[Greenbuilding] Exterior insulation retrofit

elitalking elitalking at rockbridge.net
Thu Apr 28 07:42:27 PDT 2011

Yes, I have just completed a project very similar.  My house was built in 
1995 with conventional fiberglass batt and electric plugs in wall. 
Therefore I left those walls as they were.  I mounted 3" isocyanurate boards 
over the existing plywood siding attempting perfect seals with acoustic 
caulk at seams.  I use spray foam cans to fill and seal gaps that occur. 
Recognizing the difficulty of perfection, I installed building wrap over the 
foam boards that is fastened at the top and bottom as a redundant attempt at 
continuous air barrier.  I installed 4"x 3/4" osb vertical furring strips 
using long screws through foam and into original framing.  I also dabbed 
acoustic caulk on back of furring strip where screws came out in an attempt 
to seal the hole in building wrap I was creating with long screw.  The 
furring strip created the drain plain that John Straub was talking about. 
Over this I installed textured plywood and 1x3 battens to create a board and 
batten look. Other sidings could be used.  It is stained, therefore, have 
that continuing maintenance issue.  My concern about cedar shingles is that 
they are highly flammable.  I had to install jamb extensions on all the 
windows and doors and reinstall the trim.  I was able to reuse some most of 
the original trim wood.  This only describes the walls.  I also installed 3" 
isocyanurate under existing ceiling and installed new finish ceiling below. 
Also, I am installing 3" iso boards below floor in crawl space with building 
wrap again and osb sheathing for rodent protection.

We actually removed our heatpump; therefore no longer have the option to 
cool in our mixed humid Virginia climate.  However, the 3 inch continuous 
tight foam should keep surface temp on hot side above dew point, even if we 
install cooling.  When the house envelope is completed, we can consider a 
high efficiency mini split air source heat pump and a grid tied photovoltaic 
as a path the net zero.

It was a lot of work.  However, I am practicing what I preach.  Good luck 
with your project.  You are on the right path.


----- Original Message ----- 
From: <bill.allen at verizon.net>
To: "Green Building" <greenbuilding at lists.bioenergylists.org>
Sent: Thursday, April 28, 2011 8:45 AM
Subject: [Greenbuilding] Exterior insulation retrofit

> Hi all,
> I am researching a project for my single family home. Classic 1926 stick 
> construction. Zero insulation in the walls...time yet again for an 
> external paint job. Would really appreciate comments on the following 
> plan:
> We never want to paint again...are thinking of natural cedar shingle 
> siding, maybe with preservative but otherwise left natural.  Don't like 
> the embodied energy in the various cement siding 
> solutions....alternatives?
> While the siding is down, fill the wall cavities with cellulous from the 
> outside.
> Replace original double hung windows with new construction insulated, R-5 
> if we can afford it, windows mounted "innie" on the plane of the original 
> dimensional 1x lumber t+g sheathing.
> Add external insulation....rigid styrofoam.  Since there is no interior 
> vapor barrier (plaster on wood lathe), I am worried about the dew point 
> ending up in the center of the wall...therefore, per the building science 
> article, add 4" of styrofoam to the outside to keep the dew point external 
> of the sheathing.
> Will require careful details at windows and other penetrations.
> Big job...has anyone done this?  Any comments on any part of the above 
> would be most welcomed.
> Thanks,
> Bill
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