[Greenbuilding] detailed (older) article on double-wall .77 ach50 house

[jfstraube]

There is no doubt that an unvented roof can work in a cold climate, but it requires exceptional airtightness. In practise, the most reliable way to get airtightness in such roofs is with spray foam.
Depending on how lucky you feel, a densepack roof can work.  It often does even in cold climates like Maine.  But such roofs also fail pretty often.  So the question is not "is this house working", the question should be "What percentage of roofs built this way will work".  In our experience, and that of other forensic consultants, an unvented densepack roof fails too often to be recommended.  
This is an on-going debate between people who sell densepack cellulose and those who build on one side, and those who respond to building failures and building science on the other side.
I just got back from California, a pretty warm climate, and spoke to the repair contractor of yet another densepack roof with water dripping out after the first winter: small cracks and openings around beam shrinkage and partition wall penetrations were enough to cause the very small air leaks required to cause this.  I can think of 3 other densepack roofs in sunny California with this problem, and countless in the cold northwest.  It is important to recognize that a blower door wont indentify these air leaks, as air flow from inside, through the roof back to the inside, and the exterior roof could be, and often is, perfectly airtight.

Check out BSI-043 on buildingscience.com for more.




On 2011-07-02, at 12:06 AM, JOHN SALMEN wrote:

> Good question - nice link. I'm still curious about roof assemblies. CMHC
> here did a practical study a number of years ago on vented unvented roof
> assemblies and the determining factor for condensation was air leakage from
> the interior. I'm basically detailing unvented roofs with pretty stringent
> air barriers but using materials that will allow drying to the interior -
> pretty similar to exterior walls.
> 
> I don't like the exterior wall material in this house. I have sheathing
> (standard ext. Ply) taped at all seams  with vb tape but also use a wrap - I
> think a little redundancy is a good idea. I like the 2x4 frame as that is
> what I typically use and if laid out properly can save a few trees. This
> place seems to use a lot of lumber though and probably more than he
> intended. A prefab truss manufacture for both walls and roof might have
> worked better with less lumber - but then builders in NA are generally
> framers.
> 
> John
> 
> 
> -----Original Message-----
> From: greenbuilding-bounces at lists.bioenergylists.org
> [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Kat
> Sent: July-01-11 3:17 PM
> To: Greenbuilding
> Subject: [Greenbuilding] detailed (older) article on double-wall .77 ach50
> house
> 
> From the most recent "Musings of an Energy Nerd" blog on the GBA 
> website: a link to a very detailed article from June 2009 on the 
> construction of a particular house in Maine that I hadn't seen before.
> 
> http://www.kaplanthompson.com/_images/publications/09.06-jlc.pdf
> 
> I wonder how their roof/ceiling assembly is holding up.  Does it allow 
> for enough drying potential?  Might they get condensation on the 
> underside of the roof sheathing (or is that physically impossible when 
> it's in full contact with cellulose)?
> 
> -Kat
> 
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John Straube
www.BuildingScience.com