Insulation requirements question

There is 2x10 for rafter material being used. In the northern states r-38 is required in ceilings, how would you insulate the rafters if it was a vaulted ceiling (meaning they want to attach drywall directly to the rafters for a sloped ceiling) 2x10 measures 9 1/2" in depth, what materials would you use to acheive the r-38 requirments and yet have the 1" air gap against the roof deck sheating???

Ridge insulation then batts of some sort???

Yes. That’s one way. Working from the underside of the roof deck;- staple up your vents,- then put up the batts in between the rafters- then put up rigid foam insulation sheets covering both the batts and the rafters- then drywall.It will perform better because of the thermal bridge break. If you are diligent in sealing up the seams of the foam sheets it will also provide as a very good vapor barrier.

How would this give you the required R-38??
Batt insulation (R value ??) + ridge insulation board (R value ??) = R-38
describe the types of insulation materials you would use.

Careful … because typical batts already has a vapor barrier on the back side of the paper. You don’t want a double vapor barrier that may trap moisture.


Hi. Brian;

Paul directed you well, he just did not quote you any values.
Maybe I can help you with that.

Once you have installed your baffle vents for continuous venting from soffit to ridge vent, the only comfortable space left for insulation (batt) is 8".

Batt insulation has an R-Value of 3.14 per inch so this would equal 25.12. or a high density would be r- 30.
Required R-Values for roofs in the NorthEast is R-38 38.
Deficit -12.88

Paul recommended Polystyrene, I believe and that is 5.00 p.i. 2" = 10

Polyisocyanurate has an R-Value of 7.20 P. I. 1 &1/2" = 10.8 2"= 14.4

Either one, you would have to strap the ceiling in order to sheetrock, unless one wants to use extra long screws.


Icceynene Spray foam open cell 3.6 R-Value per inch

Polyurethane Spray foam closed cell 6-7 R-Value per inch

One has more insulating values as well as price.

Hope this helps.

Marcel :slight_smile: :slight_smile:

Pssst!..Marcel. 2x10’s are 9 1/4", minus 1" for the venting equals 8 1/4"…but you are giving some valuable information. :stuck_out_tongue: :smiley: :wink:

Sorry Larry;

Thanks for the warning, just too many numbers in my head I guess.
Corrected my post, hope I got it right this time. ha. ha.

Marcel :slight_smile: :oops: :lol:

Ahhh…no “warning” meant, Marcel…just funnin’ ya…:smiley:

You are a wealth of good info, thanks. :smiley:

You can also use a foil faced ridged foam like Energy Sheild for example. Because the foil reflects radiant heat it adds an effective ~R2.2 to the value of the foam for any given thickness. Mineral wool batts also have a slightly higher R value than fiberglass.

Be aware that the code allows for R-30 (normally batts) for vaulted ceilings and R-38 (normally blown-in) for flat ceilings. The vaulted ceiling allowance is good for up to 50% of the ceiling surface area. Anything over that requires R-38. As mentioned in thread above, if there is inadequate space for conventional insulation, then higher R-value material must be used.

Two by strapping would take care of the problem. It will give you the required 1" air space and enough left to insulate to required value. The only problem I see is the strapping may interfere with the window height.

This is strange as I have not seen paper backed batts since the sixties.

Purchased at the local Home Depot when I insulated my own 2x10 cathedral ceiling: styrofoam baffles & r-30 owens corning fiberglass(kraft faced)
Code in Buffalo called for r-30.

Not sure what type of insulation is used in you area, but in my neck of the woods almost all the faced insulation blankets are made with kraft paper facing that has a tar like coating on the back side which serves as a vapor retarder which is just called paper faced batts … e.g.



Pennsylvania allows a wide variety of compliance paths for energy conservation and Code complinace, and R-38 is not always required for ceiling insulation in Pennsylvania.

One can use Chapter 11 of the IRC 2006, the 2006 Pennsylvania Alternative Energy Code, the Department of Energy RESCheck software, and many various alternative complinace methods and approaches under the 2006 IECC.

R-38 for ceiling insulation is not written in stone in Pennsylvania and most designs do not include R-38 anymore.

The IECC allows, for example, that in a cathedral ceiling, you can use R-30 and still comply with the Code provided that the area of the cathedral ceiling with less than R-38 insulation is less than 500 square feet. (404.2.2 2006 IECC)

REScheck and other performance based compliance methods allows one to reduce ceiling insulation R-values provided wall insulation, window U-factors, or heating and cooling system efficiciencies are increased to make up the difference.

If you suspect a less-than-expected level of insulation on Pennsylvania, have your client refer to the local Residential Energy Codes Inspector for the municipality to verify the structure is compliant.

You can see the Pennsylvania Alternative Energy Code online here:

You can see the Department of Energy RESCheck and other types of performance based software here:

I guess that with all of this, if the Builder were smart would install a SIF on the roof and the R-factor would not be an issue no matter what State you are from or Code regulations.

Marcel :slight_smile: :slight_smile:

That assumes the tabs on the paper facing are adequately attached/stapled to the studs/joists … :roll:

P.S. And for an HI I don’t think it really matters if you have R30 or R38, as long as it’s not a phased inspection on new construction. A home inspection is NOT a code compliance review.

As long as it’s not significantly under-insulated (e.g. only R15 or R19 for an attic in the north that would usually require around R30 or R38 it’s close enough for an HI inspecting an existing home.