Do power attic fans help?

[adrianf]

Since when did tract home builders in Texas ever think of doing anything smart when it comes to HVAC and building thermal envelope design?

Granted, most builders came of age in an era of cheap energy. Throw the ducts and equipment up into a superheated attic. Skimpy overhangs on south facing facades. A building envelope punctured like swiss cheese. Lofty interior spaces difficult to heat and cool due to gross stack effect. A roofline more complex than college algebra. An entry that could allow a giraffe to comfortably fit in. All done so when the potential buyer rolls up to the curb, he says, "Wow! I want that house!"

Gotta love it!

/rant

It's not restricted to the industrialized builders. We've had multi-million dollar custom homes fail. One in particular that got demoed had standing water turned mold colony in the elevator shaft. There was some issue with water (condensation not sure) anyway someone had put pie tins on the floor trusses to catch the water.

Even without failure they are a challenge: multiple washer and driers, bathrooms that look like car washes, 6 and 8 burner gas stoves and multiple dish washers. Now couple all this with an envelop that is under pressure outside to in 90 percent of the time. Oh yeah they're from up North and want to keep the house at 68F

[mark beiser]

Though my post was quoted you did not respond to it.

I'll pose the question again. Isn't reducing the PAV motor speed better in most cases than disconnecting it?

Brian

There isn't a way to reduce the speed of the motor in a PAV, it is either on or off.
Off is best.

Natural ventilation has no operating/repair cost.

[RIZZO5150]

Sweeping generalizations aside....my actual experience:
In my Los Angeles house (1600 square feet), I sealed all the penetrations very well, especially when I installed a new ceiling. The attic is very well insulated R30: w. vapor barrier taped.
I installed two commercial quality sidewall gable vents with gravity dampers from Grainger. On the opposite side of the house I installed two oversized gable relief vents as well as eave vents on the half of the house farthest away from the gable fan. I installed a Honeywell remote bulb t-stat (100-220 degrees IIRC) and a solid state fan speed controller in the furnace closet. I leave the controller set to about 130 degrees and the speed at about 20hz. It works great. In the summer when I forget to turn it on when I go to work the house is over five degrees warmer upon my return that evening.
I also installed a relay so that when my whole house fans ( quiet cool fan dot com) come on, the gable fans bang on a full speed and help boost the air changes.
It's a great system, but honestly it takes a lot of interaction, that Mrs. Rizzo could not do.

[Brian GC]

Keeping the term "logic" in mind, which approach is more logical?

• Install radiant barrier on underside of roof deck to minimize radiant heat gain to an attic. Radiant heat is said to be over 70% responsible for heating the insulation laying on the attic floor, which then transfers that heat to the conditioned spaces below via conduction and radiation
• Do not install radiant barrier, but rather rely on a powered attic ventilator to reduce attic air temperatures. Heated attic air is said to be under 30% responsible for heating the insulation laying on the attic floor, which then transfers that heat to conditioned spaces below via conduction and radiation

Logic would dicate the radiant barrier solution provides higher return on investment. It is a once and done proposition. The PAV needs an ongoing power source to be useful. When it is not running, it is useless. The radiant barrier never shuts down, it is always available to work, with no ongoing consumption of energy.

Appealing to logic, does reducing a PAV's CFM output from 1,200 to 300, either with radiant barrier or without it installed, make sense for increasing the PAV's effectiveness at reducing that ~30% heat gain from superheated attic air? Earlier in this discussion either I or someone else crunched numbers at what volume of air would be needed to keep an attic at outdoor temperatures on a, say, 100 degree day. The amount of air needed was tremendous. Reducing the airflow to reduce air removal from the conditioned space would be countered by the increased heat gain the attic would experience with less ventilation, resulting in greater heat gain to the attic floor insulation and structural members.

Emotions do not always get us into trouble. Combined with logic, they give us the push to see a matter through, and to do it well. Animal kingdom logic is eat or be eaten...as humans, we can do better than that.

I’ll start by saying that my first choice is a turbine fan. They’re cheap, free to operate, do not overdraw, easy to install, on all the time, and are easy to service.

I have never been in an attic with radiant heat barriers but off the top of my head I’d say it would be difficult to install in residential attics. If the +70% figure is correct, then if you took two 100 degree days, one sunny and one overcast, you’re saying the attic on an overcast day (no radiant heat) would be 70% cooler. I do not think so. Or, are you saying the attic will have 70% less effect on the house. Also, I don’t think so. IMO, two 140 degree attics with and without radiant heat would be <20% apart.

IMO, most of the heat is due to lack of ventilation. Radiant heat is a source but not the main cause of the ambient temp. Like an oven, what causes it to get hot, the hot burners or shutting the door? Answer; shutting the door or ‘lack of ventilation’. Decreasing the heat source and ventilation compared to increasing the heat source and ventilation might be a wash, but as far as ambient temp. goes, my money would be on the turbines. Without ventilation, a little heat goes a long way.

A radiant barrier would work against you in the winter when you want solar heat. And, would reflecting that heat back at the roof causing excessive heat and wear on the roofing material.

Nobody is trying to keep attic air at an outside temperature, they’re just trying to reduce it 20 degrees or so.

Logic tells me to always question figures like the 70% / 30%. It may be a fact that is more applicable in places other than attics.

Brian

[adrianf]

....

[paul42]

IMO, most of the heat is due to lack of ventilation. Radiant heat is a source but not the main cause of the ambient temp. Like an oven, what causes it to get hot, the hot burners or shutting the door? Answer; shutting the door or ‘lack of ventilation’. Decreasing the heat source and ventilation compared to increasing the heat source and ventilation might be a wash, but as far as ambient temp. goes, my money would be on the turbines. Without ventilation, a little heat goes a long way.

A radiant barrier would work against you in the winter when you want solar heat. And, would reflecting that heat back at the roof causing excessive heat and wear on the roofing material.

Nobody is trying to keep attic air at an outside temperature, they’re just trying to reduce it 20 degrees or so.

Logic tells me to always question figures like the 70% / 30%. It may be a fact that is more applicable in places other than attics.

Brian

Stand outside on a nice hot summer day. The air temperature in the shade is exactly the same as the air temperature standing in the sun. Radiant heat from the sun makes all the difference in how hot you feel - and in heat gain.

The biggest impact on shingle temperature is the color of the shingles. Radiant barriers raise the temperature two to five degrees. Going from white shingles to black shingles raises it by tens of degrees.

[paul42]

I’ll start by saying that my first choice is a turbine fan. They’re cheap, free to operate, do not overdraw, easy to install, on all the time, and are easy to service.

IMO, most of the heat is due to lack of ventilation. Radiant heat is a source but not the main cause of the ambient temp. Like an oven, what causes it to get hot, the hot burners or shutting the door? Answer; shutting the door or ‘lack of ventilation’.


Brian

Unless you stop the radiant heat, adding some ventilation to an attic just converts it from an oven to a broiler. You are going to cook either way.

[Shophound]

I have never been in an attic with radiant heat barriers...

You may wish to do the following, all on the same day. Go to a new housing tract where the homes being built use radiant barrier that is integral to the underside of the roof decking, and go into the attic. Preferably in a house that has progressed beyond framing, and has the drywall and insulation in place. Measure the temperature in this attic.
Next, go find another house nearby, on the same day, where the builder has chosen not to use a radiant barrier product of any kind on the underside of the roof decking. Measure the temperature in this attic and then compare it to the one that had radiant barrier (RB). You will note a significant difference.

An experience similar to what I just described above was my first introduction to the effectiveness of radiant barriers. Admittedly some of my observations could be interpreted as subjective, since I was not measuring anything other than how "hot" I felt when going into the upstairs regions of homes that were in an advanced state of framing (roof decks on, windows in, exterior sheathing in place, but no drywall). That being said I could not help noting that the house with the RB had little noticable temperature difference between the upstairs and downstairs regions. The one that did not have RB, the moment my head rose above the level of the top floor, the heat became much more intense.

but off the top of my head I’d say it would be difficult to install in residential attics.

For residential new construction, it is as easy as making a different choice for roof decking. Opt for an OSB sheet with RB on one side...done. For existing residential attics, the spray-on paints are effective, but at reduced levels compared to the OSB/RB products. I have the spray-on product in my own home, and it is effective at reducing cooling demand and lowering attic temperatures.

If the +70% figure is correct, then if you took two 100 degree days, one sunny and one overcast, you’re saying the attic on an overcast day (no radiant heat) would be 70% cooler. I do not think so. Or, are you saying the attic will have 70% less effect on the house. Also, I don’t think so. IMO, two 140 degree attics with and without radiant heat would be <20% apart.

An overcast day is not an effective illustration to compare RB against PAV ventilation. A hot, sunny day would be more appropriate, since these are the conditions both RB and PAV's are designed to work against. My 70/30% ratio, admittedly rough, was a statement of breaking down total heat gain to an attic (heat gain to the house is a separate but related subject). 70% is from radiant heat gain, 30% is from the air in the attic itself becoming heated, which occurs primarily due to radiant heat! The air itself would not heat over outdoor ambient if it was not overwhelmed by radiant heat from structural members that have no radiant barrier in place.

IMO, most of the heat is due to lack of ventilation. Radiant heat is a source but not the main cause of the ambient temp. Like an oven, what causes it to get hot, the hot burners or shutting the door? Answer; shutting the door or ‘lack of ventilation’. Decreasing the heat source and ventilation compared to increasing the heat source and ventilation might be a wash, but as far as ambient temp. goes, my money would be on the turbines. Without ventilation, a little heat goes a long way.

Noone is suggesting that an attic with radiant barrier should not be ventilated. A radiant barrier with natural ventilation is, IMO, more effective than an attic with no RB but has a PAV installed. As for winter solar heating, radiant barrier is most effective in primarily cooling climates, where I and many other homeowners that frequent these forums live. Passive solar heating is a hit and miss affair, due to clouds, sun angle, nearby obstructions to direct sun (in winter the sun angle is low, even at noon, so a neighbor's two story house or an evergreen tree can inhibit solar heat gain), etc.

And, would reflecting that heat back at the roof causing excessive heat and wear on the roofing material.

In winter? Nonsense. The ambient air above shingles in winter is a far better heat sink for those shingles than in summer. The sun angle is also much lower in winter, days are shorter, air temperatures colder. The main wear on shingles in cooler months is rain, ice, snow, swings in temperature, and ultra-violet light during days that are sunny.

Shingles in summer have a less effective heat sink to ambient air, but any wind will help with heat transfer, along with natural drafts of heat rising off the shingles into cooler surrounding air. With a cooler attic below due to radiant barrier, the undersides of the shingle aren't being as cooked as they would be with a superheated attic, which provides little heat sink for the shingle undersides to lose heat to.

Nobody is trying to keep attic air at an outside temperature, they’re just trying to reduce it 20 degrees or so.

Attic temperatures at outdoor ambients are the ideal. If RB and natural ventilation can get you close, you're doing very well. If the best a PAV can do is get you 20 above ambient, vs. RB and natural ventilation getting you 10 above, which is more viable...and uses less energy to boot?

Logic tells me to always question figures like the 70% / 30%. It may be a fact that is more applicable in places other than attics.

I admit I was recalling those figures from memory. If I find the source where I read that from, I will post it. Logic dictates that any posted figures are bolstered by published data.

Logic also dictates such figures be correctly understood in context, and properly applied.

[Shophound]

Sweeping generalizations aside....my actual experience:
In my Los Angeles house (1600 square feet), I sealed all the penetrations very well, especially when I installed a new ceiling. The attic is very well insulated R30: w. vapor barrier taped.
I installed two commercial quality sidewall gable vents with gravity dampers from Grainger. On the opposite side of the house I installed two oversized gable relief vents as well as eave vents on the half of the house farthest away from the gable fan. I installed a Honeywell remote bulb t-stat (100-220 degrees IIRC) and a solid state fan speed controller in the furnace closet. I leave the controller set to about 130 degrees and the speed at about 20hz. It works great. In the summer when I forget to turn it on when I go to work the house is over five degrees warmer upon my return that evening.
I also installed a relay so that when my whole house fans ( quiet cool fan dot com) come on, the gable fans bang on a full speed and help boost the air changes.
It's a great system, but honestly it takes a lot of interaction, that Mrs. Rizzo could not do.

That's a heroic effort at powered attic ventilation, for sure! You are the exception to the rule, taking the extra steps necessary to isolate the conditioned spaces from the non-conditioned attic spaces.

That being said, if you could have significantly reduced heat gain to an attic that cost you nothing more than the initial product installation, with no ongoing maintenance or energy consumption needed, would you opt for that over your present arrangement?

[adrianf]

Unless you stop the radiant heat, adding some ventilation to an attic just converts it from an oven to a broiler. You are going to cook either way.

Actually it's a convection oven for even cooking.

[adrianf]

Joe has a Phd in building science. If you ever have a chance to attend one of his presentations you owe it to yourself, your industry and your customers to do so.

http://www.buildingscience.com/bsc/topten/south.htm
http://www.buildingscience.com/bsc/topten/north.htm

[Shophound]

Joe has a Phd in building science. If you ever have a chance to attend one of his presentations you owe it to yourself, your industry and your customers to do so.

http://www.buildingscience.com/bsc/topten/south.htm
http://www.buildingscience.com/bsc/topten/north.htm

I enjoy his monthly contributions to the ASHRAE Journal. Always informative and enlightening.

[energy_rater_La]

he gave several classes here in 06 bringing information for us to build and withstand the hurricanes, most of his info was based on the studies that building science did in Florida who had two hurricanes the year before we had our two.
one of the biggest things that will allow homes to withstand hurricanes is to stop venting attics as a vented attic has much more uplift than an unvented.
I really enjoyed his talks..very entertaining man..and has common sense to go with thue
education..plus a sense of humor.
Anyone who has a chance to go to his classes should go...imo.

[Brian GC]

[QUOTE=shophound;1860171][COLOR=red]

An overcast day is not an effective illustration to compare RB against PAV ventilation. A hot, sunny day would be more appropriate, since these are the conditions both RB and PAV's are designed to work against. My 70/30% ratio, admittedly rough, was a statement of breaking down total heat gain to an attic (heat gain to the house is a separate but related subject). 70% is from radiant heat gain, 30% is from the air in the attic itself becoming heated, which occurs primarily due to radiant heat! The air itself would not heat over outdoor ambient if it was not overwhelmed by radiant heat from structural members that have no radiant barrier in place.


I misunderstood the 70/30% statement. I thought you meant radiant heat from the roof was directly penetrating the attic insulation and making it perform at 30%. Radiant heat does superheat a surface and makes temperatures go sky high, but are you also saying that an attic would not rise above ambient on an overcast day if the ventilation was inadequate? Wouldn’t an interior of a car parked in the shade rise above ambient?

Radiant heat is definitely a major cause of attic heat, just like color of shingles are, but poor ventilation allows that air to sit there and bake. Constantly recycle that air with turbines or a low speed/power PAV and the result of radiant heat has been diminished.

But with all that said, radiant barriers are very logical.

Brian

[Shophound]

[quote=Brian GC;1861209]

[color=red]An overcast day is not an effective illustration to compare RB against PAV ventilation. A hot, sunny day would be more appropriate, since these are the conditions both RB and PAV's are designed to work against. My 70/30% ratio, admittedly rough, was a statement of breaking down total heat gain to an attic (heat gain to the house is a separate but related subject). 70% is from radiant heat gain, 30% is from the air in the attic itself becoming heated, which occurs primarily due to radiant heat! The air itself would not heat over outdoor ambient if it was not overwhelmed by radiant heat from structural members that have no radiant barrier in place.


I misunderstood the 70/30% statement. I thought you meant radiant heat from the roof was directly penetrating the attic insulation and making it perform at 30%.

I went back to see where that 70/30 figure came from...it was from adrianf's post a page or two back, where he said either FSEC or Advanced Energy stated that 80% of heat gain to attic floor insulation is due to radiant heating, with 20% from convective currents from the roof deck.

The more I study this subject, the more "cathedralized" attic systems make sense, whereby the roof deck is insulated and the attic is sealed from the outdoors. The attic could then be considered conditioned space, and fed a small amount of supply air to keep it positively pressurized. Or...it could have supply and return, IMO, in order to keep humidity under control. As a service technician, working in such an attic would be a dream. 105 outside, 80 to 85 in the attic. Sure beats 130 or worse!

Practically, however, most homeowners are not economically positioned to retrofit their attics to a cathedralized approach. Therefore the discussion returns to what's the best approach for conventional attic systems with attic floor insulation, soffit vents, and either ridge vents, whirlybirds, or static vents. I still maintain increasing attic floor insulation to recommended R value for the region and installing roof deck RB is the way to go. That and getting penetrations between attic and house interior sealed, as much as possible. For PAV die-hards, do all of the above and then purchase a solar powered PAV.

Radiant heat does superheat a surface and makes temperatures go sky high, but are you also saying that an attic would not rise above ambient on an overcast day if the ventilation was inadequate? Wouldn’t an interior of a car parked in the shade rise above ambient?

The rate of heat transfer would be significantly reduced on an overcast day to an attic due to diffused line of sight between roof deck and the sun. I have been in many attics on overcast days...they do not heat appreciably until either the cloud deck thins to allow muted sunlight through, or the clouds disappear altogether.

Radiant heat is definitely a major cause of attic heat, just like color of shingles are, but poor ventilation allows that air to sit there and bake. Constantly recycle that air with turbines or a low speed/power PAV and the result of radiant heat has been diminished.

Heating of attic floor insulation via roof deck and structural member radiation is not significantly reduced by ventilation. In order for this to be true (and logical), the airflow volume would have to be quite high, enough to keep the actual roof deck cool enough to reduce radiant heat transmission to the attic floor.

Concurrently, unless the attic is to be cathedralized/sealed, it needs to be ventilated. It's a Catch 22. Don't ventilate the attic and use no RB, the attic bakes like an oven. Ventilate it naturally with no RB, the attic floor insulation loads up with radiant and convective heat during the day, reradiating and conducting that heat into the dwelling all night long. Ventilate with PAV and no RB, the PAV depressurizes the attic AND the house, whereby air conditioned air is lost to the attic. Install RB and don't ventilate, the amount of heat build-up is greater than if the attic is ventilated. Install RB and naturally ventilate nets reduced heat gain to attic floor insulation, and reduces transfer of air through ceiling penetrations to attic. Install RB and use a PAV concurrently, loss of conditioned air through ceiling penetrations is increased, possibly offsetting gains this approach is supposed to provide.

But with all that said, radiant barriers are very logical.

They are. Even more logical, for new construction, would be to cathedralize the attic so heat gains to cooling equipment and ductwork is minimal. Or...if a conventional attic MUST be used, get the equipment and ductwork OUT of the ATTIC! Use a little forethought, fur down the hallways, and run the duct there (why do hallways need stratospheric ceiling heights like the rest of the house must have these days?). Sacrifice some needless feature of the house and put the air handler in a mechanical room/closet where it belongs. Keep all that stuff out of the attic!

[adrianf]

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.

[skwsproul]

I put in a large PAV when I redid my roof and it sucks parden the pun. I will be putting in a ridge vent soon. My question is there a temperature that having both be beneficial.

[Shophound]

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.

Of course I'm interested. Email addy is in my profile.

[teddy bear]

I am also interested in viewing the data. Regards TB

[energy_rater_La]

I'd like to see it also..I'll make sure my email address is available also.
And...thanks!