Plated underjoist insulation: R-value and air gap?

Hugh Geiger

I have installed 1/2in pex on 8 in centers (16in joist spacing) as staple-up with plates (20in plates with ~8in gap between plates). This is a main floor application over a conditioned space (heated basement) where the heat loss is ~30BTUH/sf at min design temp. There are a variety of opinions on the best way to insulate underneath the tubing:

1) 1 1/2in foil-backed foamboard, foil up, pressed tightly against the tubing.

2) Fiberglass or mineral-wool insulation (or sound barrier)with an R-value of 4 times the overlying subfloor and floor (in my case, floor+subfloor=R1.7 - therefore 3in R8 would suffice), leaving a minimum 2in air gap in the joist space.

3) As 2) above, with a much higher R value (R20).

4) As 2) or 3) above, with a layer of foil above the insulation.

My initial thought is to go with the air gap (to maximize heat distribution to areas in the joist space that are not plated, such as against the end walls where the tubing loops back and is not plated) combined with the foil (to maximize reflection of radiant heat). However, I have heard that the conductive heat transfer of plates in contact with the subfloor greatly exceeds convective (and conductive) heat transfer in the air gap, and that the plates make the foil unnecessary. Also, that the foil quickly looses effectiveness once dusty (although I'm sure this is wavelength dependent).

Any suggestions?

hot_rod

R-19

which equals a 6" fiberglass batt. No foil backing, and pushed up to the plate. Do not compress the batts, let them fluff up for best performance.

Be extra carful at the rim joist (band joist). Think I would get some spray can expanding foam to seal any air leaks and then batt over the rim space with the R-19.

best to spend on the extra R upfront, especially if the ceiling below will be sheetrocked, you only get one chance to get it right!

hot rod

Hugh Geiger

Thanks for your reply.

Some basic resistors in parallel calculations suggest that, with R1.7 above and R11 below (3 1/2in fiberglass), ~13% of the heat goes down. With R1.7 above and R21 (5 1/2in fiberglass), ~7.5% of the heat goes down. At 30 BTUH/sf, that's ~4 BTUH/sf for R11 compared to 2.25 BTUH/sf for R21.

My basement heat losses are something in the order of 10-12 BTUH/sf. But then, I have 50W potlights every 32 fs. Given that 1 BTUH = .29 Watts, I'm getting 5 BTUH/sf from the lighting alone (when it's on).

Is there value in putting in the extra insulation, just to reduce the downward heating by less than 2BTUH/sf?

Second, if the insulation is pressed (loosely) up against the plates and pex, how does the heat get to the floor over areas that are not plated (such as the end of the loops, and places where the pex has to, by code, drop down to penetrate a joist through the lower 1/3 of the joist height).

hot_rod

there is always

a point of diminshing returns on insulation. Same with the foam thickness in under slabb applications.

What I have found is ocassionally homeowners will change the floor coverings, possibly add throw rugs or carpet. When this happen the supply temperatures need to be increased. This tends to overheat the ceiling below.

Looks like you have crunched the numbers and R-11 comfortably "fits" your application. In this case the extra expense of R-19 may not be worth the upfront cost. That will be your choice.

As far as loop ends, possibly you could hold the insulation to the bottom of the joist bay under these areas. Another option would be to get some foam pipe insulation for these loop ends.

hot rod

Hugh Geiger

Thanks again for your reply.

As a researcher in geophysics, I know some things about heat flow - heck I could probably code up a simulation of my basement and run it on our parallel supercomputer. But I'm also an engineer, so I know that things that work in theory and as a computer simulation don't necessarily work in practice. And I'm also a homeowner and handyman who has worked on enough projects to know when to bring in the experts (although I sometimes only know that in hindsight!). So I greatly appreciate comments from experienced hydronics professionals like yourself. In fact, I have a professional doing my basement installation. The information I'm getting from this bulletin board is helping me with the decisions.