Here are my thoughts/concerns if I were considering such a thing
So I originally posted this in "Ask a mustachian" and was advised to repost here (http://forum.mrmoneymustache.com/ask-a-mustachian/venting-warm-attic-air-into-the-home-in-winter/) -- If a mod would like to delete/lock the original thread, much appreciated
To summarize:
- I recently added about r40 blown cellulose over perhaps r12 fiberglass in my attic.
- I used foam to seal all penetrations and top plates.
- I don't have any ductwork in the attic, but I do have a single vent for my water/furnace that goes through the attic through the roof.
I noticed it's still warm up there. For example, right now it's 65 in the house, 71 degrees outside, 83 on the surface of my insualtion, and 95 on the bottom of my roof deck.
That's a nice 15 degree difference, so instead of running my whole house fan (gets me only a 10 degree difference) or the furnace (wasteful), why not take that hot attic air and bring it into the house? I do have the windows open, but convection doesn't work as well as you might suspect.
First, how large is your attic? If it is a small attic (especially volume wise when compared to the house) it might not be worth it; think how warm the air heated by your furnace is when it enters your house and how much of it there is. Without fancy things such as an outdoor reset (which you should install) it is probably ~120 degrees and it probably blows at 500 to 1300 CFM. That is a lot of thermal energy being distributed.
Second, the roof is likely vented, which means air from outside (cold) will enter as you take air from the attic into the house (unless you push air from the house into the attic) lowering, possibly rapidly, the temperature of the attic and available heat. I suppose you could rig up a heat exchanger system (but you might be better of with a solar thermal collector outside). This also depends on how much heat is generated by the roof (solar gains)
Third, I am not sure why you mention convection. Hot air rises and cold air falls . . . in a closed system. So, the hotter air in your attic will not fall into the house. In a house you are likely to see stack effect where the hot air rises and escapes through the ridge vent and is made up my air from the eve vent or lower in the house. You are going to need a fan to push the hot air down and unless you allow the house air back into the attic it will just find its way out of your house and cold air will enter attic.
Fourth, you may want to distribute it around the house. If you don't you are going to wind up with a room (where the vent is) that in markedly warmer than the rest of the house and you will still be running the furnace or furnace fan to heat/move the air around.
Fifth, any penetration of the heated envelop is a risk for air infiltration, duct dampers are no where near 100%, so you could have a cold leak (heavier air) when the attic temperature drops below the household temperature. Also, a duct damper open the attic air on one side would be very could with a low R-value and in the duct (closed system or close to it depending on the damper) you could sent up a convective current with warm house air rising to the damper cooling when it comes in contact and falling back into the house. Your gains during the shoulder season could be offset by the losses during the heating season (and as your house temperature is colder than outside possibly overnight as well).
Six, at least where I like the wind can change quickly and when it hits the side of the house it can blow right in the attic eve vents and drop the temperature up there quickly (I've been up there working before when and it is drastic). So one minute you could be dealing with a 15 degree positive and the next minute you might be looking a close to exterior temperature.
In summary, I would be concerned that in order to do what you want properly it would be somewhat expensive and complicated (open during the shoulder season during the day and closed and night and during the heating season, as well as fans, distribution, electrical . . . ), potentially for a minimal amount of heat. Before you proceed I would investigate other means of harvesting that heat more efficiently, like a solar thermal collector and/or run a calculation (manual J maybe?) to see what kind of BTUs you are dealing with harvesting from your attic.
I'm not an on-topic nazi, but the last thread got a lot of suggestions to increase venting in my attic. That's a separate discussion, and one I'd like to explore in a different thread, but would be counter-productive in capturing this free solar power.
The question is: Is it crazy to install a vent from the attic to the house?
So far, issues raised are:
- Could be dusty - would need a filter, although if the intake is high enough in the attic (say a 5 foot riser pipe) I don't think there would be much dust
- If my furnace vent is leaking, I could have a carbon monoxide issue
Regarding the dust, the attic you describe (insulation on top of the drywall) is likely vented (cool air is drawn in at the eves warms on the underside of the room and is exists through the roof vent. The dust is not a issue of the insulation on the floor, but rather the constant cycling of exterior air through the attic. If there is dust there it will be spread across the attic air. At least where I live the wind can whip up a good bit of dust if it has not rained in a while.
Regarding your furnace vent, if it is leaking you DO have a issue and it should be addressed regardless of any other projects.
All that being said if I were in your shoes and could satisfy/address all of my concerns I might experiment with the situation; still I suspect there are better return on invest/heating costs savings to be had (like your insulation upgrade and air sealing) elsewhere in the house.
