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Old 02-21-2010, 02:15 PM   #1
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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
or
b) add 1 1/4" furring strips to the rafters?

picture of r22 insulation layed over top a the rafter vent

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Old 02-21-2010, 04:10 PM   #2
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Rafter Vents and Insulation


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

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Old 02-23-2010, 11:21 AM   #3
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You could use SPF and forget about your worries.
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Old 02-23-2010, 11:48 AM   #4
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Thanks for the replies, I'm gonna stick with the 2x6 insulation I started with
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Old 02-25-2010, 11:41 AM   #5
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Rafter Vents and Insulation


Make sure you use ridge vent on that. Plus you will not lose all that much r value when you compress it that much.
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Old 03-09-2010, 01:23 PM   #6
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already ridge vented
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Old 03-09-2010, 03:26 PM   #7
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That configuration is not doing what you need it to do... only foam insulation can do that.
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Old 03-10-2010, 08:42 AM   #8
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So are you saying every roof with a vaulted/cathedral ceiling needs to have spray foam?????
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Old 03-10-2010, 11:39 AM   #9
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Well, for maximum insulating value according to current code, yes.
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Old 03-10-2010, 11:40 AM   #10
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Vaulted and Cathedral ceilings are not the same thing... so no, but fiber insulations are a HUGE waste of money.
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Old 03-10-2010, 11:44 AM   #11
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Quote:
Originally Posted by Big N8 View Post
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
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Old 03-10-2010, 12:12 PM   #12
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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.
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Old 03-10-2010, 12:38 PM   #13
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So it is the R value not the type of insulation that matters????
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Old 03-10-2010, 03:47 PM   #14
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Different types of insulation will have different R values, R values is what typically matters when you're looking at building codes.
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Old 03-10-2010, 08:09 PM   #15
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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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