Author Topic: Venting warm attic air into the home in winter  (Read 6200 times)

dragoncar

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Venting warm attic air into the home in winter
« on: November 03, 2016, 07:07:20 PM »
I just added a bunch of insulation to my attic.. great right?  Well I was up there inspecting something and noticed it's still a lot warmer than my house even in the winter (well I guess it's fall but we will see how long this lasts). 

I guess that makes sense -- in the summer the insulation should help keep my home cooler, but in the winter I'm losing solar gain. 

So I thought maybe I could install a reverse ventilation fan to pull the hot air into my house during the day.  Is this a crazy idea?  I didn't see anything about it on the internets, and maybe it's a code problem but I can always remove it if I sell.

Jon Bon

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Re: Venting warm attic air into the home in winter
« Reply #1 on: November 03, 2016, 07:22:41 PM »
You might want to post this in the diy section and wait for paddlehat.

However, this is not good. A hot attic in the winter is not good.

Did you do blown in insulation by chance? Did you inadvertently cover up your soffit vents? If your attic is hot in the winter imagine what it is going to do to your roof in the summer!


Attics are designed to pull cool air in at the soffit and vent it out at the peak. If your attic is not constantly cycling air you are in for problems in winter and summer depending on where you live. An unvented attic is going to cool in the summer and damage singles and melt the snow in the winter to create ice dams.

I don't have all the facts and I'd wait for paddlehat had but my initial thoughts are it's not a good situation.

Le Barbu

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Re: Venting warm attic air into the home in winter
« Reply #2 on: November 03, 2016, 08:06:29 PM »
Attic should be pretty much the same temperature than outdoor. If not, ventilation of the attic is poor or there is not enough insulation.

My house is 10 years old and insulated with 18 inches of blown-in fiberglass (R45) and well ventilated. The attic is damn cold in winter!

dragoncar

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Re: Venting warm attic air into the home in winter
« Reply #3 on: November 03, 2016, 11:23:36 PM »
I did not block the soffit vents, I installed baffles, but my attic has always been hot.  As I understand, most attics are underventilated, but I'm not really sure I believe that this will damage the roof.  See:

http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/31616/How-Hot-Is-Your-Roof-Insulated-Rooflines-and-Shingle-Temperature

I live in a mild climate, so there is no ice/snow

Le Barbu

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Re: Venting warm attic air into the home in winter
« Reply #4 on: November 04, 2016, 05:53:21 AM »
The way you describe it, the attic looks ok.

Back to your idea, it's called a heat pump (sucking solar heated air into the house). Maybe the code does not allows your idea where you live but if it does, this would be great to try, especially if you are a DIYer.

rhadams1988

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Re: Venting warm attic air into the home in winter
« Reply #5 on: November 04, 2016, 06:42:57 AM »
My guess is that most of that heat is coming from your living space as opposed to solar heat gain. A lot of times attics are hot because there aren't perfect seals and warm living space air infiltrates through the cracks. If that's the issue, you could air seal the joints. If this is your issue, it definitely would not make sense to recirculate that air.

If it really is solar heat gain that's keeping the attic warm, I wouldn't do anything. Since the temperature change from living space to attic is going to be low so you will lose heat really slowly. That's always nice in the winter.

J_Stache

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Re: Venting warm attic air into the home in winter
« Reply #6 on: November 04, 2016, 07:28:20 AM »
Attics are vented to reduce moisture.  A side benefit is reduced temperature, but this is less of a concern with lots of insulation.  Most roofing shingles can handle an attic being 130-140 degrees during the Summer.


Fishindude

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Re: Venting warm attic air into the home in winter
« Reply #7 on: November 04, 2016, 07:28:29 AM »
You need some attic ventilation to get the heat out.
You might also need better insulation to keep that heat in your living space.

Pumping attic air into house is not a good idea; insulation fibers, dust, etc.

dragoncar

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Re: Venting warm attic air into the home in winter
« Reply #8 on: November 04, 2016, 12:08:14 PM »
My guess is that most of that heat is coming from your living space as opposed to solar heat gain. A lot of times attics are hot because there aren't perfect seals and warm living space air infiltrates through the cracks. If that's the issue, you could air seal the joints. If this is your issue, it definitely would not make sense to recirculate that air.

If it really is solar heat gain that's keeping the attic warm, I wouldn't do anything. Since the temperature change from living space to attic is going to be low so you will lose heat really slowly. That's always nice in the winter.

I sealed all top plates and penetrations with foam before adding r40 cellulose over existing fiberglass, so it really is solar gain.  Plus, the house is cooler than the attic which is why I asked about reverse venting.

Good point about reduced thermal loss during the daytime, but at night the attic quickly cools off.

Sounds like the biggest issue would be dust so I'd need a filtered vent, and besides that might not be worth the price

I don't anticipate moisture issues, since the existing fiberglass looked fine... Of course if there is a leak all bets are off

BTW, I said "winter", but it's still sunny with highs of 75 around here.  That's great but my house is 61 in the morning, and doesn't heat up quickly during the day.
« Last Edit: November 04, 2016, 12:11:14 PM by dragoncar »

Kapiira

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Re: Venting warm attic air into the home in winter
« Reply #9 on: November 04, 2016, 01:40:12 PM »
Do you have ductwork in your attic?  It sounds like you've already considered all the sources besides solar gain.  Because of personal experience (giant holes in ductwork in the attic), I want to make sure you've checked any ductwork that might be up there.

If it is solar gain, that would be a pretty cool experiment to pump the warm air back into the house and I'd love to hear how it goes if you give it a try.

dragoncar

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Re: Venting warm attic air into the home in winter
« Reply #10 on: November 04, 2016, 01:44:20 PM »
Do you have ductwork in your attic?  It sounds like you've already considered all the sources besides solar gain.  Because of personal experience (giant holes in ductwork in the attic), I want to make sure you've checked any ductwork that might be up there.

If it is solar gain, that would be a pretty cool experiment to pump the warm air back into the house and I'd love to hear how it goes if you give it a try.

No ductwork in attic.  I mean, there's really not much else it could be -- my furnace and hot water heater vent runs through the attic to the roof, but I use maybe 1/3 therm/day right now and I installed a sheet aluminum dam around it recently so I would have hopefully noticed if it was leaking.

Of course, it it is leaking, then it would be a terrible idea to vent from the attic to living space.  I do have some CO detectors.

That reminds me that I have to check for ductwork leaks under the house, but it's a royal pain to crawl around down there and they are insulated so that makes leak detection even harder.  I don't fit under all the joists so I may have to dig down eventually to access everything.
« Last Edit: November 04, 2016, 01:49:33 PM by dragoncar »

Telecaster

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Re: Venting warm attic air into the home in winter
« Reply #11 on: November 04, 2016, 01:50:19 PM »
If the attic is venting and sealed properly, then I wouldn't worry about it.  If it is solar gain then the warm air in the attic will help keep the occupied spaces warm.   So you actually are getting some benefit from the solar gain. 

dragoncar

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Re: Venting warm attic air into the home in winter
« Reply #12 on: November 04, 2016, 01:53:05 PM »
If the attic is venting and sealed properly, then I wouldn't worry about it.  If it is solar gain then the warm air in the attic will help keep the occupied spaces warm.   So you actually are getting some benefit from the solar gain. 

Haha, I wasn't worried about it, I was just wondering if I could harness the solar gain.  With all the insulation I added, I don't think I'm getting the full benefit. 

Other posters seem very worried about my hot attic though.

On the day I blew in the insulation, it was overcast and the attic was not warm (otherwise I probably would have died -- in the summer it's unbearable)

Telecaster

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Re: Venting warm attic air into the home in winter
« Reply #13 on: November 04, 2016, 02:06:31 PM »
If it gets too hot in the summer you can and probably should add some mechanical ventilation.   They make solar powered roof vent fans for this purpose, among other options.   

dragoncar

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Re: Venting warm attic air into the home in winter
« Reply #14 on: November 04, 2016, 02:44:42 PM »
If it gets too hot in the summer you can and probably should add some mechanical ventilation.   They make solar powered roof vent fans for this purpose, among other options.

I'm not worried about how hot it gets in the summer (I never run the AC, and there are only a couple weeks per year that I run the whole house fan at night -- and that's before I added the r40).

I also haven't read much good about powered attic vents

http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/38676/Don-t-Let-Your-Attic-Suck-Power-Attic-Ventilators-Are-a-Bad-Idea

See also

http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
« Last Edit: November 04, 2016, 02:57:43 PM by dragoncar »

robartsd

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Re: Venting warm attic air into the home in winter
« Reply #15 on: November 04, 2016, 03:01:20 PM »
I also live in a mild climate. To capture solar gain, you might want to consider installing a thermal solar system. I've though about building a DIY solar thermal system like this one on Build-It-Solar. This type of system would allow you to collect solar gain while the sun is shining and use it later when you need heat. My problem is I don't have a good spot to locate the collectors - my house is on a deep narrow lot that faces south, so a solar collector would have a big impact on the front elevation. I do have a basement/cellar under one corner of the house that would be a great spot for the storage tank.

dragoncar

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Re: Venting warm attic air into the home in winter
« Reply #16 on: November 04, 2016, 03:07:45 PM »
I also live in a mild climate. To capture solar gain, you might want to consider installing a thermal solar system. I've though about building a DIY solar thermal system like this one on Build-It-Solar. This type of system would allow you to collect solar gain while the sun is shining and use it later when you need heat. My problem is I don't have a good spot to locate the collectors - my house is on a deep narrow lot that faces south, so a solar collector would have a big impact on the front elevation. I do have a basement/cellar under one corner of the house that would be a great spot for the storage tank.

Yeah, I might someday, but I've got some shading issues in the winter due to large trees uphill to the south, and my roof geometry isn't ideal.  I'm also on year 25 of a "40 year" roof, so I don't want to install anything solar until I reroof. 

If anyone has any tips on how to determine when it's time to reroof, I'm all ears... I mean do you just wait until you have a leak, go by time, or what?  I've been on the roof and I'd say the shingles aren't in the best condition, but they aren't falling apart yet (I could rip one in half when I needed to patch up the location of an old satellite dish).

dragoncar

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Re: Venting warm attic air into the home in winter
« Reply #17 on: November 04, 2016, 03:37:44 PM »

Le Barbu

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Re: Venting warm attic air into the home in winter
« Reply #18 on: November 04, 2016, 03:40:05 PM »
Why did you removed your cool avatar?

dragoncar

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Re: Venting warm attic air into the home in winter
« Reply #19 on: November 04, 2016, 04:13:13 PM »
Why did you removed your cool avatar?

Just for a joke.... I guess I'll put it back someday