Home>Discussions>INSULATION & HVAC>extending width of rafters for insulation
12 posts / 0 new
Last post
millstone
extending width of rafters for insulation

My son purchased a 1940's home and we plan on making the attic space a bedroom. The roof rafters are 2x6 on 24" centers, we would like to add as much R value insulation as possible. Our thought is to extend the depth of the 2x6 by adding or attaching a 2x4 to the rafter, making it now a 2x10. Our thought was to glue and screw the 2x4, then add 9.5" batts for an R value of 30. Then, attached to the added 2x4, we would cover it with 1/2" drywall. There would be a ceiling about 8' tall and a 4' knee wall. Is this doable?

thanks

tom

HoustonRemodeler
Re: extending width of rafters for insulation

Certainly doable but would you want to?

Normally insulation should not be in direct contact with the plywood of the roof system. You'll collect moisture there and cook the shingles off the roof.

You may find spray foam insulation a better option when combined with a venting system for the roof sheathing.

millstone
Re: extending width of rafters for insulation
HoustonRemodeler wrote:

Normally insulation should not be in direct contact with the plywood of the roof system. You'll collect moisture there and cook the shingles off the roof.

You may find spray foam insulation a better option when combined with a venting system for the roof sheathing.

Hi HustonRemodeler

My plan was to use those foam panels that allow air to move from the sofit to the ridge vent. The problem with high density foam is it must be done by a professional and the CO$T. hoping to keep the cost down and the R value high.

thanks

tom

HoustonRemodeler
Re: extending width of rafters for insulation

sounds good to me Tom:)

jkirk
Re: extending width of rafters for insulation

regarding spray foam insulation, yes it does HAVE to be done by a professional. they have the gear both to spray and to protect themselves from breathing it in. its also 3x the cost of batt insulation.. locally 2lb foam is $3.75 a sq/ft and 6 lb foam is around $5

Sombreuil_mongrel
Re: extending width of rafters for insulation

Hi,
I would put in 6" r-19 batts, then cover the whole thing with 2" rigid foam for another r14, use 3" drywall screws to hang the rock. You end up with better than r33, and save an awful lot of lumber. And the foam spanning over the rafters gives a full thermal break. So you can also decide whether to put in baffles and ridge vent it.
Casey

motoguy128
Re: extending width of rafters for insulation

Use open cell spray foam and completely seal the attic space. Any air movement through fiberglass and it's R value drops fast. Trying to vent effectively in a narrow space.

The measured shingle surface temp in the field of sealed attics vs. vented attics is 5-10F depending on conditions. That might matter in AZ, but not so much anywhere else.

Sealed attics stop air movement. You'll find that the downstairs is less drafty as well.

I've read of one person venting an attic that was sealed and finding that it made his home drafty and cold because it accelerated the natural stack effect in the house.

dj1
Re: extending width of rafters for insulation

All good suggestions. I'd choose sombreuil's, and here's why:

1. you can do it yourself.
2. not so expensive.
3. very effective.
4. no fumes, no mess.

make sure you don't forget baffels. Ridge vent may or may not be necessary depending on your current vents.

HoustonRemodeler
Re: extending width of rafters for insulation

I've done the layered insulation board method before with great success. 2 layers with staggered joints works very well.

millstone
Re: extending width of rafters for insulation

Thanks everyone, great information. never thought about what sombreuil_mongrel suggested. Have to think/plan that one out. Just to make sure, the 3" screws going through the 2" of rigid foam should hold both with no issues? Should the foam board be glued to the rafters? What type of foam board should I look for and should it be faced?
thanks

tom

Timothy Miller
Re: extending width of rafters for insulation

Howdy 8' tall ceiling are you wanting to insulate the rafter bays ? or create a 2by6 ceiling below the ridge beam?

Pages

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.