Reflective Foil with Wirsbo Joist Trac

Kent D.

Good Morning,

I've completed the tube installation for an under-floor radiant job using hepex and wirsbo's joist trac product. I'm ready to install the insulation between the joists and I'm wondering if it is worth the labor and modest expense to install a reflective foil fabric over the joist-trac and the underside of the subflooring before I push the R-38, kraft-faced fiberglass up into the bay against the trac and subfloor. Any opinions?

Dave H_2

Simple answer -

No-

More detailed as to why -

The way you are heating this project is directly heating the plates, then heating the floor.

The aluminum on the insulation will only work if there is an air gap to help refelct any energy.

In a plate system, there is no air gap and the extra expense for the foil faced will be just that, an extra expense not needed.

Dave H.

Tim Doran_2

Not Needed

Just use your R-38, Make sure that there is no air gap and you are all set.

Tim D.

heatboy

R-38?

Did you build over a glacier? What is the underside of the floor exposed to? Seems like an excessive amount of insulation for very little gain in slowing downward loss. Foil backed insulation is a pretty big waste of money for any kind of radiant, plated or not.

hb

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Tony_8

what ?

Isn't the foil for reflecting RADIANT heat ? How can that be a waste of money ? I use foil/bubble under staple-up, etc all the time and am very pleased with it's performance.

Mike T., Swampeast MO

Radiation is how heat is transferred when it can't go via conduction. Remember that convection IS a form conduction in a non-solid medium--it just happens somewhat faster than if that medium weren't free to circulate.

When you're using solid-to-solid conduction-enhancement devices, heat flows much more rapidly from the emitter, cooling it significantly thus removing much of the motive force for radiation (temperature difference). You insulate directly against the conduction-enhancement device to give heat the easiest possible path to the radiant panel. In this regard rigid insulation probably has an edge as it stops convection as well--in practice however, it can be difficult/impossible to have the insulation in perfect contact so there is always some convection and fiberglass or similar work also.

If you move a radiant barrier into contact with the conduction-enhancement device it is no longer blocking radiation--it is accepting conduction. Since most radiant barriers are very good conductors, it has no benefit.

With bare tube however, a radiant barrier offers the potential for benefit as long as it is not in contact with the tube. How much this helps in the real world with such applications is highly debatable as most bare tube systems don't radiate much to begin with.

hot_rod

Depending on who you believe

I've been told the reflective barrier needs to be as clean as possible (a layer of dust compromises reflection) It also needs to be smooth and straight, most foil backed fiberglass gets krinkled upon installation. Also the foil batts have a ton of writing on them, which blocks much of the reflectivity.

I think most manufactures agree the reflective foil backed fiberglass batts offer little if any real value. Probably why they stopped manufacturing it without a special order? Hold a plate mirror to the sun then try the same with krinkler aluminum foil!

Seems to me a krinkled surface bubble product would offer little reflective value for the same reasons.

Conduction is the best transfer, always. Some experts say conduction transfer is stronger than convection and radiation transfer combined.

That's the strong point of transfer plate application, strong conduction with simple insulation requirements. At least from my testing and observations, this seems true

hot rod

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