[Articles] Are attic fans bad?

[CT Droid]

Microsoft's Hohm blog posted an interesting article about attic fans. Many Cocooners have these installed, or are looking into installing them, so I think you might want to check this out. It definitely makes me reconsider my attic fan plans. What do you think?

View the full article

 

[apostolakisl]

Yup! Unless you have spray foam on the ceiling and don't have the HVAC units in the attic I bet it sucks tons of air through the light fixtures and other cracks. Of course copious, large, wide open soffit vents/gable vents would also be a good idea.

 

[Digger]

When I renovated my house I pulled all of the sheet rock except the ceilings where there is an attic above. Instead of having the mess of all of the insulation and dust etc fall into the house I sheet rocked over the existing perpendicular to the sheets that were already up. For the electrical boxes in the ceiling I caulked around them so there is no gap between the sheet rock and the box. I did this to minimize heat loss at the same time have fresh sheet rock everywhere.

While nothing is perfect I think that in my case at least the air loss is minimal. I also use the attic for storage so the fan is important to keep the attic temperature below 100F.

BTW the attic fan died months ago. Interestingly enough my electric bill went "UP" significantly this year. While I have no proof if it is related I will be replacing my fan.

 

[jaysonc]

This summer I addressed ventilation in my attic and found the same data this blog post details. Even the fan manufacturers state that you need at least 12 (16x8) soffit vents for each powered vent. That is a LOT when the size of your house says you need 3 or 4.

I found that the builder had only installed the minimum ventilation code requirement (no surprise) but due to subcontractor laziness the install was only providing about 50-60% of the ventilation it should have. After doing the math myself using "recommended" values, I ended up with 3X the volume of ventilation I started with.

I now have a completely passive solution using soffit vents, ridge vents an a mix of roof louvers. I saw attic temperatures drop by a consistent 15-20% immediately.

Now to finish the attic foil and blow in some additional insulation.

 

[Kevin L]

I use Icynene for insulation. No attic vents, attic fans, soffit vents, ridge vents, etc. Attic never reaches 100 degrees. Plus, an added advantage of the lack of vents is no air/water penetration in hurricanes. Much of the damage to homes from hurricane winds is the pressure exerted upwards on the roof and downwards on the ceiling from air penetration from attic and soffit vents.

 

[nsisman]

The author of that article is clueless !

Too much time in the books and not enough time in an attic !

Nuff said...................

 

[apostolakisl]

The author of that article is clueless !

Too much time in the books and not enough time in an attic !

Nuff said...................

You must never have seen any of the numerous examples demonstrating the infrared images inside of a home when a blower-door test is done. I have never seen an attic example of this but it would be foolish to not assume that the exact same thing doesn't happen and probably worse as construction guys tend to be slopier in the places that think no one will see their work. Keep in mind that those infrared photos of penetrations on outside walls of your house have a second air barrier in the sheething and tyvek that the house to attic transition doesn't have. If you want put an attic fan in you really need to make sure that you have enough soffit vents to prevent negative pressure, the HVAC unit is sealed up tight as a drum, and the fixtures are all caulked.

http://www.alpineinspection.com/AirInfiltration.html

 

[Digger]

I should probably add that I have 3 air intakes and one exhaust for my attic. The size of the fan is 1540 CFM and I remember following the manufacturers instructions when I put it in (thats why I have 3 intakes now).

Attic fans are bad if installed incorrectly. I think many times they are because people dont know that the intake is so important or its just to much work for a contractor to do it right.

In my case I need it because I use it for storage space.

 

[sda]

and the fixtures are all caulked.

Not only that, but in older homes the holes through the top plate for wires and pipes won't be sealed. The air will get sucked through the walls and into the attic from any penetration in the walls, i.e. switches, receptacles, shutoff valves, and any place the drywall/plaster isn't sealed to the framing.

 

[jharrell]

Attic venting started and was incorporated in building codes because of cold climate ice damming. It has very little benefit in hot climates, with large volume venting the heat load might be reduced 10%, while losing 10-20% is a/c ductwork losses.

The best way to improve thermal efficiency in the attic is do go with a fully sealed attic with spray foam on the underside of roof, making the roof the thermal barrier not the attic floor.

The attic becomes part of the condition envelope and any a/c losses through ducts are retained in the envelope along with leakage pathways through attic into house.

If you want to know how to properly insulate and improve the efficiency of you house there is no better place than Building Science they know what the are doing. This is the specific research report on unvented attics:RR-0301 and here is a very detailed article on attic venting:Understanding attic ventilation

In a 100 years no houses will have vented attics, it would be like having a dirt floor now.

 

[Chakara]

Almost makes me happy to have a swamp cooler on the house. Now the attic fan I want will decrease the need for other air outlets.....

-Chak

 

[Kevin L]

Attic venting started and was incorporated in building codes because of cold climate ice damming. It has very little benefit in hot climates, with large volume venting the heat load might be reduced 10%, while losing 10-20% is a/c ductwork losses.

The best way to improve thermal efficiency in the attic is do go with a fully sealed attic with spray foam on the underside of roof, making the roof the thermal barrier not the attic floor.

The attic becomes part of the condition envelope and any a/c losses through ducts are retained in the envelope along with leakage pathways through attic into house.

If you want to know how to properly insulate and improve the efficiency of you house there is no better place than Building Science they know what the are doing. This is the specific research report on unvented attics:RR-0301 and here is a very detailed article on attic venting:Understanding attic ventilation

In a 100 years no houses will have vented attics, it would be like having a dirt floor now.

Exactly the way my home is insulated with Icynene.

+1 for buildingscience.com. The best site for understanding this subject.

Kevin

 

[apostolakisl]

Attic venting started and was incorporated in building codes because of cold climate ice damming. It has very little benefit in hot climates, with large volume venting the heat load might be reduced 10%, while losing 10-20% is a/c ductwork losses.

The best way to improve thermal efficiency in the attic is do go with a fully sealed attic with spray foam on the underside of roof, making the roof the thermal barrier not the attic floor.

The attic becomes part of the condition envelope and any a/c losses through ducts are retained in the envelope along with leakage pathways through attic into house.

If you want to know how to properly insulate and improve the efficiency of you house there is no better place than Building Science they know what the are doing. This is the specific research report on unvented attics:RR-0301 and here is a very detailed article on attic venting:Understanding attic ventilation

In a 100 years no houses will have vented attics, it would be like having a dirt floor now.

Exactly the way my home is insulated with Icynene.

+1 for buildingscience.com. The best site for understanding this subject.

Kevin

Me too. I can't recommend this approach highly enough.

I posted this a couple months ago, but i want out at 5pm on a 105 degree sunny day with my IR thermometer. The roof measured 145 degrees. The attic ceiling (the spray foam surface) measured 85 degrees. The attic air was about 80. My roof complex (tiles, sheathing, OSB, Spray foam) was holding a 60 degree differential. This is with the attic being indirectly cooled by the house below it.

Curious about the ice damn issue being the reason for attic venting. Here the motive was actually to let heat out of the attic . . .in the WINTER! Make the attic colder so the snow doen't melt and then refreeze into damns. As far as efficiency, that is shooting yourself in the foot!