[Greenbuilding] Exterior insulation retrofit

jfstraube jfstraube at gmail.com
Fri Apr 29 04:34:39 PDT 2011

I am not near by: Waterloo and Boston.
If you look at at the article by Betsy Petit in the retrofit of a 1916 Sears Roebuck house, you will see one approach we have used numerous times.
strip to board, drill holes and densepack with cellulose, IR scan to confirm all cavities full, Tyvek Drainwrap taped and seal (as air barrier and secondary drainage plane), special care at mudsill and roof with tapes and foam, then two staggered layers of foam insulation, outer layer taped (as primary drainage plane), 1x3 screwed to nominal 1" sheathing boards, cladding.  I think TimberLok are too big and I know that we dont need them: we use them because they are locally available in the 6-10" lengths we need.  For other project we order roofing screws from commercial roofing supply stores.

You would be "safe" with just 1.5" of polyiso over 2x4 with cellulose, but I would recommend more, two, 1.5" layers would mean you are done for the next 100 years.  

The larsen truss on the exterior requires more care and effort to get good performance, because foam plastics are air impermeable, non-hygroscopic, and vapor semi-impermeable: using cellulose as exterior insulation is a completely different animal and requires better care and control of air flow control, reverse water vapor drive (particularuly with shingle cladding) etc.  Is, has, and can be done, but lots more things that have and do go wrong.

On 2011-04-28, at 9:43 AM, bill.allen at verizon.net wrote:

> John,
> I am in Mt. Vernon, NY, climate zone 4 about 15 miles north of New York City.
> Looking for recommendations for new barriers....some say taped styrofoam is good enough for both but I am open to suggestions....
> Are you near by?
> Bill
> -----Original Message-----
> From: "John Straube" <jfstraube at gmail.com>
> Date: Thu, 28 Apr 2011 12:57:16 
> To: <bill.allen at verizon.net>; Green Building<greenbuilding at lists.bioenergylists.org>
> Reply-to: jfstraube at gmail.com
> Subject: Re: [Greenbuilding] Exterior insulation retrofit
> I have done this. What climate are you in? To avoid condensation even 1.5" is usually more than enough but I would recommend more for energy
> what is your new water control layer?
> Air control layer?
> Sent wirelessly from my BlackBerry device on the Bell network.
> -----Original Message-----
> From: bill.allen at verizon.net
> Sender: greenbuilding-bounces at lists.bioenergylists.org
> Date: Thu, 28 Apr 2011 12:45:46 
> To: Green Building<greenbuilding at lists.bioenergylists.org>
> Reply-To: bill.allen at verizon.net,
> 	Green Building <greenbuilding at lists.bioenergylists.org>
> 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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John Straube

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