Vent or Not Vent
I was recently struggling with this question for the house we are building and found this article particularly helpful. I had decided to go with a non-vented roof assembly until I started to compare the costs. I then ran into the standard trade-offs that I’m sure most people find. I finally settled on using a vented roof assembly with batt insulation from below on my scissor trusses (gable and hip). I only have a couple of ducts that will be in the attic space and will use rigid duct. I am planning to have use spray foam at the truss blocking and will spray foam the ducts too prior to covering them in insulation as the the batts go in. I like the approach of using more standard building approach, but focusing on executing the details well to limit the typical weaknesses. I don’t think I would have gotten there without this article from Joseph Lstiburek. Thought this might be helpful for others.
https://buildingscience.com/documents/insights/bsi-074-duct-dynasty
If anyone has experience doing what I am planning, please share your thoughts.
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Replies
Kevin,
In February 2017, Allison Bailes wrote a detailed article on burying ducts in insulation. Here is the link: "Buried Ducts Allowed in 2018 Energy Code."
Bailes cites research that seems to support less stringent rules than Lstiburek's rule of thumb. (Lstiburek advises, “You just can’t 'bury' the ducts or they will sweat. You have to insulate them with an impermeable layer of spray polyurethane foam and then you can bury them. You need to do this in all climates except in the desert.”)
For specific code-approved methods of burying ducts in attic insulation, see Bailes's article.
Martin,
I just read the article and it has some great information for what we are planning. I looked up our maximum annual dew point. It is only 60F - we are in 4C near Seattle. Allison's article confirms our plan and gives me some other options for how to run and insulate the ducts. It looks like if we do a great job of sealing them, either flex or rigid duct will work and we have options for how to insulate the outside of the ducts. Thank you for pointing me to his article.
Kevin,
You're welcome. Now that GBA has been online for 8 1/2 years, we can say that GBA has published an article on almost every topic of interest to green builders.
The information is here somewhere -- the hard part if finding it. Glad to help.
According to Lstikburek:
"You have to insulate them with an impermeable layer of spray polyurethane foam and then you can bury them (Photograph 2). You need to do this in all climates except in the desert. "
Think of Seattle as that desert, foliage notwithstanding. There are places in the Gobi Desert with higher summertime dew points than Seattle. (The Atacama desert is drier though. :-) )
Desert SW cities in the US like Las Vegas or Phoenix or Barstow CA have higher peak dew points than Seattle:
https://weatherspark.com/m/2460/8/Average-Weather-in-August-in-Phoenix-Arizona-United-States#Sections-Humidity
https://weatherspark.com/d/2228/8/6/Average-Weather-on-August-6-in-Las-Vegas-Nevada-United-States#Sections-Humidity
https://weatherspark.com/m/1924/8/Average-Weather-in-August-in-Barstow-California-United-States#Sections-Humidity
https://weatherspark.com/m/913/8/Average-Weather-in-August-in-Seattle-Washington-United-States#Sections-Humidity
Unless you are air conditioning the place to temperatures where you would need a sweater, burying hard-piped AC ducts in fluff without foam in Seattle will NOT result in sweaty dripping ducts, even during the 37 minutes per year that the dew points head north of 60F.
Sealing the joints & seams with duct mastic is good enough, no foam or pre-insulated duct needed.
Dana,
Sorry for the late reply. Somehow my email notifications for these got turned off.
This is a very interesting observation and post, thank you. The counter intuitive thought for the day: Seattle has a lower peak dew point than the Gobi Desert! I played with the website and did find a that the dew point is lower in Antartica. So maybe no insulation is required in deserts and on tundra.
https://weatherspark.com/m/147510/8/Average-Weather-in-August-at-Teniente-R.-Marsh-Airport-Antarctica
I bought a digital thermometer / humidistat last year that tracks the all time high temperature and relative humidity data. I just checked it and the maximum relative humidity was 59%. So worst case if the maximum relative humidity lined up with the maximum temperature, 77F, the maximum dew point would have been 61.5F. That lines up very well with the data you found. I think we turned on the AC once last year.
I will make sure we seal up everything tight with mastic both inside the conditioned space and in the attic. The other opportunity this points out though is that the ducts inside the conditioned space only need to be insulated if we want to reduce noise. That helps out in some places where we have limited space. The HVAC contractor stated that they needed to be insulated to prevent sweating.
The air in a vented attic would track the outdoor dew points, with very low risk of sweating ducts in your climate.
The indoor dew point is often higher than outdoor dew point. (During a shower the dew point in the bathroom air often reaches the temperature of the mirror & walls no? It's prudent (though not essential) to insulate the supply ducts inside of conditioned space where possible to prevent sweating, since they are generally exposed to higher humidity air than the ducts in the attic in your climate. (In other US climates the converse is often true.)
I suspect very few locations in Antarctica are outfitted with ducted cooling systems... :-)
I can just picture now the lonely AC dealer in Antartica. Kind of like the old Matag repair man!
Thanks for the additional council. I'll insulate the ducts in the interior.