

Chart

I created this chart from RE's fea data. It's based on 1/2" original Thermofin-C below the subfloor.

-Andrew

Ted_9

Graph

That's a nice graph.

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Kniggit

Depends on what you call work.

I have 5/4 sub floor (true 1") and 3/4 hardwood over 1/2" with plates 8" on center R-22 insulation below, and If i wish to heat the home with the floor only I need a water temperature of about 185, so I added som panel rads, the floor does the bulk of the heat except in a couple of high load rooms. Ask your contractor for a copy of the design, make sure you offer to give it back after just a quick check, or if they want you to pay a deposit then thats ok too. Or you can also just download the Slant Fin software yourself, it will tell you based on the information you can get with a tape measure. Just remember to keep your floor under 80 and you'll be fine. 85 floors can be uncomfortable, and will damage the wood no matter the thickness.

MikeB_2

Thanks, Andrew - so based on that chart, am I ok? Do I need to use Thermofin? Thanks in advance

ALH_4

It all depends

It depends on your heat load. Once you have determined your heat load, you can determine whether your can get the output you need from pex with extruded plates. Most manufacturers will provide this data for their particular method of installing radiant floor heating. If you are in a colder area of the country, you may need more heat than radiant floors can provide comfortably in which case steel panel radiators would make a nice supplement. It all starts with determining your heat loss.

I would definitely use extruded transfer plates, like Thermofin, with a thicker subfloor like yours to maximize the output. Whether that is enough depends on the heat loss. If the lower layer of subfloor is 1x boards, the output is reduced more than if both layers are plywood. Hope that helps.

-Andrew

MikeB_2

I will do my own heat loss and see what happens. I have a wood stove that I can use for supplemental heating. I am looking at the Wirsbo Joist Track units, they seem expensive, but if they will make the difference, it's money well spent. Thanks.

Unknown

Hey andrew, why isn't that output linear? It's got a 'kink'.. shouldn't it be linear with the Delta-T between room and water temperature, given the same installation type and floor R-value?

ALH_4

Good Question

It's showing the resolution of the fea. If the isotherms were closer together there wouldn't be a kink. Maybe I should have done a curve fit and dropped the first two points. The graph is from the raw data I had. Here's one with trendlines, which is probably better than the other graph.

-Andrew

Pinball

Question..... What happens to all these wood floors when the ambient temps hit 90+* in the summer? Would'nt they be "damaged"?
I think it would deffinalty be uncomfortable, but not damaged. If that were true then wood floors could only be installed in climates where the temps never get above 80*
Wood is alot more resilant than that.

Al

Kniggit

Not a problem with heat its the Humidity

When you cook the wood you drive even more moisture out of it all winter, then when the heat goes off and air conditioning season comes in the wood soaks up the mositure again because you have pushed it all out. The temperature is not the real bad thing for it, its the lack of humidity in the winter and raising the temperature of the floor. I would hazard to guess that if you did something to maintain a higher level of humidity in the home all winter then the higher temperature of the wood would be less of an issue.

With my floor the gaps between the boards is almost nil during the summer, however during the winter some of them open up to as much as 1/32-1/16 might not sound like a large gap, but it can be a little ugly to you when you are looking for it. I am not saying that the radiant heat causes all of my problems as I am certain that hardwood floors all over the world shrink and grow with the seasons, but there are industry standards as to what the wooden floor should be subjected to.

K

Mike T., Swampeast MO

Are your floors relative new? Say post WW-II?

If so, the quality of wood has severely declined since. Older wood (even plain sawn) is typically MUCH more stable and the quarter sawn stuff is damned near eternal.

Kniggit

Like brand spankin new man

I understand that newer wood has a more open grain and is therefore more prone to the humidity changes, this post apeared to be based on a remodel with 3/8 prefinished (I would guess being new wood) so I thought my nearly completed remodel would be a close comparo with hardwoods just installed this winter. I saw the gaps open up all winter after the install, then this summer even with the A/C running (2 stage with variable airhandler) keeping the humidity reasonable they closed back up. It happens no matter what the heating system is. Especially if you don't do anything to keep the space humidified.

K

ALH_4

flooring

3/8" prefinished flooring is probably engineered "plywood" type flooring which would be a lot more stable than 3/4" solid hardwood. It seems to me that the issue of uncomfortably warm floors would arise before hardwood damage would occur. Although that would depend on how warm someone likes their floors. I think you're right, moisture change is the culprit, not the temperature itself.

-Andrew

Mike T., Swampeast MO

Natural wood or engineered? If engineered it really shouldn't be moving noticeably.

If natural and more much more than 1" in width at that thickness, then that's a major part of why you see the gaps. For natural wood strip flooring any width greater than 3x thickness is problematic--even with the good old wood.