Insulating and Sealing Joists with Track Radiant
As a long-time reader and first-time poster, many thanks to all who contribute their knowledge to the Q&A section of GBA.
While I recognize that insulating and sealing radiant track has been a topic a number of times before, I haven’t seen as much on how to minimize floor joist conduction while insulating and sealing the joist cavities simultaneously.
We have low temp (~120° water) radiant floor track that was retrofit between the basement joists in our zone 6 house. I’ve insulated and sealed between the floor joists below the track. However, lacking a way to decouple the floor from the joists, the exposed joist ends are conducting significant amounts of unwanted heat into the well-insulated and well-connected basement.
My primary questions are as follows:
· – Other than losing ceiling height and quick access to wire and piping runs, and possibly diminished responsiveness, what other downsides are there to just putting 2” foil-faced polyiso across the bottom of the joists to seal and insulate simultaneously?
· – I have some joist cavities with a radiant barrier sealing in nothing other than air and other cavities where the barrier is sealing in R-32 Roxul. In its current state, both of these approaches show the same surface temperature. This is giving me pause. Heat loss calculations say the Roxul should offer ~2800 BTU/Hr in savings over the polyiso on its own. Until the joists are insulated, is the basement just too warm to allow the Roxul to show its value?
· -Is it worth the time and expense to put a radiant barrier just below the PEX track and then place the R-32 Roxul between this radiant barrier and the proposed 2” polyiso at the bottom of the joists? Is the radiant floor likely to be sensibly more efficient doing all of this compared to one of the less comprehensive approaches above?
Thanks in advance for sharing your experience and perspective.
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Replies
Measuring the temperature of shiny things like a radiant barrier will usually not be very accurate. IR thermometers won't reliably read the surface temperature of a shiny metalized surface like that of a typical radiant barrier. That might be why you're not seeing what you'd expect in your readings.
I don't see any benefit to putting a radiant barrier between the PEX and the roxul. I doubt you'd see enough benefit, if any, to make that worthwhile.
Is your basement conditioned space? Is it at least sealed off from the outside? You may find that the roxul alone is fine, with no need for the extra layer of polyiso.
Bill
Thanks for the reply and the insight, Bill.
The basement is well insulated and can be conditioned, but we treat it as unconditioned space; ideally, the basement would stay around 50° in the heating season.
I initially took the basement heat load numbers from a professional whole-house assessment we did prior to replacing our heat system. The numbers from this report have proved out in the other levels of the house and thus I used what it listed for the basement without much thought - 16,000 BTU/Hr. However, I just calculated it myself using Siegenthaler's basement heat load method. Assuming I calculated right, at 70° F interior temperature, it is ~5000 BTU/Hr at the same design temp as the report used (-7° F). There could easily be 1000+ BTU/Hr in residual heat coming off the heat system board and the associated feeder tubing on an average heating season day, which is rarely -7° F...
Armed with less concerning loss numbers, as you recommended, I should ignore the IR temps from the foil, insulate any remaining open cavities with Roxul, and increase insulation on the board/feeder tubing before figuring out whether sealing the joists is worth the effort.
Thank you again for the help! I'm not sure how long it would have taken me to recalculate the basement load numbers without someone questioning whether the basement was at least sealed from the outside or not.
Ian
I should have mentioned before, if you want to measure the temperature of a shiny thing, put a piece of masking tape on that shiny thing and then measure the temperature of the tape. That's the way around the "shiny thing IR temperature measuring" problem.
Since your basement is already air sealed from the outdoors, and you're just trying to keep it a bit different temperature from the rest of the house, you don't really need an air barrier -- you only need some insulation. I'm pretty sure the roxul alone will do what you want without the need for the cost or installation labor of the added polyiso. You're basically trying to keep the heat radiated from the PEX going mostly to the upper levels, which is what it naturally wants to do anyway. The roxul underneath will help to minimize the amount of heat that goes into the basement.
Bill
There are two things to know about heat flows. First, the heat through an assembly is determined by its area, its r-value and the temperature difference between the two sides. Second, if a space is at a constant temperature it means the heat flows in and out of that space are balanced.
In your case, the heated floor is the heat source for both the room above and the basement. For both spaces, the heat flows have to balance, which means the heat flow into the room from the floor has to balance the heat flow out of the room.
You want the heat plates to be hot enough so that enough heat flows into the room above to meet the heat loss when the room is at 70F or so. At the same time you want there to be enough insulation below the plates so that the flow down meets the heat flow out of the basement when it's 50F.
So there's no one-size-fits-all answer, it depends upon the heat loss out of the basement, the heat loss out of the room above, and how much insulation is above the heating plates.
Note that if you size the insulation to keep the basement at 50F when it's your design temperature outside, -7F, at milder temperatures the basement is going to be warmer.