Insulation under Radiant Heating?
Section drawings for radiant heat (or hydronic tubing if you prefer) located in or under a subfloor usually show batt insulation under the tubing, with what appears to a ~3″ airspace between the subfloor and the batt.
I’m considering radiant heat for an addition to the first floor of a 95 year old house (Zone 5A, western Penn.) and replacing the existing hydronic system that uses a gas-fired boiler to feed large cast iron radiators. The existing basement is unheated. The addition will be atop a new garage extending the basement, also unheated. I am considering the Warmboard subfloor system for the addition, and Radiantec plates to attach tubing on the underside of the existing subfloor. The existing joists are 2×10 lumber, the addition will probably be 9.5″ engineered joists.
It seems to me that a single batt stuffed in the joist space under the subfloor will waste a significant amount of heat in both configurations. So I’m considering 4-6″ of rockwool batt over 1″ of rockwool board, which would be tucked inside the joist space, flush at the bottom. This assembly would be faced with acoustic ceiling tile or drywall. Is there any reason to leave an airspace between the tubing or subfloor and the top of the batt?
Other comments welcome.
Thanks in advance.
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Here's a relevant article: https://buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance
Very nice explanation of the counter-intuitive effect of the upper space - thanks for the pointer. It looks like the air space is about 1/3 of the joist height. I didn't see a discussion of that. So, for a 9.5" joist, either a 5.5 or 7.5" batt would work. From what I can tell, insulation on the bottom reduces the heat transfer to the cold side of the joist. I'd expect that 7.5" would be better at this, without reducing the air gap effect.
Your basement isn't unheated, it's just not deliberately heated. It's still getting heat from the space above. And that's the problem.
Until late in the 20th century it wasn't common for basements to be insulated, there was a magical belief that heat only went where it was sent. Today we know better. Putting on an addition is the perfect time to bring your basement into the 21st century. That means insulation, and most important, having a well-defined boundary between conditioned and unconditioned space.
Most likely you want the garage to be unconditioned and the rest of the basement to be conditioned. So the boundaries are the ceiling of the garage, the wall between garage and the rest of the basement, and the other basement walls. These boundaries need to be airtight, vapor tight and insulated.
Separately, if you put hydronic tubing between two conditioned spaces that are not part of the same zone, you want to have insulation between them so the heat goes only where you want it. That's what the batt insulation is for.
For our Warmboard installation, we repurposed insulation batts from the attic and installed the batts under the subfloor. The attic was then insulated with blown insulation. But for hydronic heating, you want to resist heat flow to direct it to the heated space.
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