Staple-Up Radiant Heat Fiberglass Insulation R-Value with Aluminum Heat Plates
Hello,
I’m doing radiant heat in a new build. We are stapling the pex piping to the subfloor with aluminum plates on the main floor. We are not pouring gypcrete or anything else that can be done with the design. Basement has the pex in the concrete. Two inches of spray foam is under the slab.
What R-value of fiberglass insulation should I use after the aluminum plates are installed for overall efficiency? I’m ok with heat from the basement zone coming up, but don’t want the main floor to be really inefficient with no insulation.
Do I go with no fiberglass insulation? Go with R13, R19 or even higher like R30 rolls. There doesn’t seem to be much consensus out there in this matter.
House is climate zone 7 at 11,000 ft elevation in Colorado.
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Replies
Insulation won't effect the efficiency one bit, 100% of the heat released will go into the building envelope.
It's a question of comfort. You want the heat intended for the first floor to go to the first floor, and the heat intended for the basement to go to the basement.
Do you have heating loads for the two zones? That would be a starting point.
Cheapest batts you can put there, pretty much anything works, R8 rolls are just fine. As DCcontrarian points out, this is only about zone isolation so you don't need much, worst case is your basement slab heat would have to run a bit less as you are getting some minimal heat from the main floor.
The batts should be tight against the heat plates, you'll probably need insulation wires to hold it up.
With any floor heat, make sure you air seal your rim joist area.
I would not be so cavalier about insulation
Insulation is cheap cheapety cheap.
Staple up is the worst of all possible worlds WRT radiant.
I would insulate the snot out of it so that the upstairs has the responsiveness and load capacity you expect.
If you have wood floors above the you have a not insignificant R value above.
No you do not need 8 inches of spray foam, but the difference in installed cost between R8 [no no no ] and R30 is minimal compared to the cost of doing it wrong
If you do it wrong, you will not tear out the sheetrock and reinsulate
You will raise the water temperature
So the downstairs zone never runs
So the wood floors dry out
quick look .30 a square foot max difference in material cost at HD
300 bucks for 1000 sq feet
NOTHING
The ratio of heat going up to going down is the R-value of up vs R-value of down. Let's say you have 1-1/2" of wood above the plates, that's about R-1.5. Let's say you have R-8 below, about 85% of the heat goes up. R13, 90% goes up. R19 93% goes up. R30, 95% goes up.
How important this is depends on the relative heat losses. Let's say the basement is above ground, about the same R-value as the first floor. Sending 20% of the heat down isn't going to affect things much. On the other hand, let's say the basement is completely below ground and the first floor has floor to ceiling windows, and the heating load is ten times that of the basement. Anything less than 90% going up is going to mean the basement has to overheat in order to keep the first floor comfortable.
Pretty close. Air gap is R1, drywall plus ceiling air film is about another R1, so R10 in total with R8 batts.
Radiant ceiling is about 1BTU/sqft/degF VS floor at 2BTU, so much less heat radiated bellow. Rough guess, about 10% radiated down with R8 batts. If you are designing the upstairs for 20BTU/sqft, that is 2BTU/sqft down. Something, but not a number I would loose much sleep over.
Basically anything other then empty ceiling cavity or bubble wrap works.
If you are looking for simple to install, safe and sound batts stay put from friction on their own and are about R10.
Thanks everyone. Some great insights here. I'll plant to do R19 or perhaps even R30 rolls.
Jwolfe,
"I'll plan to do R19 or perhaps even R30 rolls."
I'm not sure that's the takeaway I got from the responses you received.