Comments on: Guest Post: What Causes Ice Dams and Icicles?

By: Susan C. Kennedy

Susan C. Kennedy — Sun, 16 Feb 2014 01:22:01 +0000

Great info! To bad you are
Great info! To bad you are not in PA. Built a house 18 years ago. Installed a new roof this past year. Still ice dams. Living room with cathedral ceiling has dry wall pop open on the exterior/ceiling area and ceiling/dining room joint after every winter. Would love to fix it once and for all.

By: David Butler

David Butler — Fri, 25 Jan 2013 05:22:20 +0000

W.J.Parker: I fully concur
W.J.Parker: I fully concur with John regarding powered attic ventilators. They not only use more energy than they save but can rob the home of conditioned air.

And if ceiling is air tight (rare) and well insulated, a small reduction in attic temperature isn't going to have much impact on cooling loads. Think "diminishing returns."

As John points out, a substantial portion of the ceiling load is radiant, not conducted heat transfer. Conducted transfer is relative to top-of-insulation temperatures, not the temperature at the peak. Although you may see an 8F reduction at the peak, the impact on floor level temperature will be less than half that much.

Not sure what you meant by "reducing latent heat load in the attic." In summer, latent loads are from exterior air (in winter, the problem is moisture leaking into attic from conditioned space). Increasing attic vent CFM only moves attic closer to outside air dew point. If anything, it will cause higher latent load on equipment to the extent that attic air infiltrates through ceiling when PAV cycles off at night.

If you want to learn more, search Allison's blog for previous articles that include links to other articles resources on this topic.

By: John Ring in Charlotte, NC

John Ring in Charlotte, NC — Fri, 25 Jan 2013 00:17:38 +0000

Mr. Parker, in close to over
Mr. Parker, in close to over 2000 attics I have been in during the summer with a properly mounted (baffled) thermostatically controlled attic fan I have never seen one that that reduced the interior temperature by more than 8 degrees. And often that drop is assisted by some cool air being drawn out of the conditioned space below.
I'm a great believer in monitoring any situation to determine what is actually happening. In a number of cases I have monitored temperatures in these spaces and checked pressures to determine if the fan is sucking conditioned air out of the house. The only time I have found that air wasn't being sucked out of the house was with a few newer houses with with one of those big steep pitched hip roofs with a LOT of soffit venting.
Except in those few cases one of these fans sucking air out of the attic is not really helping (remember, you can't remove radiant heat with air flow) but actually costing the homeowner money, likely reducing comfort and in some cases providing a situation within the building enclosure that results in mold growth. Doing the right thing wrong is always wrong.

By: Robert

Robert — Thu, 24 Jan 2013 06:31:20 +0000

Nice job – here is my take on
Nice job – here is my take on the subject: http://youtu.be/MNMD3zI_vbc

By: W.J.Parker

W.J.Parker — Thu, 24 Jan 2013 03:09:07 +0000

As mentioned, air sealing in
As mentioned, air sealing in the attic is critical. Besides pipe and chimney chases, another big "leaker" is the drilled holes in the top wall plate where electrical lines come up into the attic. Also, the joint between the wallboard and the top plate of walls below the attic, can lights, ceiling registers, bathroom exhaust fans, et. al. The latter must ALWAYS be ducted to the outside, not dumped into the attic.

Since HVAC air handlers in attics are a way of life, they, and the assiciated ductwork, need all joints to be well sealed, and the ducts and plenum need to be insulated, to prevent humidity from condensing in the summer when a/c is running.

I disagree with the condemnation of gable-end power vents, since they are able to draw out attic air which can exceed 140ºF. in the summer, while drawing in exterior air which may be as hot as 90ºF., thus reducing the latent heat load in the attic, and the work load of the air conditioning system. The fan needs to be mounted on a plywood sheet which totally covers the exhaust gable-end vent, but with a round hole the size of the fan shroud cut into the plywood, so as to create a venturi effect for the exhaust air. The adjustable thermostat needs to be at the high point of the attic, so as to maximize the ability of the thermostat to sense when it needs to kick on the fan to remove hot, humid air

By: Ryan Shanahan

Ryan Shanahan — Wed, 23 Jan 2013 22:27:40 +0000

Another issue is that roof
Another issue is that roof trusses without raised heels can pinch your R-49 attic insulation down to R-15 or worse at the edges. New building codes are now starting to require raised heels but this problem is harder to solve in retrofit applications. Moving to a higher R-Value insulation product at the edges can mitigate the problem, but it's an expensive fix.

By: David Butler

David Butler — Wed, 23 Jan 2013 22:22:11 +0000

@Mark, the reference to warm
@Mark, the reference to warm-moist air condensing on underside of roof is referring to interior air (in winter) leaking through the ceiling plane, not outside air.

As for mold, I'm no expert, but I agree with Ted. Mold will grow under a wide range of temperatures, depending on the type. Moisture and food are the primary factors. Also keep in mind that even in a cold climate, the attic will typically rise above freezing during the day. Once mold establishes, daily cycles below its tolerance level won't kill it.

By: Nate Adams

Nate Adams — Wed, 23 Jan 2013 21:42:11 +0000

Mark – Good questions! And
Mark – Good questions! And ones that are tough to answer in a comment, they would require a full blog entry, and frankly I’m still learning about that area, so I don’t know it well enough to teach. The article Ted referenced will help with your initial question, check the other stuff Allison has up too.

As far as mold goes, my mold guy told me that 40 and up can be mold growing conditions. If you look at the Wikipedia article on mold growth, it says 32 to 95 degrees for some mold. I expect your knowledge is influenced by your climate zone just as mine is.

http://en.wikipedia.org/wiki/Mold_growth,_assessment,_and_remediation#Causes_.26_growing_conditions 

By: ted kidd

ted kidd — Wed, 23 Jan 2013 21:34:58 +0000

Mark,

Mark,

Check out Allison’s recent blog on humidity, it should set lightbulbs off for you:

http://energyvanguard.com/blog-building-science-HERS-BPI/bid/57151/A-Humidifier-Is-a-Bandaid-The-Problem-Is-Infiltration 

By: Mark J.

Mark J. — Wed, 23 Jan 2013 20:19:35 +0000

I would like more
I would like more clarification on how "Warm moist air... gets into a cold, insulated attic". I think in terms of dew point to measure humidity, and live in the hot-humid South where your normal conditions surely are quite different from mine.

Do you commonly have days where outside dew point is higher than the attic decking? I need some examples of temperature and dew point to understand this. I don't think it ever happens in S. Texas, not with enough persistence the attic cannot dry out. And I have been told that temperatures below 60-70F were not conducive to mold growth, can you elaborate please.