How to insulate joist bays with radiant heat tubing?
I have just installed radiant tubing with plastic clips under a plywood subfloor. Looking for some common sense insight on one or more cost-efficient ways to insulate the joist bays separating the first floor below from the second floor above. I have a well-insulated building shell (3″ closed cell SPF insulation from sill plate/band joist continuous through side walls and under the roof deck, with an added 2″ unfaced fiberglass to fill the remaining 2×6 wall stud cavity).
A few basic questions:
1. How much R-value do I really need within this conditioned envelope to create a delta between floors so that the heat from the radiant tubing is transferred up to the second floor, and not down to the first?
2. Is a reflective radiant barrier really necessary between the tubing and the insulation, if there is enough insulation below it?
3. Is foil-faced batt insulation a decent (though maybe not optimal) choice to get some reflectivity and insulation in one package?
4. If the foil face is only a minimal advantage, wouldn’t unfaced batt insulation be enough to isolate the 1″-2″ air cavity and tubing from the space below and make the system work efficiently?
5. If using foil or kraft-faced batt insulation, is there any fear of the facing acting as a vapor barrier and creating unwanted moisture somewhere in the joist cavity once it’s sealed top and bottom with floor and ceiling materials? Seems like that space would never fall below the dew point to creat any kind of condensation or dampness.
This is a DIY, budget-conscious project, and I need to finish and close up the framing. Wondering what to do amid lots of conflicting information. Any insight based on hard science, past experience, or common sense is welcome.
Thanks.
Replies
Underfloor heating pipe
Heat always moves to cold. Pipes radiate heat through 360 degrees. If you have the pipes in full contact with the floor above, most of the heat will be exchanged by conduction.
One way is to fit sheets of 2 of more inches thick polyurethane foam covered with a reflective membrane below the pipes, a small gap of 19mm will work. The foam should fill all of the space below the 19mm line.
If you have the space another method is to lay the pipe in the top of polystyrene sheet, that is at least six inches thick, this way there will be very little heat loss downwards.
I DIY'd my RFH years ago.
Staple up between I-joists. Stapled the PEX to the bottom of the subfloor.
Took 1/2" foil-faced polyiso, ripped it to width on the table saw, then friction-fit it between the webs of adjacent joists. By snugging it up to the bottom of the TJI flange, it left me with a 1-1/2" air space between the polyiso and the subfloor. You could use XPS instead of polyiso, but I would't use EPS. EPS is too fragile.
Since I used 1/2" polyiso (R3) I added unface FG batts (R19) under the first floor (basement ceiling).
You'll not have moisture problems with the foil or XPS acting as a vapor barrier.
I do think the R3 foam alone outperforms plain ole R19 FG batts. Many years ago when I had air conditioning put in, the guys running the duct removed some of the insulation in the basement ceiling to to gain access for their work. They never replaced things correctly. Some places they replaced it all in others they left the polyiso in place with no FG, in others they replaced the FG but never replaced the polyiso. When I pulled down the FG that had no poliso above it, the top of the joist bay wasn't really warm at all. In areas where there was no FG but just polyiso, when I pulled down the polyiso the top of the joist bay was quite warm.
1/2" or 3/4" foam would be easy to friction fit. Either polyiso of XPS would work fine. The thickerer you get with the foam, the more difficulterer it gets to install it.
Mongo wrote:
I do think the R3 foam alone outperforms plain ole R19 FG batts.
How's that?
My understanding is that R-value is that same metric applied to any material. The only way I could see R19 FG batt being outperformed by R3 foam is where high powered fans would be run through FG batts to a point where just about all insulating value is being lost to wind wash. Good luck finding those conditions in any home.
I hope HePex is what was use by all posters. We don't use insulation between the floors. If the shell is tight, the heat heats the heated space regardless of up or down. I believe we get more even heat this way too. Heat rises which is why floor heat is great. If there is no cold at the ceiling because the floor above is warm, the cold doesn't fall and kick on the thermo stat. The cost of the insulation is never really recovered by energy efficiency savings and the difference is not noticed in I comfort. Even when we zone the house up and down, we don't have issues.
Note that more isn''t better here -- over about R3 it's just added expense and wasted motion.