[Greenbuilding] exterior eps and drainage plane was exsulation?

Tue Mar 22 18:48:13 CDT 2011

I would agree with aiming high on the exterior insul. I pay about 17 cents
per board foot of eps so it is not a big expense compared to the labour of
an renewing an exterior.  

I have a project that I am having trouble making a decision about the
location of the drainage plane. It is 2x4 frame with eps infill and 3.5" of
exterior eps. My original plan  called for a 10mm  drainage plane between
the wall ply sheathing and the insulation board similar to eifs systems.
The drain gap was planned as bottom drainage vent only and sealed at the
top, i.e. minimal venting.  The disadvantage of the wall was a reduction in
the thermal value of the exterior insulation depending on air movement
(there will still be air movement). How much of a reduction is hard to
determine and I am wondering what other disadvantages there might be.

The reason for the drainage plane at the sheathing is that the foam is being
covered with a render so didn't require battens as a nailing surface and I
felt it is easier to detail penetrations at the sheathing level with any
potential moisture having a better opportunity for drying.

From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of John
Straube
Sent: March 21, 2011 4:15 PM
To: Green Building
Subject: Re: [Greenbuilding] exulation

Adding insulation on the outside is a a great idea if you are interesting in
reducing heat loss and avoiding condensation.

The key to avoiding condensation is to use as high an R-value on the
exterior as possible, thereby warming the existing sheathing temperature and
hence reducing the chance of condensation.  Placing enough insulation
outside the existing wall and condensation is practically impossible.

In your climate, given a 2x4 wall, even 1" of extruded would be pretty safe,
and will decrease heat flow by about 1/4.  I would try for R7.5 or so, and
thus reduce heat flow to about 60% of what it was before.  Given that you
get to do this type of retrofit once every 40-75 years, I would aim high
with the R-value, and add R10 or more.  XPS (R5/inch) and polyiso (R6/inch)
are good choices given their higher R-values per inch, but EPS is fine and
even Rockwool can work (R4 per inch for both) 

In all cases, I would try very hard to get as good an air barrier as
possible, and it is critical to get a good drainage plane.  Taping joints
will do both, but some are concerned with the durability of the tape. 

Check out our website at buildingscience.com for case studies and details.

In my own house with 2x4 framing, I added 4.5" of ccSPF, and have no
regrets.  

On 2011-03-21, at 2:54 PM, Chris Koehn wrote:

I am planning some work on a 1970's single storey home here on Vancouver
Island. Replacing windows is the impetus. The home has vertical cedar
channel siding over tar paper over sheathing (both ply and solid sawn
recycled form wood in different areas) over 2X4 framing with fiberglass batt
insulation, over 6 mil poly vapor barrier over GWB. We are in zone 4C,
"mixed marine" with roughly 5400 f HDD. Our heating season is long but
rarely severe.
<http://www.greenbuildingadvisor.com/sites/default/files/H-T%20Zones%20-%20l
egend.jpg>

I'm considering removing exterior siding and adding a layer of rigid
insulation while I'm at it. Some siding needs to be removed anyway for
window replacement and some in other areas needs to be replaced due to
flicker damage, so taking it all off (and re-using it) doesn't seem to me to
be a substantial added burden. 

Quoting the U.S. DOE site: "Foam board insulation is commonly placed between
the exterior finish (i.e., siding, brick) and the studs of exterior walls.
To prevent air infiltration, you should place rigid insulation boards
tightly together and seal the seams with tape or caulk. However, this
practice may worry some builders in cold climates since the foam board may
act as a second vapor diffusion retarder. Studies have shown, however, that
condensation rarely occurs in these areas unless something else is seriously
wrong with the wall assembly (i.e., massive uncontrolled air leakage into
the walls from the house). If the assembly is constructed correctly, the
inside surface of the foam board stays warm enough to keep water vapor in
its gaseous state long enough for it to escape."

In the few areas I have opened the walls up I have seen no evidence of
condensation, though in some areas near the top plate some of the insulation
has turned grey, indicating air moving through it. It is possible, I
suppose, the air that's moving through the wall is not coming from the
inside.

My questions to the group are:

* Given our climate and wall profile, is adding insulation to the outside a
good idea?  

* Is there a type of insulation recommended for this work, specifically in
light of any concerns over moisture drive, as well as a desire by the client
to keep the thickness build-up minimal, and of course cost.

* What are suggestions around detailing: joints between insulation panels,
treatments at the bottom (siding currently laps directly over brick, which
is applied to a concrete foundation), fastening schedules (insulation and
siding), drainage plane. The house has min. 2' overhangs. 

Thanks as always,

Chris Koehn

TimberGuides

Cobble Hill, BC

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Dr John Straube, P.Eng.

Associate Professor

University of Waterloo

Dept of Civil Eng. & School of Architecture

www.buildingscience.com

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