Brennan Less
  • Berkeley, CA
  • United States
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Brennan Less's Discussions

Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso

Started this discussion. Last reply by Mike MacFarland Aug 12. 21 Replies

Greetings, We recently placed 3.5" of RMax foil-faced polyiso foam board (air and vapor impermeable) neatly cut and placed between the rafters of our cathedralized ceiling home in the Santa Cruz…Continue

Tags: condensation, roof, unvented, Polyiso

Trends in space conditioning systems in high performance homes, looking for your insight

Started this discussion. Last reply by William H Nickerson Jul 1, 2012. 4 Replies

Can anyone comment on the common types of heating and cooling systems installed in their region in high performance homes?  More and more, I see designers and builders turning away from forced air…Continue

 

Brennan Less's Page

Latest Activity

Mike MacFarland replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"Hi Brennan, Physicist Niels Bohr is credited as saying “An expert is someone who has made all the mistakes which can be made in a very narrow field.”  Sounds like you’ve moved even one step closer to expert status with this…"
Aug 12
David N. Armington replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"I am confused.  What is the source of the moisture in your roof assembly?  Theoretically, it should “boil off” and your roof would stabilize.  If you think it is from rain, fog or dew you do have roofing problems. …"
Aug 11
Brennan Less replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"So, here's my thinking on furring down the ridge and insulating a bit from underneath. First, it might keep the ridge above the dew point, but it could very well remain in mold growth territory above 80% RH. Second, even if the ridge was…"
Aug 11
David N. Armington replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"The discussing seems to be devolving from the central problem of condensation along the ridge. I would appreciate an endorsement of a narrow ceiling along the top of the room, to isolate and insulate the thermal bridging.  Second, while…"
Aug 11
Brad Cook replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"Yes & No - We ventilate a roof to allow moisture to escape. In a well insulated roof with a vapor impermeable rigid insulation (warning- not all rigid insulation is a Class I vapor retarder!) vapor escaping is not a concern, but heat loss is.…"
Aug 11
Sean Lintow Sr replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"Yes & No - just remember that anyone in a region with a 50# PSF or greater snow load they should have ventilation as snow itself is a good insulator "
Aug 10
Brad Cook replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"Stripping a roof just to add a ventilation space is only an attempt at fixing the symptom. If you are going to strip the roofing, then add at least a few inches of rigid insulation first. With adequate vapor impermeable insulation covering the…"
Aug 10
David N. Armington replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"We've all seen snow-covered roofs where the snow has melted the lines of the rafters and the bays are still cold.  Wood, per inch, is a poor insulator compared to manufactured materials.  If you could live with an architectural…"
Aug 8
Charles Bennett replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"Anybody want to bet that if that closed cell layer had been installed on top of the roof deck that we wouldn't be having this discussion? Charles Bennett Mr Insulation;  San Rafael, Ca"
Aug 8
Kohta Ueno replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"Hi Brennan--I'm assuming you had a chance to read the cut & cobble article that Martin Holladay wrote, and that I commented on with pictures (Comment #7): Cut-and-Cobble Insulation: Does it ever make sense to cut rigid foam into strips…"
Aug 5
Jeff Classen replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"The thin application of expanding foam along the perimeter has a much lower R-value than the foam board. There are also small gaps between the foam board and the roof deck as others have pointed out. When I install with foam board and expanding…"
Aug 5
Ted Inoue replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"Tad, bingo! Nighttime extreme cooling of the structure from the roof down. Interior moisture migrates up during the morning until it reaches a critical RH and condenses on the metal enough to drip. Time delay because it takes a while for the…"
Aug 5
Kent Browning replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"I ran into something similar recently. I suspect your situation is similar but I can't say for certain from pictures and post on internet. So verify for yourself. Joe Lstiburek has written articles on this and several others have made…"
Aug 1
Ed Minch replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"You talk about interior panels that you removed.  Is the surface exposed to the interior airtight?  If not, on a hot day you could get convection-driven air leakage from the interior through the panel joints.  Depending on your local…"
Aug 1
Tad Everhart replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"Hi, Brennan, Good to hear from you.  Wish the circumstances were better.  I think you'll have an easy solution. We need more data.  Perhaps you could get temperatures of top of asphalt and bottom of roof sheathing boards for very…"
Jul 26
Chris Stratton replied to Brennan Less's discussion Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso
"Sorry this is happening, Brennan. Sounds like you're set on an unvented assembly (i.e., no air gap, soffit and ridge vents)? If it's the issue Ed is describing, maybe 1" polyiso continuous across the rafters could keep them from…"
Jul 26

Profile Information

Company
LBNL
Professional Interests
Building science research. Deep energy savings in existing homes. Performance monitoring. Ventilation and indoor air quality.
Twitter ID
@brennanless

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At 9:55pm on September 11, 2011, Diane Chojnowski said…

Hi Brennan,

Welcome to Home Energy Pros! Feel free to join the conversation in the Forum and Groups. And share your energy efficiency photos, videos and upcoming events. If you're blogging about home performance, this is the place for you!

Don't forget to upload your profile photo. And add your location on our Member Map!


Thanks!
Diane, HEP Moderator

 
 
 

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