Radiant barrier, attic fan, blown cellulose? Which should I use?
Hello everyone!
I’ll start with a bit of context…
我住在2区(佛罗里达东海岸)。我的家was built ~1960 and has a very low pitch roof (I believe 3:12) with ~1,450 sqft living space. The insulation and ductwork in the attic is original and, between critters over the years and regular settling, the insulation is now between R1-R3.
The trunk of my ductwork supply is sheet metal, with much of the insulation worn off (many spots are bare sheet metal). I asked to have that repaired when the AC company replaced my duct work, but I went up there recently and it wasn’t done (it’s not on the invoice so I don’t feel like I can call and complain). The rest of the supply (the ‘branches’ if you will) have been replaced with flex duct. The returns go through the crawl space.
我有一个人字形屋顶和低音调,it makes maneuvering the attic quite a challenge.
It appears there is space between my ceiling and the attic ‘floor’… through most of the attic it appears that there is the sheetrock ceiling, a 2×4 stud/joist, then another layer of sheetrock. Some areas don’t have this.
I put a thermometer in my attic last August-September and during the day it got up to 90F but at night it got down to the 70s… that’s with a high of 95 and a low of 75. So the AC was leaking into the attic and the insulation is awful (energy bills were high to boot).
I was looking at getting insulation blown-in, as well as potentially adding an attic fan (gable mounted) and then I saw some articles online about radiant barriers.
I’ve read Martin’s write-up on radiant barriers and it definitely made me second guess whether I should get it. However, Florida Solar Energy Center seems to indicate that radiant barriers can be cost-effective in Florida. Also, given that my duct work is in my attic, I thought it may be beneficial to use radiant barrier to help my attic be semi-climate controlled.
3 questions:
1. Is a radiant barrier (stapled to my roof joists) cost effective in Florida?
2. Is an attic fan worth it?
3. What’s the recommended way to replace insulation on sheet metal duct?
Thanks in advance for your time and insight
Ben
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Replies
In Florida, with ducts in the attic, a radiant barrier *somewhere* in the roof assembly can be helpful. But there are a lot of caveats:
1. You need to air air-seal the attic floor first. As perfectly as you possibly can.
2. Then you need to air-seal the ductwork. Make sure the air in the ducts stays in the ducts.
3. You need to re-insulate the ducts themselves in an airtight manner, to prevent condensation from forming on them.
4. You definitely want to add more insulation to the floor, ideally burying the ducts after you've done steps 1-3.
5. Radiant barriers stapled to the underside of the rafters work best when there's eave and ridge ventilation, so that the air above the radiant barrier that gets super-hot can escape. Make sure you have good-quality eave and ridge vents. And a solar-powered attic fan can help here too. With a radiant barrier that's inside the attic (stapled under the rafters), the air above it that it superheats has to be evacuated somewhere or else it just circulates back down into the rest of the attic.
但the far superior choice would be to reflect the heat before it even makes it into the house to begin with. This entails installing a bright white metal or ceramic tile roof, with ventilation channels underneath the roofing material. The place for a radiant barrier is under that roofing material--ideally installed so that it's facing down rather than up, if you can get the installers to manage that (if it's installed primarily facing up, dust will settle on it and ruin the performance).
Ben,
Nate is correct that installing a radiant barrier under the rafters in your climate would improve the situation. But there's reason to believe that you'll have a hard time finding a contractor willing to do that work, since your "home ... has a very low pitch roof (I believe 3:12) ... and with the low pitch, it makes maneuvering the attic quite a challenge."
Moreover, even if you could find a contractor willing to do a good job installing the radiant barrier, at the end of the job you would still have an uninsulated attic with ducts outside of your home's conditioned space -- which is very bad.
For information on why you don't want an attic fan or powered attic ventilator, see this article:Fans in the Attic: Do They Help or Do They Hurt?
For information on why it's important that to keep your ducts inside of your home's conditioned space, seeKeeping Ducts Indoors.
The best solution to all of your problems is to convert your unconditioned vented attic into a conditioned vented attic. Since you have a shallow-pitched roof with terrible access, the best way to do this work is to install one or more layers of rigid foam on the exterior side of your roof sheathing, followed by a new layer of roof sheathing and new roofing. These two articles describe the process:
Creating a Conditioned Attic
How to Install Rigid Foam On Top of Roof Sheathing
-- Martin Holladay
Ben,
Another thought: fixing all the problems in your disastrous attic will be expensive. It might be cheaper to abandon your current air conditioner and ductwork, and just install a couple of ductless minisplits in your house. No more ducts = no more duct problems.
Of course, if you do that, you'll still need to do some air sealing work in your attic, and you'll still need to install some cellulose on your attic floor.
-- Martin Holladay
Reply to Martin
Martin,
Thanks for the response(s). I appreciate the article on the attic fan. I guess I never thought about it pulling conditioned air from the house through cracks in the ceiling. I was just about to ask if it was worth doing what you mentioned in the original post since it seemed expensive (and not cost effective). I suppose my best course of action at this point is to repair the insulation on the duct work, then put R30(?) cellulose in my attack. I'm not expecting to live in this house for more than the next 5 years. During the Summer months here (May-September) I average $90 month for cooling costs (I did some math on AC run time, AC power draw, and total Kwh from my utility company). So my hope is that I could spend $600-800 (self install) to lower that by $20-30$ per month, it would pay itself off in the next 4-5 years... (even if I didn't, I'd fear that the next home buyer would ask what my average utility bill is).
Does that seem like the best course of action (at least to start)?
Also, is there a simple way to find leaks and air seal the attic? I'm pretty handy at DIY, unless it requires expensive tools :)
Thanks again for the response.
I respectfully disagree with Martin about powered attic fans provided they're solar-powered, self-installed, and the attic floor plane is near-perfectly sealed. All the criticisms center around being useless in the winter (irrelevent to a Floridian), cost-ineffective (irrelevant if they're solar powered) or pulling conditioned air from the house (minimal ro nonexistent if your air sealing is good). If you are willing and able to do the work yourself, they represent a lot of bang for buck.
但I agree with him that your best betperformance-wiseoff is converting to an unvented conditioned attic with rigid foam above the roof sheathing. This will of course be hugely expensive and necessitate a re-roof, but it can be worth it if you need to re-roof anyway and have the budget for upgrading to ventilated white metal or tile.
That said, this is likely not the mostcost-effectiveapproach due to its upfront expense and non-DIY-friendliness. It'll sure cost a heck of a lot more than $600-800 (try adding a zero to those numbers and then doubling them). So if your budget is limited or cost-effectiveness is your top priority, then I think it makes sense to follow more of my advice. $50 of caulk and spray foam can make short work of the attic floor leaks. Radiant barrier materials shouldn't cost more than $300 or so, and if you can install it yourself despite the bad access, that's a lot of bang for buck. A cheap solar-powered attic fan can run you $100 and will make a difference provided you've diligently sealed the air leaks and you have enough inlet ventilation at the gablesor eaves. Another $400 spent on cellulose to blanket the floor and bury the ducts should round things out, and you'll likely get a blower rental for free.
这是马丁的好方法吗?不。但it will probably be 80-90% as good, for less than 10% of the money.
Thanks, Nate.
Are there any articles you recommend on the best ways to find attic leaks? Most of the floor is covered in compressed fiberglass. Should I pull this back to see under it?
I've heard good things about Great Stuff for sealing holes. Would you recommend something different?
I was considering the radiant barrier sheets. I could get a few rolls for $200 or so and spend a morning stapling it to my roof joists. I can maneuver the attic, it's just not as easy as the how-to videos where they're standing up :)
I'll definitely consider the conditioned attic for my next house (hoping to build new when we sell this one in a few years).
Push/pull attic fans can be adjusted to reduce attichouse air flow.
Use a good, ASTM C920 high movement class sealant.
Getting the attic floor perfectly sealed (and the ducts too) to make a solar attic fan to work without inducing higher cooling costs sounds good, but can be quite an undertaking. The best way to find attic air leaks is with a blower-door pressurizing and depressurizing the house, but you also have to run a duct-blaster to pressurize/depressurize the HVAC system to fix those leaks too. Short of a blower door, a large reversible window fan can often be useful in finding the leakage in smaller homes such as this.
High solar reflective index roofing (mop-on or new shingles) will be more effective than radiant barrier. The SRIs of different roofing materials & coatings can be compared apples-to-apples using third-party tested results listed at the CRRC:http://coolroofs.org/
Vented attics in Florida brings in more moisture to the attic than it removes, and provide at best a paltry amount of cooling benefit. Sealing up the attic from the outdoors may result in higher peak attic temperatures, but can still reduce the overall cooling energy used, since all duct leakage then happens inside of the pressure boundary of the house. It depends a bit on how leaky &/or unbalanced your duct system is.
Ben,
Here is a link to an article about sealing attic air leaks:Air Sealing an Attic.
-- Martin Holladay