What should attic temp be?

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As some of you know, I moved into a new house at the beginning of the summer. It is a two story home with an unfinished attic in Stonington, CT. I mounted the remote thermostat about 4 feet above the floor joist.

Yesterday's outside temp topped at 91 F, inside temp 78 F (A/C off all windows closed), and attic 112 F. When I left for work in the morning outside was 73F, inside 74F, attic 75F

I’m happy with the interior only creeping up 4 degrees (A/C off windows closed) Is the 21 degree differential temp ideal for an attic in the N. East? The roof is black asphalt shingles and ridge and soffit vent setup. I’ll continue to take some readings and get your feedback before I look into getting additional ventilation up there.
 
You should at least figure between a 20 to 30 degree F increase in attic spaces. I keep a wireless temperature sensor in my attic to monitor because I was curious. Now I'm in Oklahoma, but my attic will hit about 120-125F when it's around 100 outside. On days when there is no wind and brutally hot, it could reach about 130. The most I've seen is about 133F. I got to vents on the roof and side vents on the eaves. I've been comtemplating on getting a attic fan, but if all it raises is 20 degrees, I don't know if it would be worth it and if the ambient temperature is in the 100's, just how much "cooling" affect would there really be???
 
quote:

Originally posted by Schmoe:
I've been comtemplating on getting a attic fan, but if all it raises is 20 degrees, I don't know if it would be worth it and if the ambient temperature is in the 100's, just how much "cooling" affect would there really be???

Heat flow through the insulation is roughly proportional to the temperature differance.

If you are keeping your house at 78F in 100F ambient and the attic is 125F, then you have a 47F temp gradient.

If you do a good job with attic fans and get the attic temp down to 5F over ambinet, 105F, then you are dealing with a 27F temperature gradient. That would cut the heat flow through your attic insulation by over 40% during peak attic temperatures.

Obviously, not all heat gain is through your ceiling and those temps only occur during part of the day, so any savings would be no where near 40%. You would need to dig deeper to be able to hang some kind of dollar value on the savings.
 
I'd say a 110 - 115 degree attic temp wouldn't be bad at all in that kind of heat. Rather than invest in some kind of electrically powered fan for the attic, I'd suggest you do two things: verify that your attic has enough ventilation and make sure the insulation is at least R50.

When I had my roof re-done a couple years ago, all three companies that quoted the job mentioned that the ventilation was woefully inadequate. I have about 4,000 sq. ft. of roof over a 1,600 sq. ft. ranch and the roof had seven vents. It now has 14. Also, when I upgraded from R-11, which was installed when the house was built, to R-50, the gas used for heating and the electricity used for cooling dropped drastically. It also virtually eliminated the hot and cold spots throughout the house.

So if you're going to focus your energies anywhere, I'd suggest those two things. They're relatively cheap and provide a big payback. If you're already good in that regard, I wouldn't worry.
 
So I take this as a 21F differential is within an acceptable range, but ideally I should strive for 5F above ambient. Is this possible with passive ventalation? I guess full ridge vents are pretty much standard now in the N.East but rely on wind to work as designed.

I don't remember the site, but one group of people say ridge vents don't work by design because the rising hot air must travel down (against gravity) before exiting the ridge. They set a smoke bomb off in the attic and said you would not see any smoke coming out. Even after 2 hours later the smoke was still in the attic.

I may have to test that theory.
 
The last two houses we've lived in had 3 layers of shingles when we moved into them and needed new shingles. My neighbor at the last house still had his original shingles, and the only difference I could see was he had much better ventilation. So my house had shingles replaced twice and he was getting ready to replace his for the first time. The house was about 20 years old. I have concluded that bad ventilation shortens the life of the roof......
 
No, I did mean ridge vent. I'm not surprised cutting a hole in an unvented roof set off a geyser of hot air. My question is, "would it do the same if you cut a hole in an attic with a ridge vent that has already been installed?" I don't know the answer.

The ridge is at the peak of the roof, but when it hits the top of the ridge, it must go down before exiting. This is because the ridge overlaps the roof line slightly to prevent water entry (see pic). Without wind, I guess the pressure from the differential temperature must be great enough (maybe 20F is the answer) to force the hotter air out. I'm don't think ridge vent, even with gobs of soffit openings, can achieve
+5F above ambient
dunno.gif

 -
 
If what this group says about ridge vents is true, then 99% of roof vents are useless, which I have a hard time believing. With any vent, you need either soffit or gable vents to allow cool air to come in from the bottom to replace the hot air leaving near the top. Convection currents will move the air through the system. The fact that the air has to move down an inch or two will be more than compensated by the force of the air current.
 
All said and done - for places such as where I live and no one has AC (except for some reason the guy accross the street), it makes some/total sense to get ab attic fan.

This last heat wave we had - it got upper nineties here, rare but it happens. Our upstairs got quite roasty and took awhile to cool down in the evening because of the hot attic. If we had a few fans, I would think running them during and after the heat build up would greatly increase the comfort in the house.
 
My father-in-law, a carpenter, believes all your insulation efforts in the attic should be on its floor. Put vents in and you need not care about the attic temperature. In winter, the attic will be wintry, and the snow on the roof will not melt and refreeze, wrecking the shingles, making ice dams, etc. Give it time and the snow will either fall off or blow away. Oh, a metal roof is a perfect compliment.

Without insulation in the rafters, there are no codensation problems. Of course, there should be a vapor barrier between the ceiling drywall and the attic floor insulation.

This assumes you won't live in your attic, or store anything that can't take temperature extremes.
 
I have a power vent in my roof along with 6 vents. My power vent is set to turn on at 105F, it has been running pretty steady the last little while but the cooler you can keep your roof, the better for the life of the shingles.

P.B.

(I get to go up once a year and oil it
fruit.gif
)
 
quote:

Originally posted by Pablo:
All said and done - for places such as where I live and no one has AC (except for some reason the guy accross the street), it makes some/total sense to get ab attic fan.

This last heat wave we had - it got upper nineties here, rare but it happens. Our upstairs got quite roasty and took awhile to cool down in the evening because of the hot attic. If we had a few fans, I would think running them during and after the heat build up would greatly increase the comfort in the house.


Your sound like a good candidate for whole house fan. The biggest drawback is the larger amount of unfltered air being sucked in through the windows. That souldn't be much of a problem is Western Washington.
 
My attic temp during high 90s to 103 was around 125-130 with a 16 inch powered fan on the peak of the roof. (in the center of the roof)

Turn off the fan and it climbs to around 140. Not too much of an improvement but it is moving around air.
dunno.gif


If I was going to do it again, I'd make sure I had plenty of soffet vents and go with the turbine vents (3 of them) across the roof.

The insulation is R50 in the ceiling and R23 in the walls.

My house is a 4 level so the top story is the only one that has the power vent. The other part of the house is just normal vents (6 of them) and I have not put in a temp guage in that attic (it's shared with the attic in the garage).

My top story runs 3-5 degrees hotter than the 2-3 level and 15 degrees hotter than the basement with the A/C on with the temp set at 76 on the 3rd level.

Take care, Bill
biggthumbcoffe.gif
 
quote:

Originally posted by eljefino:
My father-in-law, a carpenter, believes all your insulation efforts in the attic should be on its floor. Put vents in and you need not care about the attic temperature. In winter, the attic will be wintry, and the snow on the roof will not melt and refreeze, wrecking the shingles, making ice dams, etc. Give it time and the snow will either fall off or blow away. Oh, a metal roof is a perfect compliment.

Without insulation in the rafters, there are no condensation problems. Of course, there should be a vapor barrier between the ceiling drywall and the attic floor insulation.

This assumes you won't live in your attic, or store anything that can't take temperature extremes.


The arguments I've read over the past 20 years agree with this in principle.

And, with that in mind, there should not be but the barest amount of electrical, and absolutely no HVAC equipment or other in the attic.

The thought is that there are two roofs: One, of insulation and appropriate water-movement control; and two -- the outer roof -- of being the weather break, preventing any excesses. No holes cut in the outer roof is the ideal (utilites/chimneys to be a central service core, not unlike a commercial building).

The corollary is a full basement and thick walls.

In effect, the idea is to isolate the house from temperature transients; modulating them with appropriate ventilation, cooling and heating.

Imagine a box suspended in unmoving air several feet thick on all six sides.
 
I vent the heck out of our attic. I use a small household fan blowing out a 1 foot hole in the downwind corner and a box fan located 1/3 of the way down from the upwind side. It does "something".
 
we crack our attic door and have a box fan in one window venting out. This way it pulls cooler air from down below (we have window ACs in specific spaces, so there is no worry about wasting central air), and we achieve aboout 5-10F temperature gradient. Not too bad...

JMH
 
JMH. It sounds like you are operating the attic fan as a whole house fan. I hope you only do this at night when outside air is cooler than inside air. Sure 5-10F above ambient is good for an attic, but I'd go ballistic if my living space was close to ambient in the 80F+ range.

As you are describing it, as soon as it's getting hot in the attic, I would run the fan with the attic door closed (so it's pulling make-up air from the outside (open another attic window), not the cooler living space. Then, in the evening, run the fan with attic door open to pull out the warmer living space air.
 
quote:

Originally posted by Razl:
but one group of people say ridge vents don't work by design because the rising hot air must travel down (against gravity) before exiting the ridge

They must never have installed one on a hot day..

I had one installed on my rental property and the guy who did it (who does a lot of work for me, so I trust him) said it felt like the attic burped when he cut the peak off the roof to install it.

Also I'm trying to picture it and I don't see where hot air has to travel down to exit from a ridge vent. The ridge is at the peak of the roof, the highest point.
 
Razl,

Since we only use window units where we are currently doing things, and done have one in the kitchen, the draw of air is set up to pull through the kitchen and up. The air down there is ambient anyway (with a fan to make it seem cooler), and so its no big issue.

Maybe Ill give your method a try... then I dont have to leave the attic door open at least.

Thanks,

JMH
 
quote:

Originally posted by brianl703:

quote:

Originally posted by Razl:
but one group of people say ridge vents don't work by design because the rising hot air must travel down (against gravity) before exiting the ridge

They must never have installed one on a hot day..

I had one installed on my rental property and the guy who did it (who does a lot of work for me, so I trust him) said it felt like the attic burped when he cut the peak off the roof to install it.

Also I'm trying to picture it and I don't see where hot air has to travel down to exit from a ridge vent. The ridge is at the peak of the roof, the highest point.


I agree. I helped my brother install a cupola on the ridge of his roof. It was definitly an ugly hot blast of air thet greeted us when we cut through the roof.

I put three 14 inch whirly type (turbine?) vents
on the back slope of our roof in San Jose about 4 feet down from the ridge. They would spin from the rising hot air on a windless day. I didn;t do any temp measurements, but it made a world of differance in how the attic felt on a hot day.

Razl, did you mean soffit vent instead of ridge vent?
 
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