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Dave K 04-21-2011 10:14 AM

Insulation options for 2"X4" roof framing
I am getting ready to re-sheet the roof on a home I own that has 2"X4" roof framing with the sheet rock screwed to the bottom side of the 2"X4". Currently it is insulated with pink R-11 but it packs the entire bay and leaves no room for air flow. As I am going to have access to the bays when I have the sheeting off I would like to replace the insulation with something thinner so the air can flow but all the thinner foam insulation at home depo has R values in the R-8 or lower range. Is there a better option out there? I am going to sheet with Norbord Solarboard OSB so the heat should not be a big problem but would still like the R value for the cold



Grumpy 04-21-2011 11:59 AM

Install rafter baffles over the fiberglass to ensure there is air flow beneath the sheathing. You could also install spray foam, making sure to leave an 3/4" air gap between the top of the insulation and sheathing for ventilation.

mem 04-23-2011 10:54 AM

The baffles are a good idea to ensure air flow but they will probably compress the pink and therefore reduce it's R value--possibly to eight or even less. Best bet might be to fur the rafters with 2x2s and then install the baffles for ventilation.

Gary in WA 04-24-2011 12:18 PM

The Solarboard needs an airspace below it to be effective, 3/4", I believe. If you live in a mixed heating/cooling climate or just a cooling climate, it is effective. Not in a heating climate, the added insulation would be better. You need the solar gain in the attic to reduce the load on your furnace in Winter. I would use an unvented design with shingles rated for "no venting", using the saved money from S.B. on shingles for the warranty; Still need to isolate sloped ceiling from other attics with foam board.

You may still need to add furring for insulation as said, depending on your location. Where are you located?


Dave K 04-24-2011 08:01 PM

I am in Northern California. The weather here is mild with temps rarely hitting freezing in the winter almost never over 100 in the summer. Keeping cool in the summer my biggest concern as we almost never need to turn the heater on in the winter. I think I am going to fur the rafters with 2"X4"s turned on there side. Should I go to R-19 with baffles or just stick with the existing R-11?

Thanks for the help


AndyGump 04-24-2011 08:55 PM

Ooh, in California, you let the cat out of the bag there.
You need:
1. A permit.
2. A"cool roof" or "radiant barrier sheeting".
3. Probably R-30 insulation of some sort, whether by furring out the roof rafters or some other method.
4. California Energy Standards Calculations, ie. use the "prescriptive" approach or the computer calcs.
5. A set of plans.

IF, you want to be legal and above board.

Not necessarily in that order.


Dave K 04-24-2011 09:03 PM

My local building department is not all that bad. Just a $160 roofing permit and I can go about my business. The inspector comes out to check my nailing on the sheeting and then again to check off my shingles.

tcleve4911 04-24-2011 09:12 PM

A 2x4 roof system is best served with spray foam insulation, no venting.

This will give you the highest R value, zero condensation and it actually adds structural integrity.

AndyGump 04-24-2011 10:05 PM


Originally Posted by Dave K (Post 635815)
My local building department is not all that bad. Just a $160 roofing permit and I can go about my business. The inspector comes out to check my nailing on the sheeting and then again to check off my shingles.

Hi Dave, I didn't think there was any place like that left in California. LOL

A couple years ago I did some plans for a guy in Placerville, He thought that the permitting agency was really lax on requirements too. Ended up having to do all the basic stuff required in California like Energy Standards including electrical, heating, cooling, etc.
You see a lot of people do not realize that all of CA. is under the new CBC, all cities, counties etc. have to adhere to the codes including the Energy Standards, big PITA I know. So unless you are just re-roofing and not tearing out the existing sheathing then you can get by with maybe just a re-roof permit, but once you open the envelope of the building you are required to do the full compliance thing.
Anyway just letting you know, please let us know how it goes with your project. Thanks.
I wonder if Nathan would let me blog on this subject?


Dave K 04-24-2011 11:07 PM

I only talked to the lady at the front desk and not the building inspector because he works alternating days so there is a chance that she miss-led me. I am going to hope for the best. My last project in Ca cost me $12000 in permit fees just for a small addition not to mention going through design review and all the other BS that goes with building in Ca

AndyGump 04-24-2011 11:42 PM

12k?! That's out of line even here in the Olde Town Historic District of Orange.

I am stunned.


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