Radiant Barrier

[tanyalew]

Just started looking into radiant barrier for the attic (I live in Texas!). Has anyone had this installed, does it work and have you noticed a difference?

[SJF]

Where the heat flow is downward as from a hot roof to a room ceiling below (mainly by radiation) they can be pretty helpful. Good for hot summer conditions. In the winter, when heat flow is upward (mainly by convection) they are questionable.

I once had good results laying kitchen style foil wrap between the rafters in the attic above a bedroom. The bedtime ceiling temperature was very noticeably reduced and the room was much more tolerable without air conditioning.

SJF

[Darrell Dorsey]

I just put some in my attic the other day. I used Radiosity 3000 mixed in 5 gallons of semigloss paint. I needed 10 gallons total. The dealer sold me two buckets of mis-Tint paint for $15 each. The Radiosity was $80 per pail and I needed 2 pails, 1 per 5 gallons. Pretty easy to mix up. Plus I rented a sprayer for $60.

Over all the job was fairly easy, but it was very hot in the attic even though it was cloudy and rained the entire day. Spraying worked very well after I learned how to set the airless sprayer. I've never used one before. It was a very messy job. I was covered from head to toe in paint. If you do the job yourself, wear eye protection and a respirator.

So far we've had some cloudy days in Houston this week. Today is the first sunny day. I'll see this afternoon if it works any better.

I was planning to do the job in the spring or winter when it was cooler. We put it off until last week when I got the eclectic bill and it seemed that the upstairs AC unit would run all evening and night before it would get cool.

Darrell

[stevie]

will installation of a radian barrier harm the shingles (supposedly by making them hotter)?

[Darrell Dorsey]

My understanding is no. As there is nothing to trap the reflected heat. Where as it is trapped in your attic. Similar to a heat trapped in a closed up car.

Here is the product that I used:

The faq:

[trader4]

I just started looking into this myself.

First, I would not use the spray on type product, as it seems to me it would be a hell of a mess to apply. Also, I'd be concerned about it's effectiveness vs the solid foil type product. I'd go with the radiant foil type products that have a sheet of aluminum combined with a sheet of polyethylene plastic in a roll 4ft wide. Seems like that would be easy to apply.

Second, it does raise the roof temp. I saw some DOE numbers that indicated it raised the temp from a few to maybe 8 deg. They said the effect on roof life is unknown, but it doesn't sound too bad to me. Like many things, there is a tradeoff.

Third, I think a lot may depend on how the attic is ventilated. With the foil, you leave about 4 inchs open down at the soffits and 6 inchs open at the peak. That should work really well with a ridge vent. However, if you have other type of venting, like gable vents, then it's obviously not going to work as well. I would think all the hot air being channeled by the foil up to the peak could increase the temps up there quite a bit.

I'm seriously thinking of adding this to my attic. From everything I've read so far, it looks like it can make a significant difference in the amount of heat that reaches the ceiling. And it's relatively cheap. You can get 125' x 4' of the best stuff for $120. The cheaper stuff is about half that. And I think the real diff btwn the two is the better one is a heavier gauge material, so it won't tear when you try to install/staple it. In terms ability to reflect heat, I think the two are the same.

[Darrell Dorsey]

No, with an airless sprayer it was quite easy and very fast. Over spray was minimized, but I did get a good bit of paint on me. Nothing that a good shower didn't fix. Much faster than cutting sheets of foil to fit and stapling to rafters.

Given the year round temps in Houston and the complexity of my attic, it was the best compromise of installation speed vs. effectiveness. I spent no more than 2 hours in the attic. A simple gabled attic might be easier, but I have "hips" on both ends and a dormer on the front, lots of complicated angles, a bunch of vent piping, (two furnaces, a water heater and several plumbing vents).

I wanted to minimize my time in the attic and lower the potential to kick a hole in the ceiling.

This is a guy who's opinion that I trust in the Houston area.

He has two links on this page from shows discussing radiant barriers, plus some other good stuff. He seems to have the opinion that the foil is near 98% effective and the paint is about 75%.

Yes, lots of good info there:

You're talking two different things here. Radiant heat is reflected, not conducted, and not convected. Having good attic ventilation is a part of the total package. You want to be able to vent any heat in the attic and keep it cold in the winter. But you also want to reflect out as much Radiant heat as well. You can have one with out the other. But you really need both to be totally effective.

Given the same conditions, I don't see how a layer of foil will help with convective transfer. Are you saying that you get some sort of chimney effect? I don't have continuous soffet vents, but we do have a lot of area. We also have a ridge vent. Are you saying that the foil will improve the ridge vent performance?

[trader4]

The point is without the foil barrier you will likely have an entirely different air flow pattern. With the foil installed, the air is confined to flow entirely in the rafter space between roof deck and foil, and has to come out in the top six inchs, at the peak. It seems very reasonable to expect that flow to work best with a ridge vent where the air can just exist and not so well with gables, where it now must first go to the peak and then make it across and somewhat down to the gable vents.

For example, take a channel 6 feet away from a gable. Without a foil barrier, some of the hot air, from say a third of the way up the roof could start migrating over to the gable by the shortest, direct line path. With the foil, it has to travel to the peak, where there is no direct exit. From there, it now has to make it to the gable, which typically is maybe 18" down, from the peak, which is in the wrong direction.

I don;t know if it will improve the ridge vent performance, though it might. But I do think it will degrade the gable performance because it's harder for the air to make it to the gables. But I guess that assumes there is the same amount of heat to deal with to begin with. But because the foil is directing some of the heat outward to beging with, there is less heat to then remove from the attic.

[Joseph Meehan]

In the right part of the world, and I believe most of Texas would qualify, it can be remarkably effective at a small cost. However I strongly suggest that you assure that you have venting between the foil and the roof keeping an air flow thought there to carry out the heat that would be trapped in there without it. That heat can damage roof parts and will tend to defeat the value of the barrier.