insulating 2x4 trusses

[littleboss]

Thoughts on insulating the top chord of 2x4 trusses? I will probably have 2" of closed cell spray foam applied to make it airtight but would then like more insulation as well as to help with thermal bridging. After 2" of foam I am only left with 1 1/2", how would I secure fiberglass batts to the trusses? Only thing I can think of it baling wire.

 

[ace10]

Are the soffits and ridge vented?
Is there a ceiling?

 

[ericm]

I'd either go with more spray foam or skip the foam and use batts (rockwool or glass). Mixing the two seems like you'll get the worst of both- the expense of the foam and the hassle of the batts.

If you want to seal the building you could use great stuff type foam on the gaps instead of depending on the spray foam to do it.

 

[dcg9381]

From what I've seen building "green" homes the standard solution in framing seems to be installing a thermal strip over the 2x4 to help prevent the thermal bridging that occurs the top chord surface. You can buy "thermal break tape".

I've never done it...

I will say that if you're coating the trusses and roof deck 100%, you're keeping that heat on the right side of the envelope. If you're doing it that way, I think it's a relatively minor advantage. The most likely use case is in a traditionally framed wall where there is no insulation between the outside material and inside of the drywall - in those cases, the 2x4 is a pretty big thermal bridge...

 

[littleboss]

Are the soffits and ridge vented?
Is there a ceiling?

No vents and no ceiling. I might add one later

 

[littleboss]

From what I've seen building "green" homes the standard solution in framing seems to be installing a thermal strip over the 2x4 to help prevent the thermal bridging that occurs the top chord surface. You can buy "thermal break tape".

I've never done it...

I will say that if you're coating the trusses and roof deck 100%, you're keeping that heat on the right side of the envelope. If you're doing it that way, I think it's a relatively minor advantage. The most likely use case is in a traditionally framed wall where there is no insulation between the outside material and inside of the drywall - in those cases, the 2x4 is a pretty big thermal bridge...

The roof is tin over OSB. As far as I know there is no vapor barrier. So if I go with the foam the foam will provide the vapor barrier. Using only 2" leaves me 1.5" exposed on the sides of the 2x4 as well as the face. Just the faces of the 2x4 reduce the whole wall R value by a lot, add in this extra surface and it probably doubles or triples

 

[ericm]

No vents and no ceiling. I might add one later

Either put in vents and a ceiling and insulate the ceiling, or don't put in either and insulate the underside of the roof.

If it gets hot where you are, I'd do the ceiling and vented attic. A good vented attic will be cooler than the underside of the hot roof. It's also easier to insulate and to insulate to more depth on the ceiling.

 

[littleboss]

I'd either go with more spray foam or skip the foam and use batts (rockwool or glass). Mixing the two seems like you'll get the worst of both- the expense of the foam and the hassle of the batts.

If you want to seal the building you could use great stuff type foam on the gaps instead of depending on the spray foam to do it.

Foam in a can would be a big job.

I could go all fiberglass but not sure what to do about a vapor barrier. The roof is tin over osb with no vapor barrier.

In the house I used the flash and batt. 2" of foam and then R13 batts. Got R26 out of it and saved $4000 doing it that way.

Not sure what the foam would cost but I could either have the spaces filled to 3 1/2 to 4" and have him spray foam on the face of the 2x4 as well.

Either that or have him spray 2" but also have him spray the exposed sides and edge of the 2x4

 

[ericm]

Foam in a can would just be to plug the gaps air can get through, not to insulate.

Most places in the US they don't use vapor barriers, just water barriers (i.e. Tyvek) over the sheathing.

If you insulate the underside of the roof and later put in a ceiling and turn it into a vented attic, you'll be wasting the money you spent on the insulation.

 

[jabin]

Search the youtube Spray Jones for his video about flash and bat.

 

[littleboss]

Foam in a can would just be to plug the gaps air can get through, not to insulate.

Most places in the US they don't use vapor barriers, just water barriers (i.e. Tyvek) over the sheathing.

If you insulate the underside of the roof and later put in a ceiling and turn it into a vented attic, you'll be wasting the money you spent on the insulation.

When I put in a ceiling I will NOT be converting it to a vented attic.

 

[Firebrick43]

When I put in a ceiling I will NOT be converting it to a vented attic.

you can probably insulate with blown cellulose and do the a steel ceiling for the price of spray foam. You would also have the ability to have higher r value than a spray foamed roof deck.

Don't get me wrong, if you have rafters, are applying to osb, and want a catherdral ceiling i think spray foam is great. I really don't see any advantage to applying it with trusses and a ceiling.