Roof venting doesn’t decrease AC load
bobbomax
| Posted in Energy Efficiency and Durability on
Martin, in Fine Homebuilding, you wrote that roof venting doesn’t decrease air conditioning load (http://www.finehomebuilding.com/2017/07/11/roof-venting-doesnt-affect-cooling-loads#comment-13202 ).
Which leads me to ask what roof venting IS good for. Humidity control? Presumably, venting doesn’t increase heating load in the winter.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Bingo!
Robert,
You should read a GBA article titled All About Attic Venting.
In that article, I wrote:
"Here are the four most common reasons people suggest to explain the practice of venting attics:
• To reduce the chance of moisture build-up in the attic or condensation on the underside of the roof sheathing.
• To make roofing shingles last longer.
• To lower cooling bills during the summer.
• To reduce the chance of ice dams.
Although attic ventilation is sometimes able to contribute in a very small way to addressing the problems on this list, there are much better solutions to all four problems than ventilation."
For a full explanation of the reasoning behind these sentences, read the article.
Roof venting does decrease AC load, in some houses by a lot actually. Those solar attic fans are amazing.
Have you ever been to a room that is closed off for ages? Your attic/house will smell and feel like that if it's not vented.
Anon3:: Really? Show me the data!
A review the published literature of actual measurements shows attic venting in hot climates usually increases cooling energy use (with but a few exceptional cases to prove the rule.):
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
Best case with one of those "amazing" solar fans is yields only a low single digit percentage in energy savings:
http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/
Show the data? I installed one recently, the difference it made was huge. I should have gotten the bigger version in hindsight. Oh well, on my next house I'll get the bigger version.
How is it that an attic fan made a difference Anon3 ? Was the difference in a sensible feeling or in energy used ?
The AC would struggle when it's 90f outside. With the solar fan, it's now fine into 100f. I was getting massive radiant heat before, now it's mostly gone.
Note this is a older house.
Anon3,
I've asked you before: Can you tell us your name? And perhaps you can tell us whether you work for a product distributor? Many of your posts seem to promote one or two products.
The Fine Homebuilding column that Robert Fankhauser referred to in his original question concerned cathedral ceilings. Anyone who attempts to install a solar-powered attic fan in the ventilation channel of a cathedral ceiling is nuts. Such a fan would undoubtedly pull indoor air into the ventilation channel through ceiling cracks, raising the home's energy bills.
Anon3 has apparently brought up a new topic, concerning attics instead of cathedral ceilings.
Suffice it to say that solar-powered attic fans have the potential to cause more problems than they solve. They can occasionally be useful, but only in attics that (a) have insufficient insulation, (b) have ductwork located in the attic, and (c) have a very, very airtight ceiling. Such homes are rare.
If you have an older home with insufficient insulation and ductwork in the attic, the solution to your problem is (a) perform air sealing work on the attic floor; (b) seal duct seams; (c) improve duct insulation, if possible, and (d) add more R-value to the existing attic insulation.
If you have a big bank account, an even better solution is to convert the vented unconditioned attic into an unvented conditioned attic.
The steps I have described make much more sense than installing a powered attic ventilator.
Dude I should have installed the attic fan years ago, but thanks to forum like this I was mislead. Oh well, you learn something everyday.
There's another benefit, the incredibly nasty attic air is now almost fresh smelling!
Anon,
What difference does it make how your attic air smells unless that smell permeates the house below? In other threads you have suggested that the attic fan will make the whole house smell better, but if the attic air is causing odours below, the solution is to air-seal the ceiling, not to draw indoor air through the leaks into the attic.
I wish you would engage in some discussion of the advice you suggest, rather than moving on to a different discussion and just repeating. If you are right, that's how you could convince us. Just making an assertion and leaving it hanging doesn't do that.
I don't get how ANON3 has not been banned. He consistently posts conflicting posts that are confusing and incorrect. I'm sure you can handle the $16 hit to improve the quality and consistency of content. There are several reasons he doesn't want to post his name. Trolls have no interest in helping others but rather to amuse themselves. I've also not found a troll that wasn't a coward hiding behind internet anonymity. He will smile at this post knowing he made somebody post, but I hope it's the last post he can write.
Eric: This forum is not (and should not be) about promoting homogeneous thinking of some orthodoxy. As long as participants are willing to support assertions with something other than more unsupported assertions, it's useful to have real world situations to dissect & analyse.
Dana,
As you know, I've never had a problem with the posting of conflicting opinions, as long as the comments section isn't used to insult the personalities of other readers or authors.
When a frequent poster shows signs of promoting products, however, and seems unable to back up assertions with valid data, it's only reasonable for a web forum administrator to ask the person in question to identify himself or herself, and to let us know whether these product recommendations are motivated by business interests. If such a person refuses to identify himself or herself, it would be within the rights of the web site administrators to block future comments.
I expect that air sealing without correcting pressure differentials (often attic->interior in the summer) would reduce but not stop odor movement.
The distinction I'd make is between those asking and those answering questions. If the information being tendered to people coming here for advice isn't correct, the forum loses it's usefulness to all of us.
Old thread, but wondering what to make of the FSEC-CR-1496-05 report that shows in the case of insulation at the roofline, a double roof (ventilated space between 2 layers of roof sheathing) does decrease the heat flux through the ceiling, compared to the same assembly without the double roof (Figure 4 vs Figure 9). This doesn't really jive with the other published data I've seen.
If you have 2 adjacent rooms at different temps, a barrier with less insulation will always have higher heat flux. It's just thermodynamics.
But why does that matter? The only heat being "lost" is through the exterior barriers.
I don't think we're talking about the same thing. As I read the report, they're talking about the heat flux into the building envelope through the roof on a hot day.
https://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
The vented attic had R19 batts and the sealed attic had R22 spray foam. But the sealed attic had a higher heat flux vs the vented attic (figure 4) until they added the vented 2nd roof (figure 9)
Presumably this is because the hot side of the insulation is hotter in the sealed attic - it's close to the shingle temperature which was 155F on the day in question, vs the peak of 117F for the attic temperature in the vented attic. With the sealed attic, venting under the shingles reduced the sheathing temperature in contact with the insulation, reducing the heat flux. This makes sense to me.
They also note that the cooling energy load for the sealed attic house was lower. But in both cases the air handler and ductwork are located *in the attic*. So moving those inside the building envelope saved a lot of energy despite the sealed roof assembly having a higher heat flux into the building.
ETA: Martin has said that a vented 2nd roof doesn't actually lower sheathing temperature that much at least toward the ridge. I'm not sure if there is data on this?
The ceiling in Fig. 4 is uninsulated:
"Figure 4 shows the ceiling heat fluxes over the 2000 summer period. The uninsulated ceiling of the
double roof with sealed attic (Cell #2) has a peak heat flux greater that of the control (Cell #5)"
Anything uninsulated is going to have a higher heat flux unless the temp difference is very extreme:
Q = (A/R)*(T1-T2)
I'm not sure why the authors include that figure as it's confusing to compare the uninsulated ceiling to the insulated ceiling. I guess it's because they don't put supply and return to the sealed attic, so the sealed attic is not in the conditioned space but it is within the thermal envelope, which makes an apples to apples comparison difficult.
(Also, confusingly there are two figures labeled Fig. 4 on pages 18 and 23. The one on page 23 should be Fig. 9.)
The sealed attic with the uninsulated ceiling still has R22 at the roof line.
The measured heat flux at the ceiling is still indicative of the heat flux entering the building thermal envelope.
The point I'm most interested in is that the situation for the sealed attic changes when you add the double roof. The heat flux is lower, similar to the vented attic. This implies that the sheathing temperature in contact with the R22 is lower, although it doesn't appear they measured that temperature.
Adding the double roof adds insulating value to the roof, so the heat flux through the roof is reduced, which will reduce the temp of the sheathing in contact with the R22. So yes, it will reduce the temp of the sheathing, but you can reason that out without the measured heat flux at the ceiling.
But the measured value does verify the system acts as expected: The heat flux through the roof is reduced, which in turn reduces the temperature of the sealed attic assuming everything else is held constant. The reduced temp in the sealed attic reduces the temp difference across the uninsulated ceiling, which reduces the heat flux across the uninsulated ceiling and the measured value verifies that's true.
(Response to #21)
Heat flow across the ceiling has to match heat flow across the roof. Otherwise heat would be being created or destroyed.
No, the experiment has the HVAC system within the attic in all cases, and there's some amount of cooling from the ducts and leakage.
Edit to add: Air leakage is from the attic into the conditioned space according to page 18 of the paper. Fig 6 on page 21 shows the total cooling demand.