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Old 02-03-2008, 12:02 PM   #1
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Rigid Insulation Installation (R-30)


I am designing an attic build out within attached garage and want to insulate the roof between 2X6 rafters on 24IC. I am looking at R-30 rigid insulation to put between the rafters before drywall finish on the attic ceiling. My question is how is the rigid insullation attached between the roof rafters? Most importantly, do you put the insulation (4.5 in thick) directly against the roof sheeting and leave air space between rigid insulation and drywall or do you attach the insulation so that the air space is between the roof sheeting and the insullation? I have seen schematic drawings that show both.

Thanks for the help. Mark


Last edited by Mablamb; 02-10-2008 at 05:42 PM.
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Old 02-03-2008, 12:41 PM   #2
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Rigid Insulation Installation (R-30)


leave the gap between the insulation and roof. The natural air flow from the sofit vents to the ridge vent is the best insulation you can get. If you dont have those vents- Install them. I would use 3" drywall screws (toe nailed) into the rafters to hold insulation up till drywall is up.

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Old 02-03-2008, 01:50 PM   #3
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Rigid Insulation Installation (R-30)


Quote:
Originally Posted by Mablamb View Post
...do you put the insulation (4.5 in thick) directly against the roof sheeting and leave air space between rigid insulation and drywall or do you attach the insulation so that the air space is between the roof sheeting and the insullation?
First off, I would assume that your calculated R-value is correct for your region.

To answer your question regarding the foam placement; NO, do not install the rigid foam directly against the underside of the roof's sheathing. You must allow air space, or an air "channel" from the eave vents, all the way to the ridge vents. This may ba accomplished with rafter vents/ baffles.

http://www.southface.org/web/resourc...lings_4pdf.pdf
"Cathedral ceiling insulation techniques
Cathedral ceilings must provide space between the roof
deck and ceiling for adequate insulation and ventilation.
Cathedral ceilings built with 2x12 rafters have space for
standard R-30 batts and ventilation. A vent baffle should
be installed between the insulation and roof decking to
ensure that the ventilation channel is maintained.
If roof framing provides insufficient space for required
insulation, higher insulation values can be obtained, by
either framing furring strips (that permit additional
insulation to be installed) or by adding rigid foam
insulation under the rafters. Both techniques offer a
resistance to the thermal bridging effect of the wood
rafter. The rigid foam insulation must be covered with a
fire-rated material when used on the interior of the
building. Half-inch drywall usually complies, but check with
local fire codes for confirmation."

Rafter Vent/baffle picture (attached with galvanized staples to the underside of the roof sheathing in each rafter bay, from eave to ridge)


http://www.lowes.com/lowes/lkn?actio...nsulHome2.html


Quote:
Originally Posted by Mablamb View Post
I have seen schematic drawings that show both.
Most likely, the drawings were for two kinds of roof designs. One requiring ventilation, one not requiring it, for specific reasons.

Last edited by AtlanticWBConst.; 02-03-2008 at 01:54 PM.
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Old 02-06-2008, 04:01 PM   #4
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Rigid Insulation Installation (R-30)


good links
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Old 02-07-2008, 10:51 PM   #5
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Rigid Insulation Installation (R-30)


Mark-
Can you tell me who manufactures R-30 rigid insulation that is only 4.5" thick? The only rigid insulation I've been able to find (on the web) is Owens Corning (3 layers of R-10, each 2" thick). Thanks.
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Old 02-10-2008, 05:30 PM   #6
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I could not find it on the web either. I priced it through Ganahl Lumber is OC California. I just called again to get their supplier but they could not tell me other than that it is R30.3 in 4X8'X4.5". The downside is that it is $86.74/sheet. Ganahl said it's like a 2X4 - they just get what you want so it must be out there.

I did find it at Truitt and White in San Fransisco (no price or manufacturer on their website). http://www.truittandwhite.com/produc...seaction=other

You may try calling them for their supplier. I will not need much but probabaly 6 sheets so I may look at the price of the R20 R-24 and use that to reduce cost.

Last edited by Mablamb; 02-10-2008 at 05:41 PM. Reason: update
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Old 02-11-2008, 09:16 AM   #7
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Rigid Insulation Installation (R-30)


Mark-
Thanks for the info....at least I now know it is out there (somewhere). My roof joists won't accomodate fiberglass insulation in R30 (not deep enough) and I really don't want to furr them out but at $90 a sheet....ouch. I think code here is R30 in the ceiling but I will make sure to confirm that before I begin building. One thought I had was to use something less than R30 in rigid insulation coupled with reflective insulation (see: http://www.insulation4less.com/) which has a high R-value/thickness ratio but I haven't checked whether my local inspector will recognize its R-value. Do you (or does anyone else) have any experience wih reflective insulation?
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Old 02-11-2008, 10:12 AM   #8
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Rigid Insulation Installation (R-30)


Look for isocyanurate foam insulation. It has a higher R-value than common extruded foam of R5/in. Also, it usually has a foil coating on one side to eliminate off-gassing and to preserve the insualting value. It is made by a number of major producers.
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Old 02-11-2008, 10:15 AM   #9
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Quote:
Originally Posted by concretemasonry View Post
Look for isocyanurate foam insulation. It has a higher R-value than common extruded foam of R5/in. Also, it usually has a foil coating on one side to eliminate off-gassing and to preserve the insualting value. It is made by a number of major producers.
have you used it ???
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Old 02-11-2008, 02:10 PM   #10
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Here in Ohio we have what is called res check.I though it was national but I guess not.It is a building energy code program run by the U.S. department of energy.Works something like this; insulation requirements are not determined by individual components,but by the complete system,Based on your geographical region.If you don't have room in your roof cavities,you can make up for it with thicker walls or down sizing your windows.Just something to look into.Ask your local building inspector.
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Old 09-08-2008, 10:58 AM   #11
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Quote:
Originally Posted by woodworker2000 View Post
Mark-
One thought I had was to use something less than R30 in rigid insulation coupled with reflective insulation (see: http://www.insulation4less.com/) which has a high R-value/thickness ratio but I haven't checked whether my local inspector will recognize its R-value. Do you (or does anyone else) have any experience wih reflective insulation?
Yes prodex is great but beware, do not buy from insulation4less.com , their customer service is horrible!! Especially if you are only buying a few rolls. 6 months has gone by and I still have not received the roll I ordered. Insulation4less "customer support" has been rude in every form of contact phone, email etc.

I even got one email (in response to the missing roll) that was just:

uhhh.

Who does this? Why even send and email at all??

Then the insulation4less Vice president/Customer service person recently wrote me:
"Unfortunately I am not able to pull tracking because tracking info is deleted after 90 days. So please quit saying our company is trying to "screw" you. We sell enough insulation not to screw someone for 1 roll. "

I don't know what else to call it when you order something in March, they say they will send it at the end of March then they don't. Then after you call them and email them 10 times they finally tell you it will arrive in May but it never does... Sure sounds like being screwed to me (especially when the window for reporting a claim with a credit card is 60 days).

Apparently if you are not buying bulk insulation4less doesn't care much about your business. Do yourself a favor and buy the insulation from another company or home depot.

I have spent hours corresponding with information for less, and their unpleasant people and I still am one roll short off of an order from March.
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Old 03-01-2009, 12:56 PM   #12
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Rigid Insulation Installation (R-30)


I'd look at installing a radiant heat barrier esp if you're in the South. My son did a high school science project on the "effect of roof construction on solar gain". He found that a sheet of aluminium foil attached to the inside of the rafters was as effective as R-13 fiberglass. It's alot cheaper.

Most of the heat that we get in the summer is transfered thru radiant heat. Interestingly installing a fan in the attic DID NOTHING!. So only vent to remove moisture. It won't help one bit with cooling.

And you can use it with conventional insulation. I get mine from http://www.radiantbarrier.com

Their customer service is good. It's fenestrated so it will allow air to pass thru it. Currently we using it under the house to hold up the insulation and provide some additional radiate heat protection.

The attic in our experimental house was very cool also, we aproached outside air temp in the room of our experimental house.
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Old 03-01-2009, 02:41 PM   #13
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There is also nothing wrong with spraying foam directly to the underside of the sheathing. This would provide you with the greatest R-value. This method is called a "hot roof." It is pretty common for your situation. With the fiberglass, rip sheets of foam and glue a strip on both sides and tack up into place or use construction adhesive. The strip on both sides will allow the air to channel all the way to the ridge. Then fill with fiberglass. Be aware that a lack of insulation will cause ice damming and condensation on the inside of the sheathing. It sounds like you have a budget so make sure to price out both closed cell and open cell spray foam. The open cell will be a lot less.

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