Attic ventilation and Home Performance
Hello GBA,
As always, I turn to the GBA community when I have a lingering question on my mind. This one is about attic ventilation.
How cost effective/necessary is the retrofitting of attic ventilation to an existing home? I see various studies on the topic, and I am interested in a more consensus opinion. If I am properly air sealing and insulating an attic, and the current ventilation levels are below building code, is it worth the time and money to add additional passive ventilation? Will the improvements to attic moisture levels be notable if the attic has already been air sealed? Will the cooling loads decrease notably if there is ductwork and/or an air handler in the attic?
I have a good understanding of the principles and strategies of attic ventilation, so the question is about the cost-benefit of adding attic ventilation rather than the physics behind its necessity (or the lack thereof…)
Any opinions would be much appreciated, as would any articles or studies on the topic.
Thanks!
-Art
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Replies
Art,
Some of the best research on this topic has been performed by Bill Rose and Jeffrey Gordon at the Building Research Laboratory at the University of Illinois in Champaign-Urbana.
To summarize their findings: attic ventilation doesn't matter much.
A PowerPoint presentation by Jeffrey Gordon presents some of their findings:
http://acinational.org/sites/default/files/session/81008/aci11php6gordonjeffrey.pdf
One of Gordon's slides:
"What have we learned about the impact of [attic or roof] ventilation on 4 issues?
1. Moisture control – can have a some impact, but it is small, and not the determining factor in an attic.
2. Ice damming – can have a some impact, but it is small, and not the determining factor.
3. Shingle Life – It is hard to see that it makes any difference, and if so, it is slight.
4. Cooling cost – It cools attics in summertime, but no one has measured much energy impact."
Thanks Martin. I had seen Mr Gordon's PPT. I am curious if anybody has more info on his item #4: cost savings associated with a cooler attic through increased ventilation.
-Art
I am not so sure about Gordon's Point #3
Last August I saw shingles on a "famous" Super-Insulated Non-Vented Roof...
Only about 11 or 12 years old
The shingles were "toast".... looked more like 40 or 50 years old
perhaps a bad batch of poor quality shingles....maybe...maybe not
Art,
Simple energy modeling shows that if your attic floor is well insulated, the attic temperature is almost irrelevant -- assuming, of course, that there is no attic ductwork.
If you have some money to invest in attic improvements, use the money to air-seal the attic floor or increase your insulation thickness.
John,
Come on, now, John -- do you really know what a 50-year-old asphalt shingle looks like? I don't.
Martin,
I have seen homes 40 to 50 years old with what looks to be original shingles....
But you are correct ... I don't know for sure
they may not be the original shingles... so I am only guessing
I will stand by my description of "toast"
John,
Bill Rose's temperature probes show that ventilation cools shingles near the eaves slightly, but has no effect on the temperature of the shingles on the top half of the roof. The color of the shingles affects shingle temperatures far more than ventilation.
There's no doubt that 11-year-old asphalt shingles can sometimes look terrible. But trust me -- ventilation wouldn't have saved them.
I don't believe that all attics should be unvented or conditioned, but looking at Bill Rose and Jeffrey Gordon's PPT, there are some good reasons to have conditioned attics. It came to mind Ted Storm's question a few days a go: https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/20587/unvented-metal-roof
Slide 33:
Wet, Cold, Coastal Climates
Computer simulation indicates that attic ventilation results in higher sheathing moisture content, not lower
Ventilation makes the attic colder, without lowering water vapor levels very much
Colder means wetter
Forest, Walker, Attic ventilation and Moisture. Final Report, CMHC, 1993
Slide 34:
Warm and Humid Climates
No one has ever claimed moisture control benefits in venting attics in warm, humid
climates
Outside air is more humid than inside, and attic venting will tend to increase rather
than decrease moisture levels in the attic
Slide 40:
“Even an unvented attic is very unlikely to develop significant ice dams unless there are significant heat sources in the attic or significant air leakage from below,…”
Slide 60:
Cooling season energy savings
Venting will reduce the temperature in an open attic. The difference in attic temperature between a vented an unvented attic, with R-30 at the ceiling, translates into minuscule savings. No savings have ever been measured.
Usually there is a penalty with venting because venting causes greater air pulsing across the ceiling.
Slide 62:
What have we learned about the impact of ventilation on 4 issues?
Moisture control – can have a some impact, but it is small, and not the determining factor in an attic
Regarding shingles...they don't make em like they used to. Pretty much all 3 tabs are junk. Maybe you can special order good ones, but archs are where they bother.
My last house had 3 tab shingles over OSB 4'' Iso, dead, in 15 years. Shingles on the ground dead. 8 sided hip, SSE, SSW were dead, The other sides were still ok. One wing of the building had due south facing standard construction roof, soffit vents, no ridge, shingles were fine. Reroofed with sleepers and CDX vented at the peak. I'll speak to the new owners in ~9 years to see if it worked any better.
My current house, ~11 year old roof, archs over 1" iso, cracked bubbling dry crumbling shingles over most of roof SSE somewhat worse than NNW. Replaced the SSE side with 3.5" iso sleepers CDX lifetime archs. NNW is EPDM over 5" foam. We shall see, call me in 20 years.
I would not personally put shingles on a south facing roof directly over foam, that is just me.
In the OP's case I doubt it makes a difference in a attic floor insulated style roof
Martin – a classic example of how ventilation helps shingles is looking at the shingles above a sky light in a vaulted ceiling – these shingles always look much worse than all the other shingles over areas with an unobstructed ventilation chute from soffit to ridge.
We have seen this so many times, we now cut notches or drill holes in the rafters on either side of the skylights to vent the upper portions. I do agree that ventilation is not as important as some believe but to say it has no effect whatsoever is not accurate in my experience – at least down here in NC where attic temperatures can get close to 150 degrees.
I also do not understand why so many experts say attic temperature is irrelevant. We have quite a bit of discussion on exterior walls – how we need to make them thicker, add rigid foam to the exterior, be sure the insulation is sealed on all six sides. All of this for a wall that will have a delta-T of 15-20 degrees in the summer. The attic is where we could have a delta-T of nearly 75 degrees and this insulation is almost never sealed on all 6 sides. It seems like common sense that if you have R-38 insulation in your attic for example that it would be better for your attic to be 90 degrees rather than 150 degrees. We have recently begun using radiant barrier roof sheathing and the results are amazing. Out typical attics are around 130 degrees on a 95 degree day – with the radiant barrier sheathing in a vented attic (soffit and ridge, not PAV), the attic temperature is typically only a few degrees higher than the outside ambient temperature. I have a hard time believing that this is not helping keep the second floor cooler.
Keith Gustafson,
Thanks for reminding me that the Original Post was about a conventional attic.
My concerns about shingle life are for compact non-vented roofs.
(Homes with cathedral ceilings or cathedralized attics)
Danny Kelly,
I think Riversong agrees with your reasoning for the radiant barrier in a HOT climate.
With good airtightness at the attic floor and ample insulation there is still no doubt some advantage(perhaps small) to light colored roofs and/or radiant barrier sheathings.
Since You and I are in MIXED climates do not forget that the same strategy has a "penalty" (perhaps small)during the heating season.
So what good is a ridge vent with 2' of snow on top of it? And why do we vent bathroom fans straight up through that snow, which get buried entirely at times? Even worse, why would we want warm air from furnace combustion flowing through the snow for many hours a day? My brother lives in a house that was built by a contractor for himself in MN (so presumably with extra care), and there is a plumbing stack, bathroom vent, and furnace vent all within 5' of each other: result is major ice dams every winter.
Insulating at the roof plane seems to make more and more sense...