Getting R38 Into 2x10 Vaulted Ceiling

[AlleyDude]

The plan calls or r38 insulation in the ceiling. The r38 fiberglass batts are 12 inches thick, but the rafters are only 2x10 (which is actually only less than 9 1/2 inches thick. Would compressing the batts into that smaller space still yield r38? I am trying to avoid the high cost of closed/open cell foam. Thanks!

 

[LB-1911]

That is why they manufacture compressed insulation. 2x12's would allow for venting & the use of 38c.

The rafters in cathedral ceilings are usually made of 2x10 ft. or 2x12 ft. and Owens Corning EcoTouch high-density insulation products are designed specifically for these rafters.

R30C and R38C are available in both kraft-faced and unfaced.


Technical Bulletin compressed R-value
http://www2.owenscorning.com/litera...ul Compressed R-Value Chart Tech Bulletin.pdf

 

[AlleyDude]

Just a little confused. So should I go ahead and put the 12inch r38 batts between the 2x10s? I would also use the plastic channel vents against the underside of the roof. Thanks

 

[AlleyDude]

LB-1911 Thanks. Just saw the chart you provided. I am No longer confused.

 

[zmaxmotorsports]

Don't compress insulation.
You also shouldn't have insulation touching a roof,so Ive been told for many years by lots of roofers.

 

[Gerald O]

If you add the vent baffles you'll need to subtract the thickness of the vents from the dimension of the framing space to get the compressed R-value. For example, if your vent baffle is 1.5" thick then your 2x10 (9.25") cavity is reduced to 8.75". R-38 squeezed into that space is going to only yield about R-30 effective rating.

To get the full R-38 value you are going to need to fur the rafters to create more depth.
Check to see if there is an exemption in your building code for vaulted ceilings that allows about 500sqft to be insulated at R-30.

 

[DC73]

You can get R38 in 9" of space using mineral wool instead of fiberglass. However, I don't think they make R38 mineral wool batts. They do make R23 (5.5") and R15 (3.5") so you could accomplish what you want with two layers. Mineral wool is fire resistant and helps sound proof as well.

DC

 

[Scott r c]

Don't compress insulation.
You also shouldn't have insulation touching a roof,so Ive been told for many years by lots of roofers.

I have seen three projects lately, one of them a house, where they are putting the insulation directly against the bottom of the roof. Kinda different but the attic is a nice temperature if you have to get in it.

 

[Randy in Maine]

I did the closed cell spray foam in mine, but my ceiling is just 2x6s. No more ice dams or roof raking though!

 

[DC73]

You also shouldn't have insulation touching a roof,so Ive been told for many years by lots of roofers.

I have seen three projects lately, one of them a house, where they are putting the insulation directly against the bottom of the roof. Kinda different but the attic is a nice temperature if you have to get in it.

It has to do with water vapor moving through the insulation and condensing on the bottom of the roof. If the dew point is sufficient, the bottom of the roof deck is at a certain temperature, and the air has enough moisture, condensation can occur.

This is somewhat climate dependent and the type of insulation is important. You can spray closed cell spray foam against the roof in any climate. Open cell foam is fine in certain climates and at specific thickness. Fiberglass is not recommended to be in direct contact with the bottom of the roof. Neither is mineral wool. Both fiberglass and mineral wool require a vent channel between the insulation and the bottom of the roof.

DC