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Radiant barrier praise, and roof ventilation question -- again!

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Showing 1-18 of 18 posts in this discussion
Initial post: Aug 5, 2008, 2:09:26 AM PDT
Joe King says:
Many thanks for all of the great information and discussion on the previous threads on these topics, including

I have some feedback from my positive experience so far with radiant barrier in a current project, and a specific question about roof fans for the next step.

I'm having our attic rebuilt (1903 Victorian in Seattle), originally motivated by finding mold when we replaced the soffits for all of the eaves. The insulation had been stuffed in tight along the roof when a previous owner did an amateur build-out of the attic interior, leaving absolutely no ventilation inside the roof. (Oh, and even earlier, depression-era construction used 2x4s for rafters, instead of 2x6 or 2x8s!) The attic was unlivably hot on even overcast days.

Here's what we're doing: The new soffits have covered vent holes spaced all the way around the house, under the eaves. At the top of the roof, there's a ridge vent all along the peak. And then (here's the fun part) we're making the following layers, from the roof inwards.
- shingle
- roofing
- two inches of air space for ventilation, connecting the eaves vents to the ridge vent
- radiant barrier (see below)
- four inches of insulation
- drywall

Our incredibly competent contractor says the radiant barrier has an R value of 15. I don't know what that means, really, except in the earlier discussions here there was skepticism that a thin barrier would have that much heat-stopping power.

In practice, even the contractor was surprised at how effective it is already. With the radiant barrier installed half-way up the south roof face, on a sunny day, I could put my hand in that two-inch ventilation space between the roofing and the barrier. Toasty. Then move my hand out, just to the other side of the radiant barrier. Cool.

OK--it's probably not actually the difference between a toaster oven and an iced glass of lemonade. But it was a bizarre, weird experience, like doing something impossible. I have no mental model from anywhere before in my life for something that hot and something else so cool being that physicaly close to each other. I had to move my hand back and forth again and again to really believe it!

Now for the question. To aid even further in ventilation, we want to add a small roof fan. Because we're finishing the attic, most of the roof will have just the two inches of air. But at the peak, the flat interior ceiling will leave a small (less than two feet) triangular open-air space. That's where I'd like to exhaust hot air.

Most "roof fans" or "attic fans" seem to be big hulking, industrial, poke-through-your-flat-roof style, like:
Standard Aluminum Dome Roof Mount Attic Vent

I'd like instead to to put a smaller fan on one or both ends of the roof, out through the wall below the peak rather than through the roof incline. So there will be a long, triangular prism of hot air at the peak, with a fan on either end, as well as the ridge vent at the top, sucking cool air up from the soffits.

So here's my questions:
- Am I going overboard having both the ridge vent and also the fans?
- Will two fans be better than one? I only want to make sure that the air keeps moving.
- Recommendations for a *smaller* vertical-mount fan for this use? Maybe 8" or less in diameter? I'd like one that's reliable and quiet, and it's worth spending more on the "Cadillac" of fans while everything is open and easy to install. Can I just use any bathroom fan? How are they different? Heck, could I just get a "whisper-quiet" PC fan, 3" or 5", with a power transformer??

So far on Amazon, I've seen some attic fans like these:
But there isn't much information on some of them, even to tell whether they're appropriate for our use.

What do you recommend? Thanks in advance.

In reply to an earlier post on Aug 5, 2008, 9:24:49 AM PDT
Moisture control in attic spaces can be a little tricky, since you are generally dealing with seasonal variations in temperature and humidity, both indoors and out. The passive attic ventilation (soffit plus ridge) should be adequate for prevention of moisture accumulation in the attic. Fans will help, but my experience with gable mounted fans is that they are always quite noisy, plus they will pull most of the air from immediately next to them, rather than evenly along your house. One solution might be to put a gable-end fan at one end only, then providing an inlet on the opposite gable end, thus providing a "sweep" across your attic. This option would make sense if you *didn't* have a ridge vent. Overall, my opinion is that you have already addressed the moisture accumulation problem by providing properly installed soffit and ridge vents. Any fan big enough to provide significant ventilation will be noisy, and compete with your ridge vents.

As for the question about how the radiant barrier provides an R-value, that's a little complicated. Heat is transferred across the space between the roof and ceiling by three methods: radiation, convection, and conduction. Generally speaking, the radiant barrier does the most good in summer, by preventing radiation from the underside of the roof to the top of your ceiling. Bats or blown-in insulation is primarily to keep heat in the house during the winter. The combination provides year round performance. It helps to understand that fiberglass insulation is somewhat transparent to infrared radiation, so heat can "pass through" the insulation to warm the ceiling. Of course the hot attic air is also in contact with the ceiling, but that air is also stagnant, and so convective transport is limited to buoyancy effects. I generalize here only to provide a more brief response, by explaining the dominant effects involved. So, the radiant barrier provides a means of heat transfer control, which can be expressed in R-value, although the use of "R15" is true only under certain conditions, i.e. specific roof temp, air gap, distance to next surface, etc. You might be interested to know that installation of a radiant barrier prevents heat transfer from the roof deck to the attic, so necessarily raises the temperature of the roof by a few degrees. The State of Florida has studied the long term effects of radiant barriers and concluded that they have no measureable effect on roof life, while providing significant energy savings and improved comfort.

hope this helps,

In reply to an earlier post on Aug 5, 2008, 8:21:48 PM PDT
D. Neal says:
I gotta say, a gable-end fan in Seattle is ludicrous. You would be sucking in as much humidity as you would put out. Soffit vents would be drier.

In reply to an earlier post on Aug 6, 2008, 9:55:38 AM PDT
Joe King says:
Thanks, Bill (Joseph?), for the additional radiant barrier info.

You point out that the moisture issue is controlled already with the soffit to ridge vent air flow, and I agree. I was asking about the fan for heat reduction.

It sounds convincing, as you say, that the fan would just pull air from nearby, not sweep the whole ventilation space. But isn't that always true with any attic fan? Is it more true for a gable fan than a rooftop fan? Or is it just that I've already got the ridge vents, so its easy to just pull local air out without sweeping much further?

I was also concerned about the noise. That was why I was researching fans, looking for a small, whisper-quiet one. I guess "quiet" is at odds with "moves lots of air." I was hoping for some NASA fan blade technology, like they use in those huge, new-design wind turbines, on a small scale: quiet and powerful at once.

In reply to an earlier post on Aug 6, 2008, 6:10:18 PM PDT
J. Floyd says:
I wouldn't add a a fan. The fan would likely just pull air in through the ridge vents than it would from the soffits. I was thinking of something similar when I decided I wanted to install a solar powered fan in my attic space. I wanted to add it along with my existing attic fan and put the old one on a sun sensor so it would only come on at night. The manufactures stated that they would pull air through each other so I took out the old and installed the solar.

In reply to an earlier post on Aug 9, 2008, 10:39:39 AM PDT
I'd agree that any attic fan will sweep the area closest to it, then continue to pull from farther away. My point was really about your proposed combination of gable fans plus ridge vent, which would be the least effective way to use a gable fan. Not entirely ineffective, but greatly reduced in effectivity, plus you would have a fairly high noise source coupled directly to your house framing - acting as a conductor for the noise. Pretty well describes both times I installed attic fans...Once I got the ridge vent installed, I took out my attic fans. As for quietly moving large volumes of air, I agree that is an oxymoron, you can have one or the other. (Technically, the problem with moving air is that it doesn't couple well with the prime mover, i.e. compare to water, wherein you can achieve 90+% efficiency in converting motor power to water moved, whereas fans are working down around 10-20% efficiency with the balance of energy going into heat). Side note on fans, my experience is that they burn out in a few years, the combination of heat and dust, along with running at high load for many hours per day seems to limit their calendar and useful life. In summary, they have their applications, but I think they are a poor choice for your situation.

In reply to an earlier post on Aug 12, 2008, 10:22:44 PM PDT
Last edited by the author on Aug 12, 2008, 10:27:51 PM PDT
wahidstx says:
I've added an attic fan just a couple of weeks ago. I installed it in the existing triangular gable vent area. It is a 1670 cfm fan I bought from Lowes. I also installed some soffit vents by 2 patio areas. A thermostat set at 100 degrees turns it on. I think I can already tell that the use of this fan cut down on our AC running time. It is sucking the hot air out of the attic and in turn letting less heat transfer to the living areas.

I've heard from another person at Lowes to install 2 fans where 1 will pull fresh air in and the other will push hot air out creating an even exchange.

In reply to an earlier post on Aug 12, 2008, 11:57:49 PM PDT
Anthony Ward says:
Whatever you do, Don't!
In the 1980's studies in Florida proved line powered attic fans consumed more energy than they saved in a/c power. Line powered attic fans still appear to be the bane of fire departments nationwide. They are notoriously built to be cheap and their out-of-sight-out-of-mind location leads to no maintenance and many failures sucking up power due to a too frequent stalled rotor condition inducing attic fires when the thermal protection device(if present) fails. Many unexplained high electric bill complaints have been found lurking in the attic up there, humming away in a stalled rotor condition. From a green building products standpoint we should be tossing(not recycling) our attic fans and electric toothbrushed. Passive cooling is the way to go. Even solar powered fans in summer arn't very effective as they shut off before the hottest period in the attic occurs, which is typically after the sun has set. Your reflective radiant barrier on the bottomside of the rafters along with effective soffet and ridge venting is the most efficient thing you can do. An attic fan will only mess things up, although a Whole-house fan ventilation system exhausting through the attic would be just fine. Here in Socal we have a 1910 airplane bungalo craftsman with your aforementioned ridge-soffet-reflective radiant barrier system along with a 10,000cfm WH fan ventilation system and don't require airconditioning. The quiet slow turning industrial sized 42 inch fan draws directly upon the interior of the house and exhausts its air through the attic vents. Even during heat storms we night purge and thermally coast all the next day with the fan off and the house closed up until the next evening. $.06/hr vs $.73/hr ($.19/kWh local summer utility costs, 2000sq ft solar exposed house).

In reply to an earlier post on Aug 15, 2008, 4:16:30 PM PDT
Amazon fan says:
Which radiant barrier material did you choose? I'm thinking about adding one to my attic, which currently has a soffit & ridge vent combination. I agree based on extensive web research that a fan is not effective in a system with ridge vent. With ridge vent, wind powers the ventilation--and wind is free.

In reply to an earlier post on Aug 16, 2008, 2:13:24 PM PDT
Joe King says:
I'll check about the radiant barrier brand and type. If there are no rolls remaining, I'll have to ask the contractor on Monday.

But I don't think it was anything special: just R15 radiant heat barrier.

We ended up only putting the radiant heat barrier inside the south-facing roof. On the north-facing roof, we added two more inches of insulation instead--to keep in heat in the winter, vs. reflect radiant energy in the summer.

In reply to an earlier post on Aug 17, 2008, 6:48:26 AM PDT
Regarding question about "which kind of radiant barrier" to install.......any barrier installed in the attic should be perforated, so that moisture cannot be trapped. I have never heard of any attic installation that would be appropriate for non-perforated RB. This would eliminate the bubble-wrap stuff and any barrier that lacks perforation. I see a couple of sources here on Amazon for perforated barrier, they apparently come in different strengths, i.e. amount of reinforcement internal to the foil sheet. Personally, I would go with the strongest, as it is only slightly more expensive.

In reply to an earlier post on Aug 17, 2008, 10:01:19 AM PDT
Go to "" and look up your question. You'll find tons of information available to you on this and many other subjects.

In reply to an earlier post on Aug 24, 2008, 11:34:15 AM PDT
lodefinition says:
Tony is correct. Convected cooling good; self regulating (works when hot [when you want/need it], no moving parts, silent, as reliable as "gravity"). Whole house fan exhausting thru attic also good (ie, "pressurizing" attic). Attic fans generally do not pay... no matter how many, or whatever their configuration (ie, push/pull). So, cool entire house during the night; shut windows during the day, coast on stored "coolness" as long as possible, then start the daily cycle again. In this case thermal mass helps, whereas I am otherwise a proponent of low thermal mass, 'coupled' with the application of best thermal insulation you can muster.

In reply to an earlier post on Aug 24, 2008, 2:55:40 PM PDT
Joe King says:
OK, I checked the radiant barrier material: It's just plain Home Depot R15 radiant barrier, the kind with bubble wrap sandwiched between reflective silver mylar.

In response to Joseph Studak: We didn't use a perforated radiant barrier. The insulation that goes inside the radiant heat barrier includes a layer that is a vapor barrier--so it wouldnt make any difference.

Thanks all, again, for all the great feedback and discussion on this topic!

In reply to an earlier post on Aug 27, 2008, 9:57:19 AM PDT
Last edited by the author on Aug 27, 2008, 9:58:00 AM PDT
Adam Beazley says:
I agree with Joseph, a perforated radiant barrier ( is suitable for all locations and all installation locations. A vapor barrier radiant barrier installation location is highly dependent on the local climate and humidity. If a vapor barrier is installed in the wrong place, you will see condensation problems which can lead to structure rot and mold.

For instance in norther states, vapor barriers must be placed on the warm side of the insulation. In southern states the vapor barrier must be on the exterior of the insulation or not be used at all. This page has a helpful map which shows when and where a vapor barrier should be installed:

Joseph King, you may want to keep a very close eye on that bubble wrap radiant barrier and make sure condensation is not forming on the back side.

In reply to an earlier post on Feb 4, 2009, 7:51:42 AM PST
Five Angels says:
Joseph, the non-perforated bubble can be used in both the applications made to the lower portion of the roof joists or as a vapor barrier application, installed just above the sheet-rock strapping for a cathedral ceiling.

Posted on Oct 28, 2012, 7:36:43 PM PDT
bertio2000 says:
I live in an old Mill House that has four gable vents and no soffit vents and no ridge vent. We have an attic fan installed in one of the gable vents, however after reading the comments I think I will remove it. It gets very hot in there, and on very hot days my house is a hot box.... we open the attic door and all our windows and the fan does help to suck out a lot of the heat. I was wondering if I should install a radiant barrier inside the attic. If I did this, the trapped heat would have nowhere to vent out, so I don' t know if this would help or not, I would greatly appreciate any suggestions.

In reply to an earlier post on Oct 30, 2012, 6:37:41 PM PDT
spike says:
It sounds like your project is done, but in future projects, you migh want to check a spray applied barrier like Lo/Mit-II MAX ( Almost as effective, no electrical problems, and, generally less expensive.
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Discussion in:  Home Improvement forum
Participants:  13
Total posts:  18
Initial post:  Aug 5, 2008
Latest post:  Oct 30, 2012

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