Please School Me On Reflective Insulation And Air Space Requirements

[turbocruiser]

Fine folks, I need some help understanding something. I was thinking of insulating some walls with reflectix reflective insulation and saw something that made me wonder if my method would work well or not. I wanted to staple between studs and up against the inside of our plywood sheathing of our house the reflectix reflective insulation and then put the standard fiberglass batting between the studs and on top of the reflectix reflective stuff. I'm not understanding whether that works with the definition detailed below about air space requirements. Would a fiberglass batt qualify as an air space in itself or not? If not it seems somewhat unrealistic to maintain a physical separation btwn the fiberglass and the reflectix; even furring strips would allow some settling pushing the fiberglass up against the reflectix. I'm just not getting it on my own so any advice would be wonderful. Thanks.


This is from the reflectix site:


Why are Air Spaces Required (in every application)?
For either a reflective insulation or a radiant barrier, an air space of a minimum thickness is required on the reflective (shiny) side of the product. The reflective insulation benefit is derived from the interaction of the highly reflective surface with the air space. If the reflective surface is in contact with another building material, it becomes a conductor (transmitting the energy by conduction). An air space may be specified on one or both sides of the product (always on a reflective side). Enclosed air spaces, when instructed, are required to provide the stated R-value.

 

[archie]

most air spaces are required for circulation of air. This circulation prevents mold/mildew/wood rot and any other problems related to condensation (think cold beer on hot day). Placing insuation directly onto anything that could condense water would cause condensation(lack of air flow). I think this could be very minimal because there probably would be little temperature differences at the condensation point.
You could try it out and easily pull it out if it's starts any problems. As long as you don't drywall over it.

They do sell foam channels that fits between studs that gives air space for circulation. They also sell metal spring straps that also hold up fiberglass insulation to allow for air space. That's my opinion hope that helps!

 

[EricG]

That stuff is a radiant barrier - it's only going to help with heat moving by radiation, rather then conduction or convection.As the manufacturer says, unless you have an air gap, the heat will be moving by conduction. But by creating an air gap you're going to increase your convection - probably a net loss overall. This stuff does OK in attics reflecting heat away to reduce AC load, but I don't think you'll see any benefit here. I read an EPA study looking at this stuff a few months back - in all but the hottest climates, even that didn't pay for itself very quickly. And dust settling on it quickly reduced what effectiveness there was.

Radiant Barrier Fact Sheet

If you want to try anyway, I have part of a roll leftover from a project that I'll make you a deal on. I put it under my radiant floor piping to reflect heat back toward the floor, instead of heating my first floor ceiling.

 

[turbocruiser]

That stuff is a radiant barrier - it's only going to help with heat moving by radiation, rather then conduction or convection.As the manufacturer says, unless you have an air gap, the heat will be moving by conduction. But by creating an air gap you're going to increase your convection - probably a net loss overall. This stuff does OK in attics reflecting heat away to reduce AC load, but I don't think you'll see any benefit here. I read an EPA study looking at this stuff a few months back - in all but the hottest climates, even that didn't pay for itself very quickly. And dust settling on it quickly reduced what effectiveness there was.

Radiant Barrier Fact Sheet

If you want to try anyway, I have part of a roll leftover from a project that I'll make you a deal on. I put it under my radiant floor piping to reflect heat back toward the floor, instead of heating my first floor ceiling.

Thanks Eric, exactly what I wanted to know!

 

[adambeazley]

Where you install a radiant barrier depends on where you live. If you are in a cold region, your best bet is to install the radiant barrier over the insulation in your attic. However, it sounds like you want to put radiant barrier on your walls, so you need a 1 inch gap on either side of the radiant barrier foil so the foil can emit the radiant heat it has absorbed. It doesn't matter what side the gap is on and in your case you might have an easier time putting the batting in first and then stapling the radiant barrier to the inside of the studs, about 1 inch back from the drywall (my recommendation).
Another way would be to staple the RB about 1 inch from the sheathing and then putting your batting in. Now the RB will stop radiant heat, but because you have studs, you will still have thermal bridging occurring, unless you have some sort of thermal break between the sheathing and the exterior cladding. Also, if you are worried about condensation, just get a perforated radiant barrier and that problem will be solved.

In response to the posts above about the info on the "ornl.gov" website, if you really analyze the data, it basically states that the effect of RB's diminish quite a bit when insulation over R19 is used and is really not needed when an R39 is used. This would be true for walls, but not necessarily attics where most home have their cooling and heating ducts. If you read carefully, these test were only measuring the heat transfer through the ceiling and not taking into consideration the duct work in the attic.

Anyway, Radiant barriers are great and are worth the money considering your are looking at about $200 to $300. Down here in Louisiana, ours will end up paying itself off in just under a year.

Hope this helps.

Adam

PS: I'm a Green architect, so I'm not pulling this stuff out of my arse.

 

[EricG]

TurboCruiser is in Colorado, where most homes have crawlspaces or basements - very little ducting is typically run through the attic, and many homes don't have (or need) AC. I think R30 is required in attics by building code - many people go to R38.

You're correct regarding the ORNL study linked referring only to ceiling installations. Fiberglass insulation is about R3.5/inch vs. open air space is < R1. I don't have data to back it up (I'd like to see some if you have any references), but I think you're going to lose more from that open air space then you will gain from the radiant barrier. At least in predominately cold climates (in terms of energy use).