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nofx1981 02-21-2010 02:15 PM

Rafter Vents and Insulation
Hi guys, I've got a question about insulating the rafters in a cathedral roof between a knee wall and collar ties. The rafters are 2x6's and I've installed rafter vents in every one, the vents are 1 1/4" high. My question is, can I still lay down insulation meant for 2x6 studs over top the vents or will I have to do either:
a) lay down insulation for 2x4 studs
b) add 1 1/4" furring strips to the rafters?

picture of r22 insulation layed over top a the rafter vent

Scuba_Dave 02-21-2010 04:10 PM

You can still 2x6 insulation so long as it does not crush that vent
Usually compressing insulation reduces the RValue

AaronB 02-23-2010 11:21 AM

You could use SPF and forget about your worries.

nofx1981 02-23-2010 11:48 AM

Thanks for the replies, I'm gonna stick with the 2x6 insulation I started with

Big N8 02-25-2010 11:41 AM

Make sure you use ridge vent on that. Plus you will not lose all that much r value when you compress it that much.

nofx1981 03-09-2010 01:23 PM

already ridge vented :thumbsup:

AaronB 03-09-2010 03:26 PM

That configuration is not doing what you need it to do... only foam insulation can do that.

Big N8 03-10-2010 08:42 AM

So are you saying every roof with a vaulted/cathedral ceiling needs to have spray foam?????

AaronB 03-10-2010 11:39 AM

Well, for maximum insulating value according to current code, yes.

AaronB 03-10-2010 11:40 AM

Vaulted and Cathedral ceilings are not the same thing... so no, but fiber insulations are a HUGE waste of money.

Scuba_Dave 03-10-2010 11:44 AM


Originally Posted by Big N8 (Post 412511)
So are you saying every roof with a vaulted/cathedral ceiling needs to have spray foam?????

R22 is under the required insulating value for a roof here
Here R30-R38 is the Min allowed
You will be paying much more in the long run in heating costs

nofx1981 03-10-2010 12:12 PM

The attic won't be heated, just being used for storage. I'm trying to minimize the extreme temperatures we see in the summer and winter up there.

Big N8 03-10-2010 12:38 PM

So it is the R value not the type of insulation that matters????

nofx1981 03-10-2010 03:47 PM

Different types of insulation will have different R values, R values is what typically matters when you're looking at building codes.

AaronB 03-10-2010 08:09 PM

True, to an extent, nofx1981

To understand thermal efficiencies, we must first try to understand how R value is measured/determined. For this, I have a nice article that I have stolen from the web. Let me go find it....

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