We are doing hydronic radiant on both floors of a new rambler, 1900 sq ft per floor. In the basement we put down 2" foam board insulation under the tubing before pouring the slab (with the sides insulated as well), so we should be good to go on the basement level. On the main floor the tubing is laid atop the plywood subfloor, which will be covered with 1.5" gypcrete (suspended slab). The question is, do I need to insulate between the floor joists below the subfloor even though the basement level is also heated? I assume every bit of insulation helps, but from a cost/performance standpoint, would filling between the floor joists with fiberglass batting make enough of a difference to pay for itself, or would the improvement be minimal? What about foil stapled to the underside of the subfloor? If this were a staple-up installation this would be a no-brainer, but with a suspended slab above a heated basement I didn't know if the in-floor insulation would be cost-effective. Any advice appreciated!

The question is what are you putting on top of the concrete for floor finish and where are you located?

Flooring will be 5/8" engineered wood (floating). I'm in Utah.

You don't need to insulate.

However, your response time upstairs will be a lot lower than with insulation. You will also will always be heating the downstairs when you turn up the heat upstairs. No ability to zone your heating. It may seem not a problem now but when you start living in this house you will notice the difference.

So what's my best bet from a cost perspective? Stuff fiberglass batting between the joists and be done with it?

To be able to control them separately as zones you need to insulate- the heat loss characteristics of a basement are usually very different from that of an all above-grade floor with more windows & doors, etc. There's no "payback" on efficiency worth noting so "cost effective" isn't in the picture, it's strictly a matter of being able to control the temps of the zones separately.

So, what's cheap and effective?

Foil/radiant barrier isn't going to buy you much- even with perfect installation with 3" of air gap on both sides it's only buying you ~ R10 at the ASTM tested 30F delta-T between the subfloor and the basement, which will literally NEVER happen. At lower delta-Ts the performance is even lower. Cheap R11 or R13 batts snugged up to the sub-floor is enough to provide zone isolation. If it makes installation easier, kraft facers are fine. Unfaced will work too, but without the bottom side air-barrier there will be a hit in as-installed R value, but the hit is small when the warm side is up and has an air barrier (the subfloor) on the warm side. Convective forces will still keep the warm air at the top next to the air barrier, so it won't lose much to convection loops.

Thanks, Dana1, for the great info. I took your advice and started installing R-13 batts. I've read conflicting advice, however. Some say to leave 2" of air space between the floor and the batt. You advise putting the insulation directly against the floor (ceiling). I don't know which is right. I do worry about creating a haven for mice by leaving a cozy space of warm air above the insulation. What would the benefit of the air space be?

Posted By RedHotFuzz on 27 Jan 2012 11:49 PM
Thanks, Dana1, for the great info. I took your advice and started installing R-13 batts. I've read conflicting advice, however. Some say to leave 2" of air space between the floor and the batt. You advise putting the insulation directly against the floor (ceiling). I don't know which is right. I do worry about creating a haven for mice by leaving a cozy space of warm air above the insulation. What would the benefit of the air space be?

One would dare not attempt that in rural areas. It would not be a question IF mice would infest that but HOW MANY mice. Plus if you are in a semi-rural of Utah then you are most likely in Deer Mouse territory, and they carry the HantaVirus.

Attempt what? Leaving the 2" gap? Would pushing the insulation right against the ceiling make it less mouse-friendly?

Posted By RedHotFuzz on 27 Jan 2012 11:49 PM
Thanks, Dana1, for the great info. I took your advice and started installing R-13 batts. I've read conflicting advice, however. Some say to leave 2" of air space between the floor and the batt. You advise putting the insulation directly against the floor (ceiling). I don't know which is right. I do worry about creating a haven for mice by leaving a cozy space of warm air above the insulation. What would the benefit of the air space be?

Any gap over or under fiber insulation leaves a potential thermal-bypass for infiltration and convection currents to pass. In low-mid density fiberglass it also allows convection currents to move through the insulation, lowering it's effective R value. Low density fiber absolutely needs air barriers on both sides to perform anywhere near it's rated spec. High density "cathedral ceiling" batts, dense-packed fiberglass or any density cellulose are sufficiently air retardent to work OK without air barriers though.  In this warm-side-up app mid-density R13s snugged up to the subfloor will work well enough with or without a bottom side air barrier, but if kraft facers make it easier to install, go for it.

Bottom line- no gaps==> better thermal performance, and less rodent-raceway potential.

Rodents will nest in fiberglass or rock wool, but the fire retardents in cellulose and cotton insulation is an eye irritant- they tend to avoid it.

the airspace is a requirement only in plateless systems.

By "plateless" I assume you're talking only about staple-up scenarios and not suspended gypcrete slab correct? So with my gypcrete suspended slab, no airspace between subfloor and insulation below, correct?

yes, sorry, could have been clearer about that. airspace for plateless joist mounted systems only.

Thanks all for your expert advice.