[Greenbuilding] innovative solution sought

[Steve Satow]

My 2 cents worth…

Buy a load of 2x2 (or rip 2x4s in half) and then glue and nail 4" long blocks to the existing rafters at 3' or 4' spacing. Then glue and nail full length 2x2s to those blocks. Not only will this give you an additional 3.5 - 4" for your cellulose, but it will have the effect of strengthening your existing rafters to some extent while reducing thermal bridging.

Steve.


On 2016-09-10, at 12:27 PM, Rob Dickinson <robddickinson at gmail.com> wrote:

> 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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