to vent or not to vent (the attic)

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Minister of Fire
Hearth Supporter
Sep 27, 2008
media, pa
We have probably all read arguments and articles regarding, how should you vent your attic.

Well The short end is that my house is a 1950's built house. I have a ridge vent, and soffit vents, (probably could use a few more, but it is what it is), and I have gable vents. Most of the arguments are what to do when you have gable and ridge vents.

I have put a temperature logger in the attic and monitored temperatures, With 2 kids 2 and 9 months I havent gotten to analyze the data until now.

I ran 2 iterations of measurements.
1) as is - gable vents (2) open and the power gable vent fan on set at about 105F.
2) modified - 1 gable vent shut, the other has the fan and it was set at max high(130F i believe).

I was able to match up 3 pairs of days, where the weather was the same when the gables were closed and open.

2 of the 3 days, the max temperature was 3F higher, the other day it was 1F lower. humidity was lower by 5% 2 of the 3 days.

and for the following nights after these matched days (to see how the attic cooled).
The attic was cooler by 5F and 6F 2 of the 3 days respectively. and warmer one day by 9F, strange.

What is my take away from this? After recording data for a month with the attic open and another with it closed, it was very hard to find days where the outside temperature matched up. These 3 days present a pretty limited data set. However, this data shows that with the vent closed, the temperature was ever so slightly cooler (i guess the ridge vent theory is marginally accurate). However, the attic wont cool off well at night as such by a pretty significant margin.

I am sure that it varies dramatically for each house, but for mine, I am going to leave the vents open, and the power vent running. The data logger cost $14 on ebay, and not I know for what what the most efficient way to run my very inefficient house is....
I like a good dataset as much as the next guy, but I'm not sure what to conclude. Break it down for me. If the fan was drawing air out of your house due to negative pressure, the attic will be cooler, but your AC bill would be higher. If your attic floor is perfectly sealed, and decently insulated, the difference in AC load prob doesn't cover the energy to run the fan. What do you conclude?
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I like the experimentation!
I'd suggest though that a good outcome would require better data collection.
I'm thinking you'd need to measure attic temp, outside temp, wind speed, and solar insolation on the roof; then compare the difference between outdoor and indoor temps when wind and solar isolation were the same.
Sometimes its pretty tough to establish cause and effect or even accurate association between two things.
Sorry to be an azz, but your talents and efforts are better directed elsewhere. A little background reading....

Attic ventilation is to prevent moisture related problems on your roof decking, once it is good enough, it is good enough and not worth worrying about. Have soffit and ridge vents? Done.

Preventing heat and air flow between your house and (vented) attic, that's the job for your carefully airsealed attic floor and copious amounts of attic insulation.

Basic (totally hypothetical) scenario:
--End of July, a guy notices his upper floors are too hot, checks out attic, its 130°F up there and he get a brainwave....
--Installs biggest gable fan HD sells, attic is now 115°F at noon. Wahoo.
--Notices that upper story rooms are now **much** cooler. Awesome.
--Opens September bill and compares amount to August bill, is 10% lower. Score one for the good guys. Take that OPEC!

Sadly, what actually happened:
--House has 8 sq ft of opening between its interior framing and attic space (actually much larger than existing soffit vents). Upper stories get hot because superheated attic air is getting pulled down into interior wall cavities by 'reverse' stack effect caused by AC usage (indoors cooler than outside).
--Attic fan reverses flow, now pulls AC'ed air from house, up through wall cavities into attic, cooling and drying the attic in the process.
--Upper stories, with cool wall cavities instead of super heated ones, is noticeably cooler. Sadly, additional make up air is now being drawn in through the rim joist area forcing the AC and basement dehumidifier to work much harder than before.
--While electric usage under constant conditions goes up due to dehumidifier, AC and now gable fan elec usage, the attic and weather runs cooler in August than July, so the Sept bill still comes in lower than the August bill anyway.
--Homeowner enjoys higher energy bills and a moldier basement for 3 years until fan dies.

In case you think I'm a total a-hole writing this Mav....the 'guy' above is **me**. ;em
ahhh a valid comment! BUT banning it for new houses is totally different than when putting it in an old house like mine.

When i moved in, the plywood in the attic was all molded and rotten after only about 7 years installed, no power vent. the replaced stuff, not 8 years old, i working jsut fine, no moisture issues.

I assumed that there would be a more significant difference between vented and unvented. In my case, there isnt much of a difference, the more significant drop at night, in my opinion is more important than the nearly identical daytime temps. Certainly the attic isnt air sealed, but we keep the ac set very warm (80F). The ambient conditions were nearly identical, as identified by weather underground (why i only was able to get 3 days the "same" despite having 2 months of data recorded.

being built in the 1950's my house doesnt like to cool down, its common for the AC to be set at 80F and wake up in the morning to find the house still at 80F even is the outside is 10or 15F lower. I have put in a whole house fan and that thing is wonderful!

I appreciate the articles above, and I had previously found a study with videos (linked below) that shower a power vent is still helpful:

I truly feel its a case by case situation, and think power vented is the solution for me, but I will have the temperature logger running next summer as well to get some more data.
Every house IS different, but I would suspect that the previous molding issue was due to a lack of attic floor airsealing allowing water vapor to come up from below, perhaps with a previous owner that tried to humidify the air in their drafty ol' house in winter with a big humidifier (and ended up just humidifying the roof sheathing). As happened to BB a couple years ago. Or me ~2008.

My house is from 1960 just down the road from you in Wayne. Frankly, the amount of open cavity and air bypasses were astonishing to me, and way worse than standards from my 1969-built childhood house in MA (I think those guys had more of a clue than the builders down here in PA).

If your attic floor is sealed and insulated, experiment away, but I still don't know what you are hoping to achieve. With a 1000 sq ft footprint, at R-50 (current recs) and 120°F (50°F higher than inside), the heat flow from the attic should be a measly 1000 BTU/h, or 1/12th of a ton of AC, and 4% of the load on a 2-ton AC. IF you dropped the attic temp 5°F, that would save you 10% of the 1000, or 100 BTU/h, or 30 Watts (thermal). With a COP=3 AC unit, this saves you 10 W (elec) when the gable fan is running. OF course, gable fans typically pull over 100W.

In my case, I used to have a huge AC bill, and now after attic airsealing it is <$150 year. In my case the heat of the now departed oil boiler was half of it (esp the reluctance of the house to cool off at night and during mild weather). I still hear the neighbor's AC kicking on when it is 60°F outside. Effin boilers.

The whole house fan is a whole nother calculation, those can make sense (for some weather conditions) but that has nothing to do with gable fans.
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I think Woodgeek is on the right path here...seal the ceiling and stop the moisture infiltration from below. Then, natural ventilation will likely be all you need. In my 1922 bungalow, even on a really hot day, my attic temperature is very tolerable with modest gable vents and a ridge vent to provide natural convection.
I roof part-time and I am a "certainteed mastershingler", that means my knowledge set is about on par with anyone with 10 minutes of googling. That being said I was taught to either have gable vents or ridge/soffit vents. Do not do both because it short circuits the ventilation path. Cool air comes in from the gables and exits the ridge, leaving a portion of the attic unvented. Also don't combine power vents with the other techniques for the same reason. Except soffit intakes obviously. I will also state there are thousands of homes that do not follow these rules, with no complaints. I personally believe that an unvented attic will cook shingles causing them to harden prematurely if not properly insulated. Most asphalt shingles are trash though and you will be lucky to get half the recommend life out of them.
Woodgeek, you are quite close to where I live, but Wayne is way out of my price range, sectacular area!

airseelin and better insulation wuld certainly be an improvement, however I dont see that happening in th near term for me (aside from he poorly sealedlouvers for the whole house fan when in summer mode, winter they are reasonably well sealed. We find the whole house fand to bespectacular an used more oftenthan not in the summer (again, its a big prsonal preference with how you lik t cool the space, for us it work well). I appreciat eh math to, somthing that I wouldnt have put together. Surprisingly its only an argument over about 90 watts/1000ft^2 (more or less). Thats far less than I expected.

Being in the attic is one of the more unpleasnt places to be, If i dnt wear a respirator I will be coughing for a week. Fiberglass is miserable....
No worries, I'm in the 'poor' section of Wayne, not the stately Wayne manors.

The 1000BTU/h = 300W figure would be for an airsealed R-50 attic. Airsealed and R-25, it would be 600W. For un-airsealed attic, it could be a factor of several higher than that (kW) more in line with your intuition.

We all only do what we can do.

The low degree of cooling you see is in line with the 'short-circuit' hypothesis. 200CFM * 50°F (attic warmer than outside) would suggest ~10000 BTU/h rejected heat, which you would think would drop attic temp more than a couple degrees. IOW, you are likely pulling outside air into the ridge vent adjacent to your gable fan, and exhausting air that is much closer to outside temps. Maybe you should compare the thermo in the airstream and on the other end of the attic.

On the bright side, if you were pulling air from below in large amounts, the fan would also be cooling the attic more than you see. Presumably your attic vent free area is large compared to your air leaks to conditioned space.
I was taught to either have gable vents or ridge/soffit vents. Do not do both because it short circuits the ventilation path. Cool air comes in from the gables and exits the ridge, leaving a portion of the attic unvented.
This likely works ok in my attic since the gable vents are only about 24" off the attic floor. When I added the ridge vent, that really cooled the attic down in the summer.
interesting on many levels, i will definitely have to check the air stream temp, that would be interesting. I've been half tempted to fire off a smoke bomb in the attic to watch the air move.... BUT something tells me thats a fire hazard and something which would cause hate and discontent on many levels.... haha

thanks for the input. As an aside, have you changed power companies? I am a rh plan (heating) so I was enjoying the peco discount (heat pump) but likely will switch to con edison, again, something on the long list of things to do... but with 2little kids and a lot of hours and travel, the list slowly gets marked complete...
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Dad installed a powered attic vent when he lived here. When we bought the home, the upstairs was brutal in the summertime. Very hot and humid, so he recommended to use the vent. Whenever we used it, it made things worse upstairs.

The year after we airsealed our attic, I insulated. The upstairs now stays cooler then the downstairs of the home. Just like mentioned, we had a humidifier rated for 2500 sqft running on high during the winter. We couldn't get any humidity in our home. After airsealing, we no longer use a humidifier. The powered fan was pulling from over 30 open cavities in our attic. After all the improvements, I disabled the fan and use passive ventilation. I'm with woodgeek, they cause more harm than good.
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very interesting topic. I have a 1920s bulgalow home.(balloon frame). with about 4'' of vermiculite insulation and R30 ontop of it. I wonder just how much air is traveling from my basement up through the walls and through air leaks in my ceiling......

definitely would like to seal up the attic better... Whats the best method? Closed cell foam? Can I do closed cell foam with batts ontop?
You DO know about vermiculite and asbestos, right?

Basically, you are not supposed to disturb it. Getting it removed by pros will cost a bundle. Some folks say the asbestos falls down into the wall cavities and other openings and is detectable in the house dust.

I watched my dad put it in the old house back in '78 (when I was 10). We moved out before the problem was identified.

Now, just speaking for MYSELF, I think the hazards are to pros and miners that contact this stuff every day. I read somewhere that there are no documented cases of mesothelioma (a rare cancer highly correlated with asbestos) among homeowners living with and touching the stuff. Usually homeowners ARE allowed to do asbestos abatement on their own home for this reason. But you would need a place to dump it.
Cool beans. :cool:

Its not great R-value on a per inch basis, 2-2.5 IIRC, but it works.
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