The average installed R-value for closed-cell sprayed urethane is 5.9, not 6.8 (only foil-faced polyisocyanurate approaches that level). Additionally, the higher the cavity R-value the more significant, in terms of % degredation, become the thermal bridging of the framing. Also, because spray foam rarely fills the entire stud cavity, there is more 3-D thermal transfer through the studs.

With dense-pack cellulose, either damp or dry, all manufacturer's recommend (some even require) that no vapor barrier be installed. One of the many advantages of cellulose insulation is its hygroscopicity - it can buffer indoor RH by safely absorbing and releasing excess humidity (if the wall can breathe) and it protects wood framing by drawing moisture away (unlike foams which concentrate all moisture in the wood framing).

A cellulose-insulated envelope requires a good air barrier, but should not have a vapor barrier (vapor retarder latex primer is OK, but nothing more restrictive).

- Robert Riversong
  master housewright

--- On Thu, 3/11/10, Samuel Robins <[log in to unmask]> wrote:

From: Samuel Robins <[log in to unmask]>
Subject: Re: Suggestions on Insulation retrofits
To: [log in to unmask]
Date: Thursday, March 11, 2010, 9:05 AM

Tim,
Seems like R- 6.8 closed cell urethane foam would be a better / (over all) less expensive option, allowing you to forget the extra expenses involved with  wrapping the exterior. If you choose the cellulose & exterior wrap option, you probably should also include the expense of an interior vapor barrier, which would eliminate air infiltration and interior moisture escaping and becoming trapped within the cellulose filled cavities.  Just a thought.

Dave Robins

SDR & Associates
131 Cedar Street
East Hampton, NY. 11937
(631) 324-8868
(631) 324-0900 ~ fax

On Mar 10, 2010, at 11:47 PM, Ben Graham <[log in to unmask]">[log in to unmask]> wrote:

> Tim,
> Not sure if you have seen the Building Science Corp. method for this deep
> energy retrofit method, but I would check it out, they have it down pretty
> good.  Furring strips are utilized to secure the siding.  I don't think they
> would recommend polyiso or house wrap.  Essentially, if you seal the foam
> well, you don't need the wrap.
>
> Ben Graham
>
>
> On 3/10/10 8:29 PM, "Tim Yandow" <[log in to unmask]">[log in to unmask]> wrote:
>
>> Hello all,
>>
>> On the subject of green building, I am soon beginning renovation work on a
>> 1960's home in Burlington. We are adding an addition to the south and west
>> sides of the house. The new envelope will be 2x6 with damp spray
>> cellulose. The existing house is 2x4 with fiberglass and the owners would
>> like to bottom up the house more, so we are thinking about adding a layer
>> of insulation to the outside of the house since all the old vinyl siding
>> is coming down and the windows all replaced (most of them have broken
>> seals). I am trying to figure out the best way to insulate from the
>> outside and make provisions for new corner boards and hardi-plank siding.
>> With polyisocyanurate, for instance, there is the problem of not being
>> able to add house wrap and needing to use longer fasteners for siding and
>> to secure window flanges. Or can strapping be used over that? Then there
>> is the issue of extension jams either inside or outside...I would love to
>> know if anyone has worked out a good solid system to do this. What I have
>> read and seen so far does not satisfy me. Thanks.
>>
>> Tim Yandow
>
> Those who give up freedom for safety, deserve neither.
> Benjamin Franklin
> __________________________________________________________
>
> Ben Graham
> www.naturaldesignbuild.us
> Natural building/design services/workshops/consulting
>
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