In-floor / Staple-Up Radiant - how do you deal with obstructions?

[stee6043]

Well I'm getting ready to order the pex for my in-floor project. I finally decided it will be worthwhile to remove my drywalled basement ceiling for the comfort and efficiency I can gain from radiant.

I had a bit of a "never thought of that" moment yesterday, though. I have a fair amount of ductwork running through the spaces between my joists for my forced air system. My joists are on 12" centers and I have 6"/8" round ducting for all of the rooms running between the joists in several spots.

How have you guys dealt with the insulation required below the pex in joist bays with duct work? Since I will be reinstalling the drywall there is essentially zero space for insulation in these areas. Will this be a big problem if I have roughly 2 joist bays per room with very little, if any insulation below the pex? Thoughts always appreciated...

 

[SolarAndWood]

How about a layer of foam board underneath the joists? I've done this on ceilings I vaulted to get more insulation in. Works pretty well as long as you run a line to locate the joists before you rock it. Takes a little more fine tuning with the screw gun.

 

[pybyr]

Nothing against PEX ( I like the stuff, especially PEX-a ) but there's another product which I am told is easier (more flexible) for retrofit radiant called Onix (Watts makes it).

 

[stee6043]

How about a layer of foam board underneath the joists? I've done this on ceilings I vaulted to get more insulation in. Works pretty well as long as you run a line to locate the joists before you rock it. Takes a little more fine tuning with the screw gun.

My only hesitation with this plan is that I'd be adding work to the entire ceiling for a problem only with a few joist bays. I'd also be loosing tha inch or so of headroom, which isn't much a big deal I suppose.

Thanks for the thought though. I'll add it to my list of considerations.

 

[DaveBP]

Nothing against PEX ( I like the stuff, especially PEX-a ) but there’s another product which I am told is easier (more flexible) for retrofit radiant called Onix (Watts makes it).

Looks like terrific stuff but at 3X the price it's not likely to take over the DIY market.

 

[stee6043]

Nothing against PEX ( I like the stuff, especially PEX-a ) but there’s another product which I am told is easier (more flexible) for retrofit radiant called Onix (Watts makes it).

Looks like terrific stuff but at 3X the price it's not likely to take over the DIY market.

Yeah, that and I'm not willing to put something even relatively new into my ceiling.. I plan to do this once, and once only...

 

[pybyr]

Perhaps run more runs of tubing in the bays that will be hard to insulate? And put FBBF (foil bubble bubble foil) underneath-- it's not the world's best insulation, but it is not thick, and should encourage the heat to go up?

 

[coolidge]

I was looking at the Onix tubing, there is quite a price difference. Plus i was told by a contractor that it was kinda insulated thus not allowing all the btus where you need them.

 

[deerefanatic]

I second Pybyr's reccommendation, put some of that bubble/foil stuff in there...... As a radiant barrier, it's excellent and very thin....

 

[heaterman]

Onix is a poor choice unless it can be stapled directly to the bottom of the floor, which sounds impossible in your case. Personally, I don't like the thought of a rubber hose from a longevity standpoint. Watts Radiant had some major trouble with their old product, made for them by Goodyear. That's a very long story and sordid in all its details.

If you don't insulate, you will radiate just as much heat downward as you do up. You have to trap it in the joist cavity. The minimum I have seen recommended by any reputable manufacturer is R-13 with most calling for R-19. Don't cut a corner on it as your system performance will suffer. Maybe a plan B would be in order.........


edit: I just read that there will only be 2 joist bays in the room that will have marginal insulation. What will you be using for the rest?

 

[stee6043]

Onix is a poor choice unless it can be stapled directly to the bottom of the floor, which sounds impossible in your case. Personally, I don't like the thought of a rubber hose from a longevity standpoint. Watts Radiant had some major trouble with their old product, made for them by Goodyear. That's a very long story and sordid in all its details.

If you don't insulate, you will radiate just as much heat downward as you do up. You have to trap it in the joist cavity. The minimum I have seen recommended by any reputable manufacturer is R-13 with most calling for R-19. Don't cut a corner on it as your system performance will suffer. Maybe a plan B would be in order.........


edit: I just read that there will only be 2 joist bays in the room that will have marginal insulation. What will you be using for the rest?

Thanks, Heaterman. I plan to insulate below the pex in all other joist bays with at least R19, maybe even R30 depending on the cost. The only places I am challenged for insulation is where I have the forced air ducts. I rather like the bubble/foil idea so far since it will be "better than nothing" and will still allow me to drywall as if it were a normal bay. Perhaps I could throw an extra loop in those bays also, as suggested, but this will hose up my math for total loop lengths! ha.

 

[SolarAndWood]

Are you running one line down the 12" bays? Is there a rule on how close you can run the pex lines? I have 24" bays and need to heat through 3/4 inch T+G subfloor, hardwoods and area rugs. I assume I need to get as many runs in each bay as possible and max the insulation underneath it?

 

[stee6043]

I'm currently planning on running 2 lines of 3/8" pex down each bay. I would prefer 1/2" tube but I think it's too tight to bend it without killing myself....

If I had 24" spacing I think I'd go 2 lines of 1/2" and call it good....but I'm no expert.

 

[SolarAndWood]

Thanks. I assume the idea with staple up is to heat as much as through direct contact and then encourage as much of the heat as possible that makes it into the air space up? Minimize the air space/maximize the insulation underneath it?

 

[DaveBP]

I’m currently planning on running 2 lines of 3/8” pex down each bay. I would prefer 1/2” tube but I think it’s too tight to bend it without killing myself....

stee6043, I can't say this from personal installing experience but parallel tubing spacing of 8" or 9" is pretty standard with 1/2" tubing and those layouts have 180 bends at the ends. It might not be any more trouble to use 1/2" rather than 3/8". It sure would save a lot of pumping electricity and maybe a smaller pump over the lifespan of the system to use the larger tubing. Probably allow you to put more heat into the room above on those colder nights, too.

 

[Chris S]

From what I've seen the btu output of 3/8 & 1/2" is the same. 3/8 is much easier to install & work with. You will however use shorter loops than you could with 1/2" to still produce the same head loss. I stapled up 1/2" once PITA 3/8 we do all the time.

 

[Fred61]

I hope you folks aren't forgetting thst you need a reflective surface beneath the tubing. I used a fiber reinforced reflecting sheet. When running staple-up, you don't need to bend into thr next bay. You can skip a bay and circle back to reduce the sharpness of your bend. I also stapled up aluminum flashing to spread out the heat.