Do power attic fans help?

[ampulman]

Wow: that was quite a dissertation--thank you.

A little more than I can absorb at one time, so your post has been memorialized as a Word document entitled: Shophound's Roof.

Is that device known as an IR thermometer is it sensitive enough to detect cold spots?

Amp

[Shophound]

Wow: that was quite a dissertation--thank you.

A little more than I can absorb at one time, so your post has been memorialized as a Word document entitled: Shophound's Roof.

Is that device known as an IR thermometer is it sensitive enough to detect cold spots?

Amp

Hot dog I made the big time..."Shophound's Roof".

The IR thermometer should pick up cold spots as well as hot spots. It is reading the surface temperature of whatever you aim it at. While not accurate for certain applications, it is plenty informative for sleuthing around a house when there's a significant temperature difference between indoors and outdoors.

Here it is 4 PM local time. Outdoor temperature 96 degrees, attic temperature 128 degrees, ceiling temperature over my head averaging 76 degrees, room air temperature 75 degrees, average surface temperature of single pane, aluminum frame window to my left is 88 degrees. In essence I have a hot attic overhead that is 32 degrees warmer than outdoor ambient, but is transferring less heat into the room than the window to my left.

[energy_rater_La]

very well done shophound.
seems I'm always telling folks that the windows
are the weakest part of the wall.

carnuk..I didn't do option 4 because we were talking ventilation
non vented attics are recommended for resistance to uplift here
in hurricane alley. I'm just learning to apply to outside of my climate..
but while we are here..
option 5 nonvented attic, foam on attic floor,with foam sealing
attic floor to soffits to roofdecking.
ducts in attic with radiant barrier.
not as costly as foam roofdecking (depending on pitch of roof this can be very costly) but air seal on attic floor, insulation value with foam - or foam combo..(some people install 2" foam with blown or unfaced batts on top). ducts are consitered "under insulation" with RB. (not to be confused with ducts buried in insulation, which we can't do here due to humdidty).
while the benefit is not as great with this install as with foam roof decking, there is a middle..and this is it..before just insulating attic floor with conventional insulation.

boy this is kinda fun...what if we built a sips house...sips roofline,
sips walls, sips floors. then we put ductwork inside the building envelope I'd bet you could cool 3000 sq ft with 3 tons of a/c
..with the right windows.

[Carnak]

vnon vented attics are recommended for resistance to uplift here
in hurricane alley.

I think it is dumb to give the storm "an in" myself.

Even if it is just soffit vents, it gets like blowing steam, the water defies gravity will go up in those soffits, soak ceiling insulation and bring the whole ceiling down before the roof ever failed.

But if there were no hurricanes in Louisiana, there is still no reason to vent an attic, it is a strategy to stop icycles

[mark beiser]

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.

This is pretty consistent with the results I got when I did a spray on barrier in my last house. The temperature in the attic stayed pretty flat until mid morning, peaked mid afternoon, then fell quickly as the sun set, and was at/near ambient before midnight. The peak temp was about 20-25º lower than before the spray on barrier.

In the same 100º weather and sun conditions before spraying, the attic temp climbed quickly shortly after sunrise, stayed in the 150º range from around 1pm until the sun actually started to set, then cooled slowly. It would still be rather warm in the attic at midnight.

I actually sprayed the barrier on in the middle of July a few years ago when we had a long string of virtually identical cloudless 100º days, and data logged the attic temperature for 2 weeks before and after spraying.

Unfortunately the logs died with my computers hard drive not long after recording.
No changes to ventilation were made when the spray on barrier was put in.

I have all these plans for recording with multiple sensor location and types on my current house, but so far I've been way to lazy to work on it, lol.
Every time I get up in my attic to do much of anything, my spirit is quickly broken by the fact that this house is built with an annoying truss system, and has a lowish pitch roof.

[pstu]

This is pretty consistent with the results I got when I did a spray on barrier in my last house. The temperature in the attic stayed pretty flat until mid morning, peaked mid afternoon, then fell quickly as the sun set, and was at/near ambient before midnight. The peak temp was about 20-25º lower than before the spray on barrier.

In the same 100º weather and sun conditions before spraying, the attic temp climbed quickly shortly after sunrise, stayed in the 150º range from around 1pm until the sun actually started to set, then cooled slowly. It would still be rather warm in the attic at midnight.

I actually sprayed the barrier on in the middle of July a few years ago when we had a long string of virtually identical cloudless 100º days, and data logged the attic temperature for 2 weeks before and after spraying.

Unfortunately the logs died with my computers hard drive not long after recording.
No changes to ventilation were made when the spray on barrier was put in.

I have all these plans for recording with multiple sensor location and types on my current house, but so far I've been way to lazy to work on it, lol.
Every time I get up in my attic to do much of anything, my spirit is quickly broken by the fact that this house is built with an annoying truss system, and has a lowish pitch roof.

I am curious about just how you measured the temperature. In particular where did you take the measurement. Over time I have had 3 different locations measured, and have found that each reports a different number. One is a wired probe just above the insulation in a closet, another is a wireless weather sensor hanging a foot below the radiant barrier, and the last one is on the hot side of the RB.

Thank you -- Pstu

[paul42]

boy this is kinda fun...what if we built a sips house...sips roofline,
sips walls, sips floors. then we put ductwork inside the building envelope I'd bet you could cool 3000 sq ft with 3 tons of a/c
..with the right windows.

You don't need sips to do that.

2 ton heat pump, N. Central Texas
2,230 sq. ft. living space.
1,800 sq. ft. conditioned walk out basement - 85% above grade.
According to HVAC calc, largest heat gain is through the walls at 4,500 BTUH
windows contribute 2,100 BTUH.
R49 cellulose in the attic with radiant barrier sheathing, 1,500 BTUH

inexpensive low-e vinyl double pane windows - completely shaded in the summer.
all the windows are either casement or fixed to keep infiltration to a minimum
All the duct work is in conditioned space in the basement.

The nice thing about all of this is that we can ignore the cost of setting the thermostat to what is comfortable for us. Our average monthly heating / cooling cost is about $50 a month at $.14 Kwh. We keep the thermostat at 74 degrees and 42% humidity in the summer.

[Shophound]

I am curious about just how you measured the temperature. In particular where did you take the measurement. Over time I have had 3 different locations measured, and have found that each reports a different number. One is a wired probe just above the insulation in a closet, another is a wireless weather sensor hanging a foot below the radiant barrier, and the last one is on the hot side of the RB.

Thank you -- Pstu

The numbers will likely vary with the sensor location, but I think the pattern both Mark and I reported seeing would hold true even with the sensor placed at various locations in the attic. What I have seen so far in my attic (and am pleased to read Mark's similar experience) has caused me to reevaluate how the spray-on radiant barrier performs. While I imagine it has reduced peak solar gain for each daily cycle, what I did not anticipate was how it also appears to assist nightly cooling of the attic.

Yesterday I was home a good part of the day, waiting for departure time for the airport to arrive for a much needed vacation. My attic sensor behaved as Mark posted about his...temperature stayed pretty flat after sunup (and in the seventies, no less) until mid to late morning, where it then began rising. I have been in many attics at mid to late morning hours in my time in summer, and they weren't even close to being that comfortable. Between 11 and noon the temp was over 100, and peaked at 128 with outdoor ambient at 96-97. The highest temp I saw last summer was during our hottest day of the year, where the outdoor ambient was 105 and the attic was 135.

At the attic's peak temperature yesterday, ceiling surface temperatures averaged 76 degrees throughout most of the house, with my remaining single pane windows doing much worse, with surface temps around 88, even in deep shade. The low e double pane sliding doors were about the same surface temperature as the ceiling.

[Garyl53]

Hi,
What is the recommendeed ventilation with a cement tile roof system. Currently I have gable vents with some soffit venting. Is it possible to install ridge vents with a tile roof?
Thx,
Gary

[elkhornsun]

Costco is selling a solar powered attic fan assembly all in one unit ready to place on a roof for $300. The solar power takes away the need to run electrical though it also means the fan is going to be effective only during daylight hours so if there is any heat load being transferred from the house to the attic the fan will not be extracting it during the night.

It is not unusual to have poorly installed or incomplete flashing for ducting that ends up pulling attic air into the house which is not good in the winter or the summer and one of the first things I check when installing a new HVAC system for a customer.

[dan sw fl]

Costco is selling a solar powered attic fan assembly all in one unit ready to place on a roof for $300. The solar power takes away the need to run electrical though it also means the fan is going to be effective only during daylight hours so if there is any heat load being transferred from the house to the attic the fan will not be extracting it during the night.

The attic fanshoudl be able to run at least 14 hours a day which should be more than adequate. Any battery capacity on this unit?

[dan sw fl]

as a Word document entitled: Shophound's Roof.

Is that device known as an IR thermometer is it sensitive enough to detect cold spots?

Definitely, my beer is 34'F.

[Joe Mascitti]

Just wanted to throw this out here...I was a firm believer in PAV's until reading along on this forum. I have done a lot of work over the winter in the attic. Adding insulation...making sure all soffet vents were not blocked. Also, my roof has many ridges, all have ridge vents. Taking the advice from people on this forum, I have added an attic tent, caulked all the penetrations through the top plates and disconnected my PAV. Today it was 96* and sunny...my attic temp reached only 110* Now it is currently 88* outside, my attic temp is also 88*. My upstairs Temp was 76* throughout the day, although my upstairs AC runs contantly from 1pm till 10pm, the humidity is very low. Every pro I have spoken with saids this is normal for a properly sized unit. I have 2 high efficency units(Ruud, 3 ton upstairs 2 ton downstairs) and cannot complain about my electric bill which runs about $50-$60 more than my electric bill in the winter. With my PAV, I could not achieve a temp below 78*. Thanks to all on this forum who know what they are talking about.

Ask yourself this. Can a sealed attic with a PAV achieve only a 14* temp difference? Or is a well ventilated attic better (and no $$$ wasted on running the fan) Ditch the PAV

[Carnak]

if you sealed your soffit vents and went with something like icynene under the roof deck your attic would be 78F

[Joe Mascitti]

I have heard that icynene is great stuff...very expensive...would you remove the ridge vents as well?

[Carnak]

if you seal the attic you seal it
no ridge vent

if it is gabled insulate the gable also

[Joe Mascitti]

How does it preform vs. radient barrier?

[Carnak]

Sealed attic is like wearing a condom, radiant barrier is like taking valtrex

You vent an attic to prevent icycles.


Now think about it in cooling.

An attic is a solar collector, the sun beats on it and it gets way hotter than the outdoor air. Then the vents let in humid air. So you have basically have a hot suana above you.

Now when you seal the attic and insulate the roof deck, you are reducing the solar heat getting in. Radiant barriers make a difference but inside the attic is not the best place for them. If you want something to reduce the solar radiation, put it on top of the roof, it is going to reflect away heat in the first place. Then with insulation under the roof the attic will not be much warmer than the air conditioned space.

I built a fourplex in hurricane alley. I put the insulation on top of my roof deck, then white roofing above it. It is a gabled roof, so I insulated the gable walls on the inside.



I only have approximately an R7 roof, so it averaged 81F up there in the attic. Then I conditioned it with about 30 CFM, stays about 79F. I think if you had your AC ducts up there, the duct leaks would suffice

After a major hurricane here a lot of places had new roofs. A common retrofit was about 6 inches of icynene, so must be roughly equivalent to R19 or so. Typical temperature up there was 78F

I am in the tropics, been in the attics where they draped bubble wrap under the trusses then nailed down the sheating above the bubble wrap. One had a continuous perforated soffits and then R19 at the bottom of the trusses. Low/mid 80s and humid as hell in there. Was up in one particular attic like that as part of a major investigation into high humidity, When a couple whole house dehumidifiers could not keep up. Culprit was a major return duct leak, drawing in that humid attic air.

Dehumidifers were wired to run the air handler fans to circulate the dry air. Unfortunately the system with the big return leak was one wired to run with the dehumidifier. Turned out to be a 4 ton drawing in 25% of its air from the attic

[Shophound]

How does it preform vs. radient barrier?

Bottom line is this...if you have ducts in the attic, which many homes do, icynene on the roof deck beats radiant barrier hands down. Radiant barrier will keep an attic cooler than without, but a foamed roof deck and sealed attic makes the attic capable of being conditioned space, and not much variance from indooor ambient without it.

For those owning homes with marginal attic insulation on the attic floor, ducts in the attic, and a budget that excludes foaming the roof deck, you should bear in mind that radiant barrier yields higher energy reduction with marginal insulation on the attic floor than with higher levels of insulation. The spray-on radiant barrier in my own home has at best an emissivity of .22, compared to foil based products better performance at .05. The company who installed the barrier also increased attic insulation to ~R40, which in my opinion is where the largest reduction of heat transfer from attic to house occurred. The radiant barrier helps the ducts deliver more capacity from the cooling coil.

When you crunch some heat transmission numbers, using a formula like U factor * area * delta T, and measure known R values of various walls and ceilings in your house, it often becomes clear where the largest sources of heat gain are to a house in summer. Transfer from the attic to the house through the ceiling, while noteworthy, is often considerably less than through single pane aluminum frame windows, which went into so many homes over the past forty years or so. Even a shaded single pane job can actually contribute more heat load to a room than the wall and ceiling surfaces combined.

[Carnak]

The big gains are the glass and the infiltration or ventilation. Elite spits out a pie chart, pretty good visual. Really emphaszied the window issue like you did on another thread.

But still, when you look at numbers like that, the attic is a big part of the infiltration, and air from a vented attic will have a lot more enthalpy than 'your design dry bulb and wet bulb'

My office has the worst scheme possible for an attic here. If you pop a t-bar tile, and get lucky, you see some R-19 friction fit between a couple trusses. The attic is vented, black shingles and a black gable. I am on the west end of the building.

I was missing a lot of batts and a quick fix was the big sin, staple bubble wrap acorss the bottom of the trusses. Big difference, no penalty from dust yet.

I should tape some black tape on the under side of the bubble wrap, and then apply some black tape on the under side of the R19 in an adjacent truss space.

Then get the temperature of the tape with an infrared thermometer.