[Greenbuilding] exulation

[John Straube]

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-%20legend.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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