Reflectix/bubble wrap for staple up joist insulation

baltik

My radiant installer swears by reflectix for insulation. We are in a very mild climate but temps do fall into upper 30s in the winter and I'd love to be as efficient as possible. A vented and unfinished crawl space is below the joists

So I've done my reading here and am fully aware that Reflectix is only rated to R1 but I have also read that if you do factor in the air pocket in a joist sealing scenario that the performance can be much better

Their literature claims r19 as a single layer and r21 as a double layer for radiant floor joist heating
https://www.reflectixinc.com/wp-content/uploads/Click-Here-to-Download-DIY-Rad-Fl-Wood-Joists-Installation-Instructions-Rev-0417.pdf

would love to hear some real-world feedback here. My inclination is to force him to stuff r19 fiber insulation but and eat the extra cost, but again would love some real world feedback

Larry Weingarten

Hi, Here's a link: https://www.greenbuildingadvisor.com/article/stay-away-from-foil-faced-bubble-wrap You get the idea...

Yours, Larry

baltik

Larry Weingarten said:

Hi, Here's a link: https://www.greenbuildingadvisor.com/article/stay-away-from-foil-faced-bubble-wrap You get the idea...

Yours, Larry

Thanks Larry, I read the article before and it accurately concludes that it is terrible slab and duct insulation material and needs air space to work, which in the specific case of joists, it theoretically would?

kcopp

Junk, snake oil IMO.
General contractor tried that on one of my staple up jobs years a go and it did not work.
The basement was warm though!
Stick w/ tried and true. I would use rock wool.

BlueGreen

Two layers with an air gap between them works well from my own experience in my home. It's easy enough to DIY without the mess of fiberglass bats, foam, fitting rigid foam, or spray in fiberglass/cellulose.

Paul Pollets

R21 batts are the most effective. R38 in crawlspace applications. Stay away from snake oil.

hot_rod

Good info at healthyheating.com also

Zman

BlueGreen said:

Two layers with an air gap between them works well from my own experience in my home. It's easy enough to DIY without the mess of fiberglass bats, foam, fitting rigid foam, or spray in fiberglass/cellulose.

Please define "works well". How do you know it works better than no insulation at all?

Real insulation is rated using scientific testing procedures. These reflective product salespeople are just making up numbers.

If I called in an insulation inspection in my area and the inspector saw reflective insulation, he would fail me and make me tear it all out. If I showed him the brochure from the manufacturer stating "effective R-values" he would laugh and ask which ASTM tests they used? A real insulation report looks like this http://www.spf.basf.com/DOCS/TDS_Walltite_US.pdf, what are they using?

People use reflective insulations for 2 reasons: it is easier to install and it costs less.

GroundUp

Anybody who says it serves any more of a purpose than regular poly, has never done any sort of comparison to real insulation. It's a vapor barrier at best- from an insulation perspective, it's useless.