Do power attic fans help?

[Brian GC]

Then you roll right into making a broad recommendation for a retrofit to homes you haven't seen, one known to cause health, safety, and energy issues, and the only upside is a hypothetical "might offer some comfort improvement"? Wouldn't you agree it is more "irresponsible" to recommend something known to cause harm and little good than recommending against it?. THE POINT BEING: MESSING WITH YOUR HOMES SHELL AND VENTILATION CAN CAUSE REALLY NEGATIVE UNINTENDED CONSEQUENCES, AND IN THIS CASE THE UPSIDE IS SIMPLY NOT THERE!

My recommendation for attic ventilation is for specific homes just as your cautions against harmful effects should be for specific homes.

If AC is very seldom used, then spending money in an attempt to reduce that use further is pretty dumb, wouldn't you agree? I mean, if your total AC bill is $70 per year, how much are you going to spend to reduce that to $60? Your roof is for keeping weather out, the more holes you cut in it the more you risk it won't do it's job. This is a potentially hazardous "improvement" that does not save measurable energy or provide measurable comfort.

Money is not spend to further reduce cooling costs when they are zero to start with. It is spent to slightly reduce interior heat. A roof is also to keep sunlight from beating on your ceiling directly and to insulate the attic deck. Heat extraction is a form of insulation.

Here's a much less costly, less "spend some money, maybe this'll work" method. Offers greater control and is likely MUCH more effective: Box fan in a window in one side of the house, open window at the other. (Go ahead, open the attic hatch instead of the window.)

Closing all windows except two with box fans will not compare to all windows being open. Opening an attic access hole will allow hot attic air to enter the living space. Both are silly ideas.

[geoss54]

Hi,
So we decided that we are getting the attic air sealing, and adding up to R-49. The diff. between R-38 and 49 was not much, so we will do the 49. Also getting soffits cleaned out, and removing the single baffle and adding the double between each 24" rafter.
I can get away with this roof for another year-no leaking, roofer said some are starting to show the fiberglass a bit. There is a PAV fan there now that has a seized motor, but still provides access for air to escape, and there is a higher vent that is one of those small squarish one.
If I do get a new roof, they would take out the old PAV and other small vent, and put in a ridge vent. Really doing the roof now is preventative, but it is 21 years old!
Love to hear your comments/opinions.
Thanks
George

[Brian GC]

Then you roll right into making a broad recommendation for a retrofit to homes you haven't seen, one known to cause health, safety, and energy issues, and the only upside is a hypothetical "might offer some comfort improvement"? Wouldn't you agree it is more "irresponsible" to recommend something known to cause harm and little good than recommending against it?.

I have put clear conditions on when attic ventilation is safe and not safe. It is you that says that accelerated attic ventilation is always bad. You won’t even admit that your cautions only apply to tight homes with AC operating.

We are professionals and should know how to install something that is potentially dangerous. Framing, electrical, natural gas, and hand rails can kill people too, but it should be our jobs to know how to install them safely. No difference with attic ventilation. I would not give advice to a non-professional how to do my job. Heck, they would probably fall off the ladder or cut off their finger before they ever asphyxiated themselves.

[tedkidd]

Great George! What is the process for air sealing? Are they performing a blower door test before and after? Are they giving you a leakage reduction target/goal/minimum? You really need these measurements so you know how effective their air sealing efforts are!! Sometimes you get a challenging house and have to go back and do "blower door assisted" air sealing.

Brian, not sure why depressurizing a house is a good idea, but pressurizing it is a "silly" one. Because the idea is mine rather than yours? Actually, I'm sure I read it somewhere, so it's not really mine if that makes you feel better.

And, I actually use the box fan in the window trick. Set it on a timer to run all night and lock the house down during the day. The mass in 100 year old homes with plaster walls is pretty significant and effective heat sink. Not sure why you thinks it's a silly idea as it dramatically improves comfort, which is your standard of measurement.

I'm sorry, I didn't realize this thread was specific to a single home. If you started this thread, and were looking for some advice for a specific home, you should be aware that advice can't be given without going to the home, inspecting and testing. So, if this is a specific home, probably should stop talking about it.

I think sh's calcs show that ventilation in the hopes of removing much heat to save money is silly. Rather than accept that as good information, you struggle to prove this silly idea of yours has comfort merit? If it's going to save very little money with ac, how much comfort is it going to provide?

Tell me installing fans in roofs costs less than a cheap window shaker...

[Shophound]

Heat extraction is a form of insulation.

If that is really true, then air conditioning is also a form of insulation.

After all, it extracts heat!

[Mr Bill]

High John!

[Brian GC]

Ted,
Forums can be like a courtroom. Neither side will ever give credit to the other side for making a single factual statement. Johnny Cochran would never give Marsh or Chris any recognition for fear of loosing ground. It proves to be a winning strategy. So my question is…Who’s being Johnny?

[DavidNJ]

If that is really true, then air conditioning is also a form of insulation.

After all, it extracts heat!

Is active noise cancellation acoustic insulation?

insulation

the act of protecting something by surrounding it with material that reduces or prevents the transmission of sound or heat or electricity
Does cold air 'insulate' the occupants from the hot air and radiant heat outside?

[commerce48]

Closing all windows except two with box fans will not compare to all windows being open. Opening an attic access hole will allow hot attic air to enter the living space. Both are silly ideas.

If all windows are open, the box fan will only draw air from the nearest windows, cooling only one room. I close all windows except two on opposite ends of the house - sort of a poor man's whole house fan. It also creates light and delightful air movement through the entire house as a plus. Obviously, temp changes are slow. But this is not a silly idea.

Is active noise cancellation acoustic insulation?

insulation

the act of protecting something by surrounding it with material that reduces or prevents the transmission of sound or heat or electricity
Does cold air 'insulate' the occupants from the hot air and radiant heat outside?

I'd say yes. It reduces sound energy, or insulates the user from the sound energy. You could quibble about whether the effect is just at the user's ear, or environmental cancellation.

[tedkidd]

I have put clear conditions on when attic ventilation is safe and not safe. It is you that says that accelerated attic ventilation is always bad. You won’t even admit that your cautions only apply to tight homes with AC operating.

We are professionals and should know how to install something that is potentially dangerous. Framing, electrical, natural gas, and hand rails can kill people too, but it should be our jobs to know how to install them safely. No difference with attic ventilation. I would not give advice to a non-professional how to do my job. Heck, they would probably fall off the ladder or cut off their finger before they ever asphyxiated themselves.

Brian, the problem isn't the "professionals" ability to install something safely, it's the assumption the homeowner will operate it safely.

Furthermore, ac is not the condition that applies. Do you know how to perform worst case CAZ testing? If not, then maybe your ability to safely modify a homes envelope is worth a look, if you have decided to get into that line of work. If you have never heard of this test then what you don't know may hurt others. I think you'll find that is the minimum test required to determine if envelope modifications are safe.

The "does it pay" conversation is much less important than the "might it make people sick or dead" question.

[mark beiser]

I know of at least one case where a roofing contractor was held liable for over $1 million in real and punitive damages in a case involving damage caused by house depressurization due to PAV's they installed.

Admittedly, the level of ventilation they installed was on the extreme side, but still...

It is an issue that NEEDS more real research, but so far ALL of the small scale studies I've found by building science organizations and public utilities are negative on PAV's.

[energy star]

I know of at least one case where a roofing contractor was held liable for over $1 million in real and punitive damages in a case involving damage caused by house depressurization due to PAV's they installed.

Admittedly, the level of ventilation they installed was on the extreme side, but still...

It is an issue that NEEDS more real research, but so far ALL of the small scale studies I've found by building science organizations and public utilities are negative on PAV's.

What exactly happened?

[Shophound]

Here's a link to an article that contains a formula I've long sought after; how to calculate heat transmission from a hot roof deck!

http://webcache.googleusercontent.co...&ct=clnk&gl=us

Which, should you choose to read this, would dovetail right into our discussion about PAVs. Let's use a BTU per square foot the author derived from a roof deck with no radiant barrier attached, 27 btu/ft^2. Let's give the total roof deck area of our earlier example of an 1,800 square foot ranch house a times 1.5 size increase, or 2,700 ft^2 of roof deck area. Let's also assume it is uniformly adding 27 btu per square foot to attic air, and that the surface temperature of the decking facing the attic is 150 degrees, as in the link's example.

This means this attic, with 2,700 square feet of roof deck at 150 degrees, and at 27 btuh/ft^2, is pumping 72,900 btuh of heat into the attic through radiant heating. Um, that's equivalent to over six tons of a/c capacity. Wow.

Let's add the air volume of the attic over this house, which is 20 feet wide by 90 feet long (not an unusual shape for a ranch house) and has a gable roof with a peak six feet over the attic floor. This gives a total attic air volume of 5,400 cubic feet. Earlier we discussed lowering the attic air temperature from 130 degrees to 100 degrees, and tossed around some cfm number like 250 to do it. Let's see...to completely turn over the entire 5,400 cubic feet of air volume of the attic at 250 cfm would take about 21 minutes to pull off. To lower 5,400 cubic feet of air from 130 degrees to 100 degrees would require the removal of 38,880 btu to do it. 38,880/(1.08 x 30) = 1,200 CFM to move enough air for lowering the volume of attic air 30 degrees F. At that rate you're turning over the entire volume of attic air in about 4.5 minutes.

However, we still have an attic roof deck cooking hot, and pumping 72,900 btuh into the attic. To cool it down to 130 degrees would require the removal of 48,600 btuh. To move enough air at 130 degrees to get a roof deck surface temp to drop from 150 degrees to 130 degrees would take about 1,500 CFM. To lower the attic air volume temperature from 130 degrees to 100 degrees takes another 1,200 CFM.

Are we now talking about a PAV moving 2,700 CFM, or about 1 CFM per square foot of roof deck, to accomplish what a spray-on radiant barrier can possibly do (pertaining to roof deck temperature reduction) without any fan at all??

Here's another link to ponder on:

http://webcache.googleusercontent.co...&ct=clnk&gl=us

[mark beiser]

What exactly happened?

Mold in the walls and poor temperature/humidity control were the big problems.
I believe there were some IAQ and health issues too.

I don't know the full details on it, but the HVAC contractors feet were being held to the fire over it until someone actually went in and diagnosed what was causing the problems and redirected blame to the actual parties responsible.

[DavidNJ]

Here's a link to an article that contains a formula I've long sought after; how to calculate heat transmission from a hot roof deck!

http://webcache.googleusercontent.co...&ct=clnk&gl=us

Which, should you choose to read this, would dovetail right into our discussion about PAVs. Let's use a BTU per square foot the author derived from a roof deck with no radiant barrier attached, 27 btu/ft^2. Let's give the total roof deck area of our earlier example of an 1,800 square foot ranch house a times 1.5 size increase, or 2,700 ft^2 of roof deck area. Let's also assume it is uniformly adding 27 btu per square foot to attic air, and that the surface temperature of the decking facing the attic is 150 degrees, as in the link's example.

This means this attic, with 2,700 square feet of roof deck at 150 degrees, and at 27 btuh/ft^2, is pumping 72,900 btuh of heat into the attic through radiant heating. Um, that's equivalent to over six tons of a/c capacity. Wow.

Let's add the air volume of the attic over this house, which is 20 feet wide by 90 feet long (not an unusual shape for a ranch house) and has a gable roof with a peak six feet over the attic floor. This gives a total attic air volume of 5,400 cubic feet. Earlier we discussed lowering the attic air temperature from 130 degrees to 100 degrees, and tossed around some cfm number like 250 to do it. Let's see...to completely turn over the entire 5,400 cubic feet of air volume of the attic at 250 cfm would take about 21 minutes to pull off. To lower 5,400 cubic feet of air from 130 degrees to 100 degrees would require the removal of 38,880 btu to do it. 38,880/(1.08 x 30) = 1,200 CFM to move enough air for lowering the volume of attic air 30 degrees F. At that rate you're turning over the entire volume of attic air in about 4.5 minutes.

However, we still have an attic roof deck cooking hot, and pumping 72,900 btuh into the attic. To cool it down to 130 degrees would require the removal of 48,600 btuh. To move enough air at 130 degrees to get a roof deck surface temp to drop from 150 degrees to 130 degrees would take about 1,500 CFM. To lower the attic air volume temperature from 130 degrees to 100 degrees takes another 1,200 CFM.

Are we now talking about a PAV moving 2,700 CFM, or about 1 CFM per square foot of roof deck, to accomplish what a spray-on radiant barrier can possibly do (pertaining to roof deck temperature reduction) without any fan at all??

Here's another link to ponder on:

http://webcache.googleusercontent.co...&ct=clnk&gl=us

The level is about right. It may actually be a bit higher if at a point in the day the roof points at the sun. If the roof didn't point at the sun it would be less. The numbers are pretty widely published with regards to roof mounted solar photovoltaic panels.

However, if that radiant heat went directly into the attic, the roof shingles would be at ambient temps. If the roof shingles are asphalt they have about .25 btu/lb/F, and wood is about .6 btu/lb/F.

Wood is about 2.7lb/board ft, asphalt shingles are about 2 lb/sq ft. If 1/2 decking is used that would be 2 btu/sqft. If you 2700sqft roof was 50F above ambient, that would represent 100btu/sq ft, 270mbtu, or the full load from 4 hours of your calculation.

However, now the roof is 50F above ambient. Quite a bit of heat transfer will occur at the surface of the roof. If the attic was 30F above ambient, that would represent 38,800 btus according to your calculation. Much less than the heat in the roof itself.

BuildingScience.com has in one of its papers statement that 5% of the shingle's heat is dissipated indoors.

Not to get into this discussion, but to say the amount of radiant that makes it through may be quite modest. It is that moderate amount that the radiant barriers are dealing with.

[Shophound]

Not to get into this discussion, but to say the amount of radiant that makes it through may be quite modest. It is that moderate amount that the radiant barriers are dealing with.

I think this is why many think that if there are no a/c ducts in the attic, just increase the insulation level on the attic floor, in particular to cover the ceiling joist tops to reduce thermal bridging. With a/c ducts, radiant heat transfer is much more significant. Therefore a radiant barrier would be more effective.

All my mental doodling in my preceding posts, I hope, was to show that to achieve roughly the same effect of a radiant barrier by instead using a PAV, it would require a lot of energy and moving a lot of air. The passive option won't be a constant drain on an energy grid that will only become more expensive to draw from in the coming years. Which will I advocate if I have half a stitch of sense? The passive options, of course.

[tedkidd]

I don't know the full details on it, but the HVAC contractors feet were being held to the fire over it until someone actually went in and diagnosed what was causing the problems and redirected blame to the actual parties responsible.

Every problem is clearly the hvac guys fault. I went into a home with condensation on the windows @ 40f oat. Mentioned they might want bath or kitchen exhaust.

Replaced equipment. Later, Mold growing on window sills was my fault. Even had recommendation in writing.

There are very few true adults, we are mostly big children.

[Shophound]

There are very few true adults, we are mostly big children.

Sadly, yes.

[DavidNJ]

Consultative selling. The customers have minimal domain knowledge and talk to several vendors in different specialties providing conflicting information. Then the vendors wonder why they don't trust what they say or think they may be at fault?

If vendor doesn't take the time to educate the customer then the problem you describe would happen in any industry.

[Mr Bill]

"Worldly wisdom is a futile attempt to know more and more about less and less until one finally
learns everything about nothing."