I am curious/concerned about advising a tight house (addition) with a cathedral ceiling. I understand that normally, one should keep a 1-2" gap in the rafter bay above the insulation as an air gap. But, if the construction is tight, to achieve a low ACH50 or 1-2, what is the point?How is air (moisture) going to escape if the enclosure, including the roof sheeting, is sealed tight? Also interestingly, the plans did not include any mandatory ventilation for this 780 sqft addition in So. Cal. by the ocean. 

Thank you. 

Dav  

Tags: Tight, control, enclosure, moisture

Views: 250

Reply to This

Replies to This Discussion

It depends, are they going with a hot roof system? If so no air gap is required

The reason for the gap is because even with super low leakage numbers, you still have leakage & the reason the bays get ventilated is to eliminate condensation issues with the sheathing

Most places will not require "ventilation" for additions as they assume the house has it already & it is not a mandatory no matter what if you do X you must put Y in like smoke detectors / carbon monoxide

I agree with Sean, that the main concern is condensation, and in northern climates frost accumulation in the attic.  In southern CA, I am sure those are not a big considerations. You should still have some low attic venting, but it is not as critical.  

Paul

With a vented cathedral ceiling, getting a tight envelope is difficult, but not impossible. Various products are available to maintain the vent gap, some of which are easier than others to air seal.

It's easier, and I believe better, to go with an unvented assembly, in which case the insulation must be fully in contact with the roof, and depending on climate zone, some portion must be air impermeable (less in milder climates, more in colder climates). The purpose is to prevent house moisture from coming in contact a cold roof.

House moisture is removed by spot exhaust in kitchen & baths. Whole house ventilation has a different purpose.

I'm not familiar with California Title 24 requirements for unvented roofs but here's a link to the IRC Chapter that covers that detail (scroll down to Section R806.5): IRC Chapter 8

It is all about the dewpoint. Think of the roof system as a big box. Air can circulate in, out and around the inside of that box. If you could possibly air seal the entire inside plane, and install a durable, continuous and effective vapor barrier over the entire inside plane, you still have the other 5 sides where air and/or moisture can enter (or leave) the box. When the outside temperature falls by a significant amount (as when it is cold enough for snow in the South), you are most likely to hit the dewpoint on those 5 sides. When that condensation pools up in the air permeable insulation, it will be difficult for that water to get out of there unless there is a reasonable connection for air to move in and out. By the time the water eventually gets out, it could promote the growth of fungi, such as wood rot. That is why we ventilate roofs. If there is enough air and vapor impermeable insulation on the roof deck, then the temperature inside the box should stay above the dewpoint and ventilation is not needed (or desired).

I would refer you to this document by Building Science Corp., which tackles the issue of ventilation in cathedral or flat roofs. You don't need to ventilate, but you need to carefully design the assembly.

https://buildingscience.com/documents/insights/bsi-043-dont-be-dense

"Why such a difference between walls and roofs? Walls in most older houses can dry out to the exterior because they rarely have exterior vapor barriers. Roofs have either roof membranes or asphalt shingles that are exterior vapor barriers, and they can’t dry out to the exterior unless the roof assembly is vented to the exterior. We have developed ways of dealing with unvented roof assemblies and insulation, but dense packing them is not one of them. I’ll talk more about the alternative ways later. Walls have “wiggle” room. We don’t have to be perfect because some drying can occur to the outside. We do not typically have the same “wiggle” room with unvented roofs."

RSS

Forum Discussions

Overheating garage

Started by Chris Bellanca in Best Practices 1 hour ago. 0 Replies

GSHP for domestic hot water

Started by David Holtzclaw in General Forum. Last reply by David Holtzclaw 4 hours ago. 3 Replies

Log home roof insulation

Started by Bob Reuther in Best Practices. Last reply by Daniel Baur-McGuire yesterday. 8 Replies

Latest Activity

Chris Bellanca posted a discussion

Overheating garage

Hi all, I'm looking for potential solutions to a problem at a client's house.  The south facing 2…See More
1 hour ago
Landon added a discussion to the group Energy Auditing Equipment for Sale, Trade or to Purchase
Thumbnail

Brand new retrotec blower door system $3000

Hi, I’ve got a brand new blower door system from retrotec that I got 2 months ago, and never got…See More
1 hour ago
John Redmond posted discussions
1 hour ago
David Holtzclaw replied to David Holtzclaw's discussion GSHP for domestic hot water
"Thanks John & Bill. Bill your experience is one reason why I wanted a larger heat pump with…"
4 hours ago
John White posted photos
13 hours ago
Diane Chojnowski posted events
yesterday
Rob commented on Bob Krell's group Healthy Indoors (IAQ)
"Hello, Has anyone tried custom filtering of window AC units? Specifically concerned with…"
yesterday
Michael Dunseith's 2 photos were featured
yesterday
Franco Oyuela commented on Perry Ning's blog post Warnings About Portable Air Conditioners
"Here are the top five benefits of owning a portable air conditioner: 1. Low Price – All…"
yesterday
Daniel Baur-McGuire commented on Kurt Shafer's status
"Whole house fans are a scam if you use air conditioning. Seriously "
yesterday
Daniel Baur-McGuire commented on Michael Dunseith's photo
Thumbnail

NYC DEPT OF BUILDINGS TR8 ENVELOPE TESTING

"Looks fun. How did you connect all the fans together or did you adjust the flows manually? The…"
yesterday
Daniel Baur-McGuire replied to Bob Reuther's discussion log home roof insulation
"I would look to stop air from penetrating the ceiling. I would guess hot air is able to rise…"
yesterday
Jay Lillien posted a blog post

How Changing Temps Affect Your Home’s Windows

In Colorado, there’s a time-honored saying:“If you don’t like the weather, just wait a few minutes……See More
Tuesday
Matt Peiris replied to H.O. Electric's discussion Stop Drinking Bottled Water! in the group News & Announcements
"Cities like Los Angeles now use chlorine AND chloramine to treat their water. While chlorine can be…"
Tuesday
Matt Peiris joined Diane Chojnowski's group
Thumbnail

News & Announcements

Share your news and announcements with the community in this group.To post news or an announcement,…See More
Tuesday
Michael Dunseith posted photos
Tuesday
Ron Jones added a discussion to the group Job Board
Thumbnail

Residential Construction Superintendent for Net Zero Energy Homes: Northern California

At Acre Designs (acre.co), we're bringing Net Zero Energy homes into the 21st century, and we're…See More
Monday
Kim added a discussion to the group Job Board
Thumbnail

Job Opening - New Construction Specialist - Philadelphia or Pittsburgh, PA Location

New Construction SpecialistAre you looking for a collaborative work environment, colleagues who are…See More
Monday
Profile IconSteve, Todd Crawford and Evangeline Spence joined Home Energy Pros Forum
Monday
Steve liked Alex Jonson's discussion Constant Airflow Regulator
Monday

Photos

  • Add Photos
  • View All

© 2018   Created by Home Performance Coalition (HPC)   Powered by

Badges  |  Report an Issue  |  Terms of Service