Aluminum foil roof?

[The Rooster]

I found this really cool link about radiant barriers that both educated me, and demonstrated theory in a way I could understand it.

The physics of foil: how radiant barrier stops heat gain & loss in buildings.

[rmay635703]

Originally Posted by cfg83

WaxyChicken -
I wouldn't want a tin roof in direct contact with the house. The metal will heat up and convect into the building. The good part would be that it would cool very fast at night and radiate to the "infinitely cold" night sky.

Assuming your roof is a "normal" shingle color, I would beg the wife for permission to paint the roof white with some kind of elastomeric paint (i.e. something like this : Roof Coating Products & Cool Roof Coating | Elastek | Arizona). A white roof is the only way to go in the desert (aesthetics be damned).

CarloSW2

Actually given the sucess of an envelope home keeping heat out I've often wondered if a simple "Envelope Roof" so to say would be almost as good, AKA if you want a reflective roof, also offset it a 2"x2" or more above the existing roof, my guess is more is better, at least for the envelope home effect, probably a foot or more is ideal so the heat can "blow" away and not penetrate the main roof.

And I also agree with having a white roof, I've often wondered if there was anything purpose made that would do better than plain white in terms of keeping the hot outside. I know silver/reflective surfaces get blazing hot compared to white though.

[The Rooster]

Theoretically, the aluminum foil dosen't need to be on the outside of the house. Most of the heat absorbed by your house through the roof from the sun is not through the conductunce of heat from the roof downward into the home, it's the invisible ultraviolet radiation that passes right through the roof into the house.

Using white paint will reflect a lot of the visible light and lower the surface temperature of the roof, and through conductance (or lack thereof), somewhat lower the temp in the attic, but it won't stop any of the infrared light passing right through.

Ever see an infrared camera work? They see right through walls, doors, and can see engine heat through car hoods. Space Telescopes take photographs of stars through dusty, gassy nebulae with ultraviolet cameras. The reason it works is because the energy in that wavelengh, passes through most solids, liquids and gasses with ease.

Aluminum has the property of reflecting roughly 97% of all radiant (ultraviolet in this example) energy, so your idea is very sound, just impractical as it'll be subject to the elements and pissy neighbors.

Three types of heat;

Conductance: This is how heat travels through solids. Your walls, studs, joists and even fiberglass insulation are all conductors of heat. (Fiberglass insulation just happens to be a very, very poor conductor, which is why it works...for conductance.) However, fiberglass is a very poor insulator against radiant heat...which is what you mainly need to insulate against.

Convection: This is how heat travels through a liquid or gas. Heat rises for this reason: As a liquid or gas is heated, it expands and thus becomes "lighter" and moves to the top of the medium (air/water/whatever)

Radiation: This is in the form of invisible light. Every object not at absolute zero radiates heat in this fassion. Hotter objects radiate more heat (Like the sun, a lightbulb, a heater, fireplace, etc)

(Technically radiant energy has no heat, it must be absorbed by an object, then that object gives off the heat...this is how heat gets from the sun, through the frigid vaccuum of space to warm the planet...the space between the sun and earth is cold, but there is a lot of energy in the form of radiation (light of variying wavelenghts) from here to the sun.)

So since what we really need to block from the sun is radiation, though it may be easier, putting aluminum foil on the outside of the house isn't necessary. I propose this idea...if you can somehow find a way to apply it in your tiny attic;

Staple your aluminum, shiny side out, to pieces of cardboard. Do this to both sides, as in the winter time, you'll also retian the radiant heat you create inside the home and reflect that back down into your house.

Then, using 2" nails, hammer your cardboard into the underside of your attics joists, but hammer the nails in only 1", leaving the cardboard/aluminum foil suspended and creating about a 1" air gap. For better performance, find a way to add additional layers that don't touch each other.

It's important that the foil not touch anything but air as much as possible. From what I read, it'll need a minimum of 3/4" of air space to work, and the more, the better. Also the single aluminum panel wont do anything for convection, however, a few layers an inch apart can reduce most of the heat loss through convection.

You can buy products that are already made for this purpose, but cardboard and aluminum foil is way cheaper, if that's the route you want to go.

[Ford Man]

Since I made my original post I've studied this more and also contacted several people who install radiant barrier about the application I am planning to use and they have told me that as long as you have an air space on one side of the foil you'll receive the benefits. Since making the post I have also talked to people that have actually used the barrier and most have told me it lowered their attic temperatures by 20-30* during the summer and lowered their cooling costs by about 20%+. I was also concerned about whether the radiant barrier would help in the winter with heating with it being installed under the metal roof to the exterior of the house and the answer to this has also been yes because even though it's on the exterior it will still block the heat from radiating to the exterior and cold from radiating to the interior. This in itself should be very beneficial since they claim that up to 60% of heat loss is though the roof.

After talking to several people about this I have decided to go ahead with it. I can do the roof, interior of my garage, and the interior walls of all closets in the house that are exposed to an outside wall for under $400. in material (3000 sf).

I used a radiant barrier on the inside stud walls and bottom of the trusses in my garage at the house where I use to live and didn't use any other insulation. The garage was exposed to the sun from sun up till sun down with no shade during any part of the day and even on 95-100* days without the use of a/c it was cooler feeling in the garage than it was outdoors. I never checked the interior temperature prior to or after the installation, but I would guess there was at least a 20* drop after installing the barrier.

[bennelson]

I had a weird experience with radiant heat recently...

It's been cold lately, around 0 degrees F.

My car had been sitting outside all week and was covered with snow. I brushed all the snow off, and then raised the hood to check the battery.

The car was parked facing north, with the hood in the shade. Even though it was cold, it was a very sunny day, and the hood is black.

As soon as I raised the hood, the little bit of snow left there nearly instantly melted! Then, all the water left began to steam off!

What was weird was just how fast it all went. The whole hood was just steaming, and perfectly dry in a few minutes.

Behold! The power of radiation!

The air temperature hadn't changed. The only difference was sun on the hood or not.

I tried taking some video of it, because it looked so cool, but it didn't show up on camera very well.

[Ford Man]

In case anyone else is interested in buying any of the radiant barrier material, here's the best price I was able to find. Several companies offered the same weight material with the price ranging from about $116. per roll including shipping up to about about $135. per roll (1000 sf) + shipping. I requested samples from different companies and all seemed to be the same.

Texas Heat Management — Store Home

[SoobieOut]

I stapled this to my roof rafters about 2 years ago. Purchased at Lowes, forgot the cost. It appear to help on the days where it was really hot, above 95 degrees. At least the a/c unit does not run constanly.

[Danncomm]

I had a house with a lighter color, white and gray mix. It was time to replace the shingles and rather than do that I bought bright white metal sheets. I installed them to 1 X 4's spaced 2 feet apart with the boards running horizontally. The year before I was using 12,000 and an 8,000 btu AC units downstairs and a 8,000 upstairs. I now use an 8,000 downstairs and a 4,800 upstairs and if you set them on high and leave for a while and then come back a few hours later, like my wife did, you can end up seeing your breath while it is 95 degrees outside. Heating bill went down about 20% during the winter too. We have no access to the attic because the way the roof is built so the radiant reflectors wouldn't work, thought doing that a couple of years prior but couldn't find anyone willing to even try to install. The metal roof ended up costing about $3500 less than new shingles. With the one inch separation between the roof and the metal makes even the thunderstorms much quieter too.

[euromodder]

Originally Posted by Danncomm

I had a house with a lighter color, white and gray mix. It was time to replace the shingles and rather than do that I bought bright white metal sheets.

Keep it clean.
Their whiteness helps a lot with reflecting the heat.

It's a weird sight, but more and more people & companies are constructing white roofs.

[Geonerd]

Bare metal, be it aluminum foil, tin, or whatever, do not do well. While they do reflect a lot of heat, they also tend to be very poor radiators. The metal gets hot, and that heat works its way down into the house. Most of the 'cool roof' elastometric coatings are designed to be both highly reflective AND highly emisive. I live in Aridzona, in an old mobile home. The metal roof's original white paint had largely worn away, leaving bare, fairly bright, 'tin' in most places. Painting this with two thin layers of generic cool goop made a vast difference, reducing my summer cooling bill by ~50%. The roof itself is thin, without much insulation, so the 50% is probably a best-case situation. Now, if I can only do something about the poorly insulated walls...

Save the aluminum foil, it's too valuable as an anti-mind control device to waste on the roof! Get decent paint or cough the $ for light colored cool-roof tiles.