Too hot at night!
Too hot at night! ATTIC EXHAUST FAN vs RADIANT BARRIER vs INSULATION between the rafters – Which should I install in my attic?
I live in Massachusetts (not near the coast). In winter outside temperatures can be below freezing for weeks and sometimes below 0F degrees. From June to August daytime highs are in the 80s and 90s (even 100) with often high humidity. I live in a 2-story wood frame colonial house built in the 1940s which has very poor insulation. The exterior has vinyl siding over ½ inch XPS foam over painted cedar shakes. The walls are insulated with bags of balsam wool which has an R-value of 2 or 3, and they have settled leaving gaps around them. I know this because I’ve been doing minor remodeling, and I removed a large area of plaster and lath from my interior (attached) garage wall, and also put holes in some other walls to add new electrical boxes. The attic has probably 5 inches of loose fill fiberglass. There is no air sealing in the house. In the future I plan to have the attic insulation removed and cellulose fiber blown in. I have a forced hot air heating system with central air. In the winter it is cold in the room above the garage, but most of the house is okay, although the furnace runs a lot. In summer the first floor is cool but the second floor is very warm especially at night. The room above the garage is the worst. The attic is super hot and the heat from the attic seems to radiate down into my second floor.
I want to do something in the attic before I have the old insulation replaced. I have been thinking of three ideas: install an exhaust fan in front of a gable vent, install a radiant barrier below the rafters, or install vent baffles and fiberglass batts between the rafters. In the attic there is a 1 foot x 1 foot gable vent on each end. There seems to be a narrow soffit vent and ridge vent. There is almost no overhang on my roof (rafter tails), so the soffit vent is very narrow. The roof has low slope (about 6/10) making it impossible to physically get near the soffit from inside the attic. I don’t think I want to turn the attic into conditioned space, but maybe semi-conditioned space.
I bought an electric attic gable fan last year but returned it without installing it because of all the bad things I read about them creating negative pressure and sucking the air conditioned air out of your house. But I’ve been thinking that as long as make-up air can enter through the other gable vent and soffit vents then what is the problem with that idea? I think I want to install a gable vent fan with a switch on the second floor and just manually turn it on for a couple hours at night in the summer. I do worry a little about sucking humid air into the attic from outside (along with dust, pollen, and spores) and growing mold. I remember when I was a child that my grandparents had a giant fan in their hallway ceiling that they would turn on at night to suck hot air in the house into the attic. I know that’s a bad idea if you have central air, but the principle of sucking hot air out of the house or attic seems logical. Should I install a gable vent fan in the attic?
The rafters are only 2×6. Would I be better off stapling a roll of (perforated) Silvertanium radiant barrier onto the rafters or putting in fiberglass batts (unfaced?) between them? The roof, by the way, is dark gray asphalt shingles.
There must be millions of older poorly insulated houses like mine in New England. Much of what I read on the Internet is confusing and conflicting and often applies to newer homes. I need some professional advice.
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Replies
A pair of identical gable fans one blowing in, the other out, would lower the peak attic temperature withtout grossly depressurizing the attic, but it's kind of a hack.
Radiant barrier installed on the underside of the rafter edges would keep the fiberglass from being heated up. Old-school blown fiberglass is too translucent to infra-red radiation, and under a hot roof deck the insulation an inch into it can reach peak temperatures higher than the peak attic air temperature (!).
Using perforated radiant barrier would prevent that, and could be left in place when doing the eventual air sealing and insulating at the attic floor (except where it would have to be trimmed or folded back near the eaves to allow deeper floor insulation). It should drop both the peak and average temperatures of the attic air, but will increase the average temperature of the roof deck and shingles a bit.
With perforated RB it's fine to make it full coverage from the attic floor to the ridge making it reasonably air tight, whereas with UN-perforated goods it would need a gap at both the bottom and top edges to allow some convection drying of the roof deck, and that convection would defeat some of the summertime benefit, but not all.
If you're going to install batts, install them on the attic floor, not the rafter bays, and use kraft-faced, with the kraft side facing up (as in IR blocker and air-retarder). A kraft facer is not a true vapor barrier, and cannot trap moisture in the fiberglass.
For a ridge vent to work properly requires having at least as much free area input at the soffit vents, and closing up any gable vents. The gable vents short-circuit the air flow at the soffits, making it so that essentially no air flows along the roof deck to either dry or cool the roof deck except near the top of the bays nearest the gables.
Thank you for the reply. I didn't know that loose fill fiberglass is translucent to infrared radiation. I also didn't know that gable vents should be closed up after installing soffit and ridge vents, although in my house I don't think there's enough airflow in from the soffit to matter. I had a big package of R-11 lying around I had aquired years ago and was thinking of a use for it, but I guess I'll just forget about using fiberglass in the attic and stick to the radiant barrier on the rafters and future cellulose fiber on the floor/joists. Maybe I'll get a couple fans like you suggested for intake and exhaust (but small and lower CFM) unless the radiant barrier makes a big difference.
Dave,
The right answer is that you need to perform air sealing work at the ceiling level, and then add insulation to your attic floor. These two articles describe the necessary work:
"Air Sealing an Attic"
"How to Insulate an Attic Floor"
Installing an attic fan is more likely to cause problems than it is to help. For more information on why this is so, read this article: "Fans in the Attic: Do They Help or Do They Hurt?"
The cost and hassle of installing a radiant barrier provide little benefit for the effort. For more information on why you need insulation, not a radiant barrier, see this article: "Radiant Barriers: A Solution in Search of a Problem."
I guess you are right. It's been so hot lately that I've been thinking of do-it-myself stop-gap measures because it will be many months before I can hire a company to remove the existing loose fill fiberglass, air seal, and install blown in cellulose in my attic. I'm hoping they can dense pack cellulose around the balsam wool in the outside walls too.
> the first floor is cool but the second floor is very warm...stop-gap measures
For an easy, temporary fix, use dampers (or duct boosters) to provide additional cooling upstairs.
I keep my MA home cool by closing and shading all windows during the day and leaving the highest window open, in your case the attic, and open a basement window. Then the hottest air has no choice but to exit the highest spot and draw cooled air from the basement throughout the house. At night, slightly open other strategic windows to allow cooler night time air to enter. Then close all windows again in the morning. If you open both gable windows in the attic, it will only draw hot air in through the other window that is at the same elevation leaving the rest of the air stagnant throughout the house.
My experience with this "basement as AC" approach was mildew on the basement walls.
In response to Dana's recommendation of leaving the kraft facing on insulation batts "up": While true, kraft facing is not a vapor barrier but rather a vapor retarder, kraft facing cannot be left exposed in any application. A warning to that effect is clearly printed on kraft and standard foil facings by every fibrous insulation manufacturer. Unfaced or FSK faced batts may be left exposed due to their class A ratings.
Chris Brown
Director: Knauf Insulation Academy