[Greenbuilding] innovative solution sought

[Rob Dickinson]

We have a 1900 farmhouse and had the same problem with having only 2x4
rafters. They were a true 4", but still way to shallow for the insulation
levels that we wanted to achieve.

We solved it by building a parallel set of 2x4 rafters with a 4-1/2" space
between them and plywood gussets connecting the two sets of rafters. The
plywood gussets are about 4" x 12".  I called these rafter extensions, or
insulation trusses. The new depth of my rafter cavities is now 12" (or
12-1/2", I can't remember).

The space between the rafter sets provides a thermal break between them,
preventing heat from transferring via the wood framing members.  We sprayed
5" of high-density closed-cell foam against the original interior sheeting,
completely covering the old rafters and providing an additional thermal
break as well.

Then we installed about about 7" of dense-pack cellulose over the spray
foam to the edge of the rafter extensions.

The final R-value is around R-63, and the whole assembly is super air-tight.

Rob

On Sep 10, 2016 11:15 AM, "Sacie Lambertson" <sacie.lambertson at gmail.com>
wrote:

> In response to my query looking for innovative inexpensive ways to extend
> old 2x4 rafters to allow for about 8" of dense pack cellulose, I've
> received a number of responses.
>
> Included is the suggestion we rebuild the roof by placing more insulation
> on top and make it vented.  Am afraid this is a non starter because we
> don't want to have to rebuild the existing soffits AND vented roofs are not
> a good idea in an otherwise tightly built house.  Also rec. the suggestion
> we sister/scab wider wood onto the 2x4s, which is exactly what I want to
> avoid.
>
> OTOH scabbing cheap plywood onto the rafters is a good idea.  We wouldn't
> have to do this for every rafter nor for the full length.  Maybe we could
> install cables across the scabbing to hold the netting in place after it
> the dense pack cellulose has been blown in.
>
> Welcome further comments/ideas.
>
> Sacie
>
> On Fri, Sep 9, 2016 at 11:32 AM, Nick Pyner <npyner at ihug.com.au> wrote:
>
>> Cheap plywood shot to the side of the rafters?
>>
>> On 10/09/2016 12:59 AM, Sacie Lambertson wrote:
>>
>> All, We have an old house in which we want a well insulated unvented
>> attic.  We are replacing the existing roof (with metal) and installing a
>> minimal layer of XPS on top of the sheathing. We plan to use closed cell
>> spray underneath followed by dense pack cellulose between the rafters.  The
>> problem is the rafters, which are only 2x4s in this old house.  I am
>> looking for a creative least expensive way to provide more rafter depth for
>> the insulation.
>>
>> (We do not want to build up the insulation on the top of the roof
>>  sheathing because doing so requires the added cost of completely
>> rebuilding the existing soffits, which are in good condition).
>>
>> Will appreciate all your useful suggestions.
>>
>> Sacie
>>
>>
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