Do power attic fans help?

[Shophound]

I live in Lousiana and during the summer my AC runs constantly on hot days. This year I had the AC system checked and all checked OK except for the fact that my attic is very hot (over 130) even though I have ridge vents and soffit vents. I think that my soffits vents area is sufficient but suspect that the ridge vent area is lacking because my home has four hip roofs(short ridges). I was considering installing a power vent (recomended by one AC contractor)but after reading here I have the feeling that installing wind turbines may be a better option. I have seen some homes in my area with a combination of ridge vents and wind turbines. I am also not sure how many turbines I should install. Can there be to many? Any thoughts and help would be appreciated?

In all honesty, I think best bang for the buck is THICK insulation on the attic floor, and AIRTIGHT ductwork if it is in the attic. Bottom line. Unless you insulate the roof deck, or unless you employ a radiant barrier with a 99.xx% radiant heat reflectivity, a ventilated attic will not stay near ambient temperatures, which is the ideal goal of any attic ventilation scheme.

For much residential construction, the largest contributors to heat gain into the structure (which determines the quantity and capacity of cooling needed to keep the interior comfortable) are windows and infiltration (leakage). A nominally insulated attic floor will keep heat gain from the attic to the interior below the percentages gained through single pane windows (particularly unshaded windows) and infiltration, Airtight, well-insulated ducts that run through the attic will assure the a/c system delivers more of its actual cooling output to the rooms vs. being lost to the attic via air leaks and heat gain into the ducts from the hot attic air and surfaces.

[Shophound]

To explore this idea of heat gain source distribution further, consider the following heat gain calculations to a 144 square foot room (12 x 12, a common bedroom size) with a standard eight foot ceiling and a six foot by four foot window, single pane glass aluminum frame, shaded. Above ceiling is R40 insulation in attic. Only one wall adjoins the exterior, and has an R value of 16. Attic temperature is 128 degrees. Outdoor air temperature is 90. Outdoor humidity is 40%. Indoor air temperature is 75 degrees at 50% relative humidity.

Heat gain via the ceiling with R40 insulation is 190 btu per hour (btu/hr)
Heat gain via the wall with R16 insulation is 64 btu/hr
Heat gain via the single pane window is 443 btu/hr
Heat gain (sensible) via infiltration @ .5 air changes/hr is 155 btu/hr
Heat gain (latent) via infiltration @ .5 is 143 btu/hr
Heat gain via two people seated in room at rest is 500 btu/hr

So...apart from the occupants of the room, which above factors contribute the most heat gain to the 144 square foot room? Answer: it's not the ceiling, nor the walls. Combined, these two equal less heat gain than that one single pane window! The one window adds almost as much heat to the space as two people. Even total infiltration lags behind the one window's sensible heat gain by 145 btuh.

Note that the attic to room temperature difference is around 53 degrees. The outdoor to indoor temperature difference is only 15. The one window transfers more heat for its small size, with a lower temperature difference, than 144 square feet of ceiling with R40 insulation underneath a 128 degree attic!

Bottom line: if you have money to improve your home energy use and comfort level, skip the powered attic fan, insulate your attic floor well, and replace crappy windows.

[rubberduck]

whirlybirds

[dan sw fl]

If anyone is interested I have HOBO data logger recordings of my semi-conditioned (formally known as attic) space in Excel format that I can email.

Use of PAV and semi-conditioned space discussion really confuses the real subject of this thread.

[dan sw fl]

To explore this idea of heat gain source distribution further, consider the following heat gain calculations to a 144 square foot room (12 x 12, a common bedroom size) with a standard eight foot ceiling and a six foot by four foot window, single pane glass aluminum frame, shaded. Above ceiling is R40 insulation in attic. Only one wall adjoins the exterior, and has an R value of 16. Attic temperature is 128 degrees. Outdoor air temperature is 90. Outdoor humidity is 40%. Indoor air temperature is 75 degrees at 50% relative humidity.

Heat gain via the ceiling with R40 insulation is 190 btu per hour (btu/hr)
Heat gain via the wall with R16 insulation is 64 btu/hr
Heat gain via the single pane window is 443 btu/hr
Heat gain (sensible) via infiltration @ .5 air changes/hr is 155 btu/hr
Heat gain (latent) via infiltration @ .5 is 143 btu/hr
Heat gain via two people seated in room at rest is 500 btu/hr

So...apart from the occupants of the room, which above factors contribute the most heat gain to the 144 square foot room? Answer: it's not the ceiling, nor the walls. Combined, these two equal less heat gain than that one single pane window! The one window adds almost as much heat to the space as two people. Even total infiltration lags behind the one window's sensible heat gain by 145 btuh.

Note that the attic to room temperature difference is around 53 degrees. The outdoor to indoor temperature difference is only 15. The one window transfers more heat for its small size, with a lower temperature difference, than 144 square feet of ceiling with R40 insulation underneath a 128 degree attic!

Bottom line: if you have money to improve your home energy use and comfort level, skip the powered attic fan, insulate your attic floor well, and replace crappy windows.

Thanks for the realistic presentation and recommendation for existing homes ! !!

Use foam insulation in new construction to make the attic semi-conditioned space.

[rnevil]

Thanks for your insight shophound. I currently have R19 insulation in the attic and if my research is correct the recomendation for our area is R49 so I should add an additional layer of R30. My windows are double insulated so hopefully they are not contributing to my problem. I have checked my ductwork for leaksand all are sealed tight. As far as the radient barrier I have found RB foil on line that is to be stapled to the underside of the rafters. Is this the best system you are aware of that can be added to an existing attic? Thanks again

[Shophound]

Thanks for your insight shophound. I currently have R19 insulation in the attic and if my research is correct the recomendation for our area is R49 so I should add an additional layer of R30. My windows are double insulated so hopefully they are not contributing to my problem. I have checked my ductwork for leaksand all are sealed tight. As far as the radient barrier I have found RB foil on line that is to be stapled to the underside of the rafters. Is this the best system you are aware of that can be added to an existing attic? Thanks again

From the website at this link:

http://www.ornl.gov/sci/roofs+walls/radiant/rb_02.html

Experiments with various levels of conventional insulation show that the percentage reduction in ceiling heat flow due to the addition of a radiant barrier is larger with lower amounts of insulation. Since the fraction of the whole-house heating and cooling load that comes from the ceiling is larger when the amount of insulation is small, radiant barriers produce the most energy savings when used in combination with lower levels of insulation. Similarly, radiant barriers produce significantly less energy savings when used in combination with high levels of insulation.

Another link with good info:

http://frontierassoc.net/greenafford...ntBarriers.pdf

In particular, the radiant barrier you select should have an emissivity rating of 0.05 or lower. Be sure you check this, and be sure the shiny side is installed toward the attic, not the roof deck.

[energy_rater_La]

rnevil,
there are 3 strategies for ventilation
each one stands alone..meaning there is good proven reason not to mix them.

1- gable end vents
2- soffit and ridge venting
3- soffit and turbines (not power attic ventilators)

when you mix these strategies, bad stuff happens.

southface inst. has an excellent air sealing guide downloadable as
a pdf file. very detailed pictures defining air leakage sites,
and best ways to seal them.

you should air seal leaks between attic and living spaces
before insulating. insulation does not air seal unless it is foam insulation.
air moving through insulation reduces its R-value. you can add
more insulation but it will not perform if you don't stop the
leakage sites.

I'm a big fan of radiant barriers.
southface also has a downloadable on this. I find it to be comperable to florida solar energy centers info.

best of luck.

[Carnak]

rnevil,
there are 3 strategies for ventilation
each one stands alone..meaning there is good proven reason not to mix them.

1- gable end vents
2- soffit and ridge venting
3- soffit and turbines (not power attic ventilators)

when you mix these strategies, bad stuff happens.

southface inst. has an excellent air sealing guide downloadable as
a pdf file. very detailed pictures defining air leakage sites,
and best ways to seal them.

you should air seal leaks between attic and living spaces
before insulating. insulation does not air seal unless it is foam insulation.
air moving through insulation reduces its R-value. you can add
more insulation but it will not perform if you don't stop the
leakage sites.

I'm a big fan of radiant barriers.
southface also has a downloadable on this. I find it to be comperable to florida solar energy centers info.

best of luck.

I like option 4, don't vent it, seal it up blow the foam on the underside of the deck, on the gable walls.

From the point of view of keeping the heat out, I would take a sealed attic with bubble wrap insulation only over any of those vented options. The heat part is peanuts, the problem is all the humidity and infiltration the vented attic causes in your home

[Carnak]

I have a white roof, it is about R7, it keeps heat out in the first place. Proably a 2F temperature differential through 2 air films and a 1/2" layer of sheet rock, that is the heat that makes it into the living space.

The air handler and ductwork up there is not being irradiated, and there is no mositure to sweat on it because it is sealed. The "10%" penalty for ductwork and an airhandler is reason enough to seal up the damn attic.

[Carnak]

If anyone is interested I have HOBO data logger recordings of my semi-conditioned (formally known as attic) space in Excel format that I can email.

I will trade you data adrian, address in my profile

[Carnak]

From the website at this link:

http://www.ornl.gov/sci/roofs+walls/radiant/rb_02.html



Another link with good info:

http://frontierassoc.net/greenafford...ntBarriers.pdf

In particular, the radiant barrier you select should have an emissivity rating of 0.05 or lower. Be sure you check this, and be sure the shiny side is installed toward the attic, not the roof deck.

What I do not get is the mentality in trhe second link. If it does not instantly pay back it is deemed not to be a good investment.

I believe it is mainly radiant heat in an attic, and radiant heat is based on temperature to the power of 4. So the other thing I do not get, is why let that roof deck get that hot in the first place so that it radiates like hell?

[Shophound]

What I do not get is the mentality in trhe second link. If it does not instantly pay back it is deemed not to be a good investment.

I believe it is mainly radiant heat in an attic, and radiant heat is based on temperature to the power of 4. So the other thing I do not get, is why let that roof deck get that hot in the first place so that it radiates like hell?

I reread the second link and didn't see what you were referring to. Maybe I missed it, but I skimmed over it several times.

I agree about the folly of letting a roof deck cook the crap out of an attic, and ducts mounted in an attic. My own home has the spray-on radiant barrier. It does not have as good of an emissivity number as the foil based RB products do, but in all honesty I think the biggest difference it makes is reducing heat gain to my supply ducts in the attic. The supply ducts are round metal wrapped in fifty year old foil faced fiberglass. I do not ever feel a hot blast of air come out of the registers when the a/c kicks on after being off for awhile, and I do not read a considerable sensible temperature gain between the supply plenum and supply diffusers in adjacent rooms.

Yesterday the attic reached 128 with 94 degrees outdoor ambient air. The insulation level is probably around R-40. I went around with my IR thermometer, shooting the ceiling surfaces throughout the house. Nothing read over two degrees above room temperature except in the master bath, which has no supply register. I measured several single pane aluminum frame windows remaining in the house, sat down with the known u factors of my ceiling plane and single pane windows...the percent of gain from the ceiling vs. the window was astounding. Since my house has three foot overhangs the entire perimeter of the structure, most of the windows are in deep shade most of the day. Even so, a single pane aluminum frame window is just an ass kicker when it comes to heat gain vs. a well insulated ceiling plane with spray-on radiant barrier on the roof deck.