Home Radiant Barriers--Good idea or bad?

[tsunamisurfer]

I recently clicked onto a web site to find some information about home "radiant barriers," specifically, a high-tech, dual-sided bubble wrap that promises big savings on AC and heating bills.

Another product offered is a Harry Homeowner kit in which you blow what looks like scraps of thick foil onto the fiberglas batting in your attic, which then rejects the solar load from above (the roof), as well as keeps the heat (or A/C air) from escaping from the living quarters below. Using what looks like a fiberglass blower, you just blow several inches of the stuff onto your existing insulation.
The attached scientific literature says this technology is tremendously effective in reducing a/c and heating bills.

Anyone have any experience with this stuff? Any downsides?

[tsunamisurfer]

Correction: the bubble wrap is coated on both sides with a silverized reflective film.

[ZenBeam]

Bump. I'd be interested if anyone knows anything about this, too.

[Engineer Don]

I know this stuff! Really. The answer depends on the geometry of your attic and the level of improvement depends largely on the rest of your home. If you are gaining lots of heat through the roof then it might help. If you have a newer house, and have R19 batting or blown insulation in your attic, then you would be unlikely to notice a difference. Scraps of foils blown loosely on top of batting would be bad, and would be unlikely to do anything constructive due to a phenomonon called cavity absorbtivity.

Heres the deal: A bunch of heat in the form of sunlight is absorbed by your roof. The roof gets hot and some of this heat is transfered by the roof radiating heat in the infrared spectrum to the top of the insulation above your ceiling. All the radiant barrier does is to decrease the emissivity of the attic side of your roof, or in the case of something you would place on top of your ceiling insulation, it would decrease the amount of IR radiation absorbed. The question is how much is the heat gain being decreased? Lots of the roofs heat is probably being transfered convectively, by air in the attic getting hot heating the ceiling. Heat moves conductively through the rafters as well, but probably not a whole lot. If you don't have many rafters, and the attic is well ventilated either through sofit vents or with a powered attic fan, then the radiant barrier might be somewhat effective.

Do a test. On a day when it is hot outside, in the late afternoon touch the ceiling inside and see if it is hot. If it is then you are getting some significant heat gain through it. If it isn't, then a radiant barrier probably won't do much.

Personally, I think radiant barriers are overrated. Once they get dusty, which anything in an attic will, then they will lose their effectiveness. Get a big tree to block the sun from hitting the roof and the rest of the house. If you get one that is leafy in the summer but bare in the winter than you won't increase your heating bill.

If I was spending the money I would go with a powered attic ventilator and/or additional insulation first. That and sealing / caulking / weatherstripping the windows, doors, attic access hole, switchplates, ceiling light fixtures and smoke detectors. Also change or clean your filters regularly, and check out your ducts and make sure they aren't leaking. If they are don't fix them with duct tape! Use real serious foil tape for ducts and put it on right. Then check out your return-air plenum and seal any joints in it that lead to the attic, exterior or an interior wall space. Builders don't do this because the A/C guy says it's the framers job, and the framer says it's the A/C guys job. Sometimes plenums have huge holes dirrectly leading into the attic or outside, and they pull hot dirty air in from there which sends A/C bills throught the roof. It is insane, but it even happens in new houses. That and people cut holes to run wires or cable, and then don't seal them up right.

Also remember than any air that leaves the house will be replaced by outside air. The bathroom fans should be used to get rid of humid air during and after a shower, but if someone is stinky then light a candle for a little while. Also don't leave regular fans on when they aren't blowing on anyone. They add heat to the space and make it warmer. I have heard of people trying to use ceiling fans in a closed up house, and not understanding why they get warmer.

[tsunamisurfer]

Hey, Engineer Don, you know your stuff!

Now: Another related question. What about my wrapping that reflective bubble-wrap insulation around the HVAC ducts in the unfinished part of my basement? My thinking (or, at least, WISH) is that by wrapping these ducts with the hi-tech barrier wrap ("r" rating is about 9), less heated air in winter will be lost to an unused portion of the basement and will instead be sent to those portions of the house I actually use.

Don't want to use fiberglas duct wrap. Don't like to work with the stuff and I'm convinced it's bad for your lungs--even with a (leaky) mask.

A last question: my new hot water heater has factory-installed foam insulation with a 16.7 "r" rating--does it make sense to put a wrap around it anyway? Thanks!

[tsunamisurfer]

BUMP...

Anyone know about radiant barriers and insulating HVAC ducts?

Also this: foam pipe wrap has an "r" value of about 2. Good grief, isn't that next to nothing?