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

Sat Jul 2 10:07:48 CDT 2011

I have a thing against using cellulose insulation - though it has little to
do with its insulating properties. I should have mentioned that I just use
foam board stock for roof insulation (not as air barrier) - laid on a
substrate. Stability in a material needs to be predictable - sheet stock is
produced with uniform conditions, materials and can be somewhat relied upon
for dimensional stability, even deformation is predictable. Foaming around
structural components on site is just not reliable. Thickness and density
can vary, equipment can be faulty, operator error or inexperience - too many
variables. Also framing is predictable in that it will shrink and move (that
is what wood does).

Same with cellulose - subject to oxygen, moisture, vibration and heat
(conductors being pipes, framing, etc) it will become embrittled, shift,
settle - that is what the material does. 

Insulation shouldn't be relied upon as the primary air barrier at a ceiling.
Poly vb should also  not be relied upon. For residential work drywall,
gaskets and caulk can work and be easily maintained. I don't do a lot of
conventional residential but on the projects I've done I've detailed all the
ceiling penetrations with sealable pipe gaskets backed on ply. I think a
layer of xps sheathing boards at the ceiling with seams taped would be an
excellent approach behind drywall but there is a lot of resistance to
altering framing dimensions that much.

-----Original Message-----
From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of
jfstraube
Sent: July-01-11 9:29 PM
To: Green Building
Subject: Re: [Greenbuilding] detailed (older) article on double-wall .77
ach50 house

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

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