Do power attic fans help?

[mark beiser]

I'd like to see it, email is in my profile.

[rnevil]

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?

[Znobyrd]

Mevil, sounds like you have alot of vents, good. I would definitely install the attic fan. I installed just one 1350 cfm gable fan for my 1250 square foot attic on the west end of the house. It has a thermostat, so it will turn itself on at 90 degrees, and off at 90 degrees. Alot of air movement up there. It works great, and the cooling bill showed it.

[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.

[Shophound]

I've also found that my attic cools off fairly quickly after sunset. The peak temperature occurs mid-afternoon, and begins tapering off toward sunset. It is now three hours past sundown and the attic temperature is now two degrees above ambient temperature, at 88 degrees.

Before observing this pattern I always thought attics took considerably longer after sundown to approach ambient air conditions. Perhaps more voluminous attics do, but my own attic, at least where my temperature sensor is, indicates otherwise. I may move the sensor deeper into the attic at some point to see if the pattern remains or changes.

[Carnak]

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.

my bad it was the first link, the DOE one

[ampulman]

I've also found that my attic cools off fairly quickly after sunset. The peak temperature occurs mid-afternoon, and begins tapering off toward sunset. It is now three hours past sundown and the attic temperature is now two degrees above ambient temperature, at 88 degrees.

That is amazing.

What do you attribute it to, ventilation setup, orientation, etc.?

I have full length soffit vents, front and back w/ ridge vents, and can't detect any airflow through the system. So after sundown, the heat radiating from the ceiling is relentless (R30 in ceiling). I'm at 40 degrees latitude.

When OD ambient decreases, shouldn't I expect a chimney effect (air pulled in through soffits and out the ridge)? Doesn't seem happen.

Amp

[Shophound]

That is amazing.

What do you attribute it to, ventilation setup, orientation, etc.?

I have full length soffit vents, front and back w/ ridge vents, and can't detect any airflow through the system. So after sundown, the heat radiating from the ceiling is relentless (R30 in ceiling). I'm at 40 degrees latitude.

When OD ambient decreases, shouldn't I expect a chimney effect (air pulled in through soffits and out the ridge)? Doesn't seem happen.

Amp

I am at 32° latitude. Here we sit less than a week away from the summer solstice, so the midday solar angle is nearly at its annual peak. This means opportunity for solar loading of an attic is also prime.

To answer your question pertaining to my attic's performance, you should know a few things about it, as you ask in your post. From mid-morning to mid-afternoon, the roof has a good amount of exposure to the sun. Afterward, trees to the west of my house begin shading the roof. This allows the attic to begin cooling down prior to sunset.

The house is oriented southeast to northwest. It is a single story mid-century modern with a three foot overhang the complete perimeter of the building. Roof is hip design. It has two whirlybirds and soffit venting. Spray-on radiant barrier paint on the roof deck underside, a blizzard of fiberglass blown-in insulation estimated at R-40. About every penetration between ceiling and attic throughout house has been air sealed. As energy_rater_la notes in another post, attic insulation is considerably less effective if the ceiling plane is not airtight to the attic and to the house.

The trees shading the roof partially explains the cooldown prior to sunset, but after sunset needs explanation as well. I will offer that the volume of my attic is not large. The house is 1800 square feet, the roof pitch either 4/12 or 5/12. It's not an attic you can stand up and walk around in. After sunset, there obviously is no more heating of the roof shingles from the sun. Instead of being a heat absorber, the roof deck and shingles are now a heat sink for the hot attic, transferring heat to a cooler sky. To what extent the soffit vent openings and whirlybird devices contribute to nighttime attic cooling I have not measured, but I wouldn't count them out. Nevertheless, one can't overlook how the roof itself must contribute to heat transfer in BOTH directions. Inward during the day when solar loading is high, outward at night when the sky above the roof is cooler than the attic volume underneath it.

The spray-on radiant barrier must also be considered, although at best it may have an emissivity number of .22 or thereabouts. Compare that to foil based products of around .05, and you can see that the spray-ons are not as effective at barring radiant heat transfer as the foil based products. Nevertheless, if it retards heat transfer over bare wood below a blazing hot roof during peak solar loading, it also does so as the sun angle declines and the roof receives more shading. After sunset, its theoretical the radiant barrier might actually be a bit counterproductive, in that it retards heat transfer from the hot attic via conduction through the roof deck (since it reflects radiant heat striking the barrier). This is partly why manufacturers, proponents, and installers of radiant barrier recommend attic ventilation alongside the barrier for the best effectiveness. I don't know if the nighttime cooling of an attic with a radiant barrier has ever been compared to a similar attic with similar solar loading without one, but it would be interesting data to look over.

Nevertheless, as you mention, one would expect a degree of natural ventilation in an attic at night via soffit inlets and ridge, gable end, or whirlybird outlets. I've been in some attics of homes built more recently that strike me as a ventilating nightmare. These homes have steep roofs with voluminous attic space beneath, and meager soffit/peak ventilation provision. More volume = more material that can be heated via radiant heat transfer. More heated material means more material that must be cooled via ventilation and nighttime radiant heat loss to the atmosphere.

If you are as curious about heat transfer through residential building envelopes as I am, get yourself one of these:



Home Depot has this one for around fifty George Washingtons. Walking around your house on a hot day with one of these, aiming it at ceilings, walls, windows, doors, etc. is an eye opening experience.

What I did this past weekend is observe my attic temperature, convert my ceiling's R value to U factor ( 1/R = U ) and then use the formula U factor times Area times Temperature Delta to estimate rate of heat transfer through my building surfaces in btu per hour per square foot. For Area, use the number 1. Once you obtain a number, multiply it by the square footage of the ceiling in a given room, and you can get an estimate of rate of heat transfer from hot attic or outdoors to your room. My calculations revealed that my remaining single pane, aluminum frame windows were transferring much more heat for their size, than the ceiling above, which has considerably more square footage and a much wider temperature delta between the hot side and the conditioned side. For example:

Ceiling of room measures 12 x 12 (144 square feet). R value above ceiling is 40, which translates to a U factor of 0.025. Window in room measures 4 x 6 (24 square feet) and is single pane, aluminum frame. ASHRAE Handbook of Fundamentals states U factor for a window of this type is 1.23 (!). This window is shaded all day for ease of illustration. Temperature outdoors is 90 degrees, indoors is 75. Temperature in attic is 130.

Heat transfer from attic into room from ceiling = .025 x 55 x 144 = 198 btuh.

Heat transfer through window into room = 1.23 x 15 x 24 = 443 btuh.

That's a mind blower! We spend so much time here discussing the merits/demerits of attic ventilation, but gloss over the dirty rotten bandits of single pane windows with conductive frames! Also note that the attic to room temperature delta was 55 degrees, whereas the outdoor to room delta was only 15. Again, all this worry about ventilating attics, and right in front of your nose is the worst bandit of all, your windows.

To further illustrate, let's take the same window opening and improve its U factor to 0.30 and then rerun the numbers:

.30 x 15 x 24 = 108 btuh.

Holy cow! The window now contributes less heat load to the room than the ceiling!

Now, let's bump the attic heat level up to 150 (say it is poorly ventilated and has no radiant barrier) and rerun the numbers:

.025 x 75 x 144 = 270 btuh

That still outperforms the single pane aluminum frame window by a comfortable margin.

Conclusion: thick attic insulation and good windows make for a comfortable house (in addition to controlling other factors regarding the building envelope).