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Community and Q&A

Radiant Barrier vs. Attic Exhaust Fans

jplee3 | Posted in General Questions on

Hi all,

I keep getting conflicting information and opinions about this (as well as whole house fans).

Between HVAC contractors and attic insulation contractors, some seem to swear by radiant barriers and slam attic fans as useless whereas others don’t think radiant barriers work very well at all but strongly advise using attic fans. Not sure what to make of it.

I figure with attic fans, the placement matters and makes a difference. In our case, we are west-facing and very slightly south-oriented. I think the prevailing winds are from the west here (I’m located in South Orange County, Southern California). The attic/gable vents for our house though (and there are three of them) are oriented more north-south so I don’t think you get too much of a natural breeze or airflow going through. In this case, it seems like an attic fan would help – I’m just not sure whether to position it on the north-side/north-facing vent or on one of the south-side/south-facing vents (see attachement)

As far as radiant barriers, I just haven’t seen or heard of this being a super popular thing down here. But maybe I just don’t know because it’s not like I’m going into my neighbors’ attics 🙂

As a sidenote: that’s a Spanish clay tile roof and we will be having solar installed on the frontside in a couple weeks. Insulation in the attic is R19 currently so I’m also considering replacing or supplementing (as well as rodent proofing). There were signs of rodents up there (droppings, etc) but I don’t think there has been any current activity in a while (trees surrounding the home have been removed since we moved in and the droppings appear to be older).

Another thing to note: Some HVAC contractors I’ve spoken with swear by things like whole house fans where others (like my current contractor) strongly advise against and say they can cause negative pressurization as well as expose an opening in the insulation (where the fan intake is) and allowing conditioned air to escape.


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  1. user-2310254 | | #1

    Attic fans are usually a bad idea. As Martin Holliday notes: "In many homes, powered attic ventilators pull conditioned air out of the home and into the attic through ceiling cracks. The net result: powered attic ventilators increase rather than decrease cooling costs."

    The full article is available at

    It's not a clear cut on radiant barrier. They can work when combined with an appropriate air gap, but the use case has to be appropriate. Allison Bailes provides his guidance here:

  2. jplee3 | | #2

    Thanks! I totally forgot to mention attic sealing but if we were to have the attic flooring/joists/etc sealed properly, would that mitigate the concerns with attic/gable exhaust fans pulling conditioned air out of the home?

  3. user-2310254 | | #3


    There are lots of pathways for air. So it's hard to know. (But air sealing your home is a good thing FWIW.)

    The bigger issue is that the fans probably won't make your living space feel markedly cooler. It's also possible that you won't reduce energy costs enough to offset the expensive installing and operating the fans.

    Keep in mind that the folks who sell attic fans and radiant barriers are probably NOT going to give you any reasons to buy there products.

  4. jplee3 | | #4

    I was planning to buy one of those QuietCool fans ($100-150) and have my HVAC contractor or electrician just install it ($200-300). Seems like a relatively small expense but I thought the intent was to reduce the temp in the attic so that your HVAC ductwork would have less strain on it (And yes, the ducts are in our attic - I've had this discussion/debate here in the past and there's just no way that we're going to change that and build plenums on the interior of our house, etc. Yes, I agree that having ductwork inside the conditioned space is more efficient. No, it's just not viable *for us* to go through all the work of re-ducting our home to make that happen haha.)

    Anyway, it sounds like the best things we could do now would be A) air seal the attic and B) either supplement the R19 up there with more R19 OR change it all out to R30 or R38. I've still been deliberating over whether to have batts laid up there or to deal with blown-in cellulose.

  5. DC_Contrarian_ | | #5

    The correct answer is neither.

    The point of attic ventilation is to protect your roof from moisture, not to cool the attic. Attic fans make the problem worse by sucking air from inside the house, which is the source of the moisture you're trying to get rid of. Radiant barriers don't really work.

    More insulation is what you want. Along with air sealing of the living space, and adequate vents so that natural ventilation brings in enough outside air to get rid o moisture.

    Is it just your ductwork that is in the attic, or the whole HVAC system? If it's just the ductwork, can you bury it in batts of insulation?

    You don't have to remove the existing insulation to add more. However, if you want to air seal the ceiling that usually requires moving the existing insulation.

    1. kurtgranroth | | #7

      I disagree with the statement "[r]adiant barriers don't really work." They absolutely do work and even do so effectively in certain specific cases!

      Anecdotal "evidence": I'm building a small house and currently there are no ceilings or attic insulation but I did use roof sheathing with a foil radiant barrier on the bottom. The in-progress house is consistently a good 5-10 degrees cooler than ambient! The sunnier the day, the bigger the temperature gap. I can also directly compare the temperature to my workshop, which is right next door and is similarly uninsulated but with no radiant barrier. In that case, the internal temperature is typically 5-10 degrees (or more) hotter than ambient. So yeah, I'm a firm believer in how well radiant barriers can work.

      But I do recognize the limitations. This is in Phoenix AZ, first of all, where radiant energy from the Sun dominates the energy conversation. Second, this is in an uninsulated house -- eventually I'm going to blow R-60 into the attic and am not going to put any HVAC components up there. I'm expecting to see zero benefit from the roof radiant barrier inside of the house at that point. Finally, it didn't cost me any more to use the foil backed panels vs plain panels so I get the temporary benefit "for free", so why not enjoy the temperature drop during construction.

      So yeah, in a Sun-dominated climate, radiant barriers will absolutely drop the temperature when its sunny out vs no barrier. What happens with that temperature drop may or may not be actually useful.

      1. Expert Member
        KYLE WINSTON BENTLEY | | #10

        Agreed, Radiant barriers do work, especially in southern climates. The economics is what's debatable.

  6. jplee3 | | #6

    Thanks. What R value would you recommend? R38?

    We only have those gable vents AFAIK. I think there are a few soffit vents too but mostly int he back of the house if anything.

    It's only the ductwork that's in the attic. The furnace/handler are in the garage (unconditioned but partly insulated) and the condenser is outside (sitting in an alleyway that's shaded most of the day).

    Yea, I was thinking we would get the attic cleaned, rodent proofed, sealed and then put the insulation in. If they're going to clean/rodent-proof/seal it I'm wondering if new insulation would be the best thing to do. There's definitely old rodent droppings and surely urine up there and probably on the existing insulation, so not sure if it would make sense to discard all of it. One contractor was saying all he would suggest doing is doing an attic cleaning, sealing and rodent proofing. I guess he would remove the existing insulation, clean it off, clean the attic, and then put it all back in after? At that point it seems like getting new insulation makes sense.

    My HVAC contractor was saying he hates cellulose and has a couple clients that had it blown in and it was getting all over the place inside their home. I'm guessing this is because they didn't have everything adequately sealed up in their attic before they blew the insulation in. Seems there's no issues with cellulose as long as you have everything thoroughly sealed up there.

  7. exeric | | #8

    Cellulose is excellent in almost every respect. It's cheap, easy to install in an attic, doesn't allow air infiltration like fiberglass does, and it sequesters carbon so it's good for the planet. The only problem are individuals who install it who are not educated. If he was at all competent, your HVAC contractor would know that he needs to seal the attic floor before installing it. It really pisses me off to hear what he's saying.

    I would run away, not walk, from any HVAC contractor who is that ignorant.

  8. Deleted | | #9


  9. jplee3 | | #11

    Too late now. I've already signed the contract with him a while ago. In any case, I'm going to have another company deal with the insulation and air sealing regardless. That said, should I have the insulation/air sealing done *before* replacing the ductwork and HVAC system? Or should the insulation/air sealing be done after?

    EDIT: I just looked at a Yelp correspondence and one insulation guy suggested that he come in *after* the ducts are replaced to do the cleaning/sealing/blown-in cellulose because there will be a mess made after the ducts are replaced.

  10. exeric | | #12

    Just my intuition for the order here -
    1. Install ducts.
    2. Seal ducts to themselves, seal ducts to ceiling registers and seal registers to the actual ceiling (no holes in ceiling).
    3. Try to have HVAC guys do a duct blaster test on ducts. Also a blower door test done now on the house should show up any missed holes in the ceiling. That's provided there aren't huge leaks elsewhere.
    4. Clean up mess made from 1,2, and 3.
    5. Blow in cellulose making sure ducts are well buried beneath the cellulose. Where you can't bury them make sure they are using R-8 duct insulation. Maybe double insulate those sections with fG bats wrapped on top.

    Just my 2cents

  11. jplee3 | | #13

    Thanks. I think they do the duct blaster testing (as part of HERS testing?) but not the blower door. Blower door testing is like $395 or something - seems like that might be a good idea and presumably they list out all the areas where there are holes/intrusions in the ceiling so that whoever comes in to seal will cover those spots for sure? Or is it better to do a blower door test *after* air sealing is done to check (and then ask the insulation company to come back out to fill any gaps they missed?)

  12. jplee3 | | #14

    The other thing I'm kind of wondering about is the Inflation Reduction Act and credits. From what it looks like, there's a $1600 credit on insulation but it has to be done in 2023. We were planning to push our HVAC system/ductwork install to 2023 for the other credits anyway, so maybe we'll just have the insulation stuff taken care of then.

    Looks like they also are crediting up to $4k on electrical panel installs - we just had ours replaced. Unfortunately, if the credits/rebates are effective starting 12/31/2022, I won't be able to capitalize on that :(

  13. exeric | | #15

    The problem would be if there are too many leaks to fix while the blower door guys are on site. The safest method would allow for that contingency and then have them come back at a later time for a second check after you've fixed the leaks you've previously identified. You might get lucky and have such few leaks you can easily fix them without a second blower door test.

  14. Expert Member
    BILL WICHERS | | #16

    Cellulose is dusty when blown in. Fiberglass is dusty when blown in. I haven't tried loose fill (blown) mineral wool, but I'm sure it's dusty when it's blown in too. Batts are dusty when installed. Insulation installation is messy. Just plan ahead, and seal off the space first so that you don't "leak" insulation into the living spaces. This is no different from renovating a room and dealing with drywall dust. It's just a fact that many construction operations are on the messy side, so just plan ahead and you'll be fine.

    Eric covered a lot of the benefits of cellulose, but there is another one: cellulose is better about infrared heating than fiberglass because IR doesn't penetrate the upper layers of cellulose the way it does with fiberglass. I'd go with blown cellulose if it were my attic. Just make sure to air seal the attic floor FIRST, then install the cellulose.

    I like whole house fans, but you need the right climate for them (cool evenings, usually). They are great for quickly cooling down a house, or bringing in frush air (I love running them after a rain storm, for example). You do have to be careful not to have insulation fall into the fan, but you can build insulation dams with plywood or pieces of rigid foam to deal with that. You do get negative pressure in your house while the fan is running, but that's the whole point of the fan! You just have to remember to open some windows first, otherwise the whole house fan can't really do it's job.There are some good whole house fans that are insulated, one particular company comes to mind EXCEPT, of course, for their name! you should be able to find them though -- they make a fan with a sort of insulated door that opens on top while the fan is running.

    Regarding R value, you need at least the minimum code allows for your climate zone, but the more the merrier. Those of us living up here in the frozen North, which seems to be many/most of the people on GBA, would go for at least R49, and often R60+. That much might not make sense in your CZ, but R49 might -- especially if you're using something relatively cheap like loose fill cellulose. You'll need to make sure you have some kind of insulation dams out at the eaves to prevent soffit vents from getting blocked though.


  15. DC_Contrarian_ | | #17

    Bill -- Are you thinking of Tamarack?

    I also am a big fan of whole house fans, except that I still haven't come up with a good way of sealing them when they're not running.

    I looked into Tamarack fans maybe a decade ago. What I recall at that time was they had some sort of babble on their website about "effective" CFM being higher than their rated CFM. I didn't buy it so I looked no further. But I just looked at their website and the babble is gone, either they have derated them or they no longer try to justify their ratings.

  16. Expert Member
    BILL WICHERS | | #18

    >"Bill -- Are you thinking of Tamarack?"

    YES! That's the one. They make well-insulated whole house fans. I agree they are light on CFM though, but that's probably because they use muffin fans instead of the typical large belt (or direct) drive fan most other whole house fans use. That would probably also make the Tamarack fans higher pitched while running, since muffin fans tend to run at higher RPM than the larger fans. Personally, I like the lower pitch sound of the larger fans, especially at night. The downside is it's difficult to put together an insulating system for such a fan as a DIY project, at least if you want it to operate automatically.

    I hadn't heard about a claim of "effective" CFM ratings. I don't like anytime a manufacturer claims their rating isn't really a "rating" and is actually something better than it appears. Too many bad memories of radiant barrier BS maybe...


  17. jplee3 | | #19

    Interesting! The Tamaracks are designed differently than the others I've seen (e.g. QuietCool) where they don't use a duct (with the fan at the end of it) that you need to end up hanging. Seems this just draws the air from the home up into the attic. And it sounds like when the doors this close, it also seals off everything from the attic space effectively so that you're also not pulling any of that air back into the living space at any time?

    Perhaps this is something we'll consider later down the road. With the recent heatwave, it would stay at 80-85F overnight though, so something like this definitely wouldn't have helped hahaha.

  18. exeric | | #20

    I'm in California also. I installed a Quiet Cool WHF and they are good units because they are quiet because the fans themselves are at the end of the insulated duct and not right at ceiling level. My original plan was to use it as a replacement for air conditioning. Well, that didn't work out so well as things have gotten hotter. Now I use it as a backup if the heat pump goes out.

    I ended up putting up 5Kw of solar panels and installed a heat pump. The solar panels cover the cost of running the heat pump, so I no longer use the WHF. Some climates can justify a WHF instead of a heat pump, but I don't think most of our state can do that anymore. Since you are getting solar panels you might be in the same boat and the WHF ends up being redundant. I can see getting a WHF if you don't have solar though to save on electricity at times when it's not blazing hot.

  19. jplee3 | | #21

    Yea, I think the WHF may not make sense in our case. I was originally just thinking about keeping the attic generally cooler with gable exhaust fans or radiant barriers so that there's less of a strain on the ductwork itself. But it sounds like the better alternative might be just to change the insulation in our attic to cellulose and bury the ductwork in the cellulose as much as possible.

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