# Incremental benefit of insulating rafters in addition to ceiling joist bays?



## benjamincall (Apr 25, 2008)

Since I'll be adding 2x10 ceiling joists, I'm contemplating partially finishing the attic space. The attic currently has two gable vents and no soffit vents. I would like to keep this area maybe 20 degrees warmer than the outside air during the winter. 

My plan is to rip out the plywood soffits and create a 1-1/2" channel up to a new ridge vent using 4 inches of xps foam insulation. I would use 2x3s as spacer blocks between the rigid foam and the roof sheathing and use a vertical piece to isolate the soffit from the joist bay. I would also close off the gable vents and leave the insulation in the joist bays for the ceiling below. I would, of course, seal with PL300 or expanding foam. 

Do you think this type of installation will provide the 20 degrees I'm looking for? Would foil-faced poly-iso be a better choice than XPS?


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## Windows on Wash (Aug 30, 2011)

foil faced iso would be better to face to the vent space to take advantage of the radiant barrier aspect of performance.

There is no making up for R-value though. As long as you have enough and it is airtight, doesn't really matter what you use.


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## benjamincall (Apr 25, 2008)

Overall, does my concept seem sound?


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## AGWhitehouse (Jul 1, 2011)

benjamincall said:


> Overall, does my concept seem sound?


*NOPE*...and not because of venting or insulating...but because of fire codes. Foam cannot be left exposed within an attic space without an approved thermal and ignition barrier. Approved barriers can be sheetrock, plywood, sheet metal, or mineral fiber insulation (NOT fiberglass). 

If you really want to do it, I suggest using luan instead of foam to create the air channels and fill the rafter bays with un-faced fiberglass batts held in place by tyvek sheeting stapled to the rafters.


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## benjamincall (Apr 25, 2008)

AGWhitehouse said:


> *NOPE*...and not because of venting or insulating...but because of fire codes. Foam cannot be left exposed within an attic space without an approved thermal and ignition barrier. Approved barriers can be sheetrock, plywood, sheet metal, or mineral fiber insulation (NOT fiberglass).
> 
> If you really want to do it, I suggest using luan instead of foam to create the air channels and fill the rafter bays with un-faced fiberglass batts held in place by tyvek sheeting stapled to the rafters.


I was planning on putting the xps foam in the rafter bays with 1/2" plywood affixed to the rafters.


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## AGWhitehouse (Jul 1, 2011)

benjamincall said:


> I was planning on putting the xps foam in the rafter bays with 1/2" plywood affixed to the rafters.


Ok then. So long as the foam is sealed from exposure once the job is complete, then you'll be o.k. in the fire code realm.


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## benjamincall (Apr 25, 2008)

Since I'm upgrading from 2x4 to 2x10 joists, I was thinking of putting down a layer of 5-1/2" and a layer of 3-1/2" Roxul to replace the compressed R13 fiberglass that's up there right now. I'll be closing off the gable vents, but the only regular heat will be what escapes from the main floor. Do you think I'll need to place strips of poly sheeting between the joists, or will the tighter envelope and the rafter insulation keep the attic sufficiently warm to avoid a condensation problem?


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## AGWhitehouse (Jul 1, 2011)

benjamincall said:


> Since I'm upgrading from 2x4 to 2x10 joists, I was thinking of putting down a layer of 5-1/2" and a layer of 3-1/2" Roxul to replace the compressed R13 fiberglass that's up there right now. I'll be closing off the gable vents, but the only regular heat will be what escapes from the main floor. Do you think I'll need to place strips of poly sheeting between the joists, or will the tighter envelope and the rafter insulation keep the attic sufficiently warm to avoid a condensation problem?


I'm confused...you're going to have insulation at both the ceiling and rafter planes? I wouldn't recommend that...pic one or the other...


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## benjamincall (Apr 25, 2008)

AGWhitehouse said:


> I'm confused...you're going to have insulation at both the ceiling and rafter planes? I wouldn't recommend that...pic one or the other...


I want to keep the attic at least 20 degrees warmer than the ambient air, but I don't necessarily want to have the attic at the same temperature as the rest of the house. I would also like to take advantage of the deep ceiling joists. My rafters are only 2x6, which leaves me with about 4" of insulation after I create the channels for the soffit vent.

What's the problem with an insulation sandwich? I have insulation in my ceiling and between the floor joists in the basement.


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## Gary in WA (Mar 11, 2009)

http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

http://docs.google.com/viewer?a=v&q=cache:EJENK_uA9RQJ:www.cr-ar.com/pdfs/Building%20Code%2

Gary


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## benjamincall (Apr 25, 2008)

So, based on what I read in Gary's articles and in R806.4 IRC, I would need to ensure that the underside of the roof deck remains at 45 degrees. 

If I put R20 of impermeable insulation on the underside of the ceiling deck, R15 Roxul in the gable ends and R38 Roxul in the attic floor, will I achieve that temperature on the underside of the rigid foam attached to the roof deck? I would obviously remove all venting.


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## AGWhitehouse (Jul 1, 2011)

benjamincall said:


> What's the problem with an insulation sandwich? I have insulation in my ceiling and between the floor joists in the basement.


The air between the ceiling and floor is tempered...you live in it. The attic is not tempered and you'll be creating a class I vapor retarder outside of the thermal envelope, which, by code, already has a class II vapor retarder in it. To avoid excessive moisture build up within the attic space you'll need to temper it and exchange it, which just adds cost and maintenance. Your money & time would be better spent adding insulation at the ceiling plane and leaving the roof un-insulated.


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## AGWhitehouse (Jul 1, 2011)

benjamincall said:


> So, based on what I read in Gary's articles and in R806.4 IRC, I would need to ensure that the underside of the roof deck remains at 45 degrees.
> 
> If I put R20 of impermeable insulation on the underside of the ceiling deck, R15 Roxul in the gable ends and R38 Roxul in the attic floor, will I achieve that temperature on the underside of the rigid foam attached to the roof deck? I would obviously remove all venting.


You would have to do an energy analysis considering HDD, orientation, average wind speeds, etc. Unless you do this, you really can't accurately estimate the true performance of a system.


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## benjamincall (Apr 25, 2008)

AGWhitehouse said:


> You would have to do an energy analysis considering HDD, orientation, average wind speeds, etc. Unless you do this, you really can't accurately estimate the true performance of a system.


So, I don't have an easy way to determine the temperature of the insulation on the underside of the ceiling deck?

I'm not trying to be a pain in the neck, I was just hoping I wouldn't have to relegate my attic to 16° temperatures each winter. I guess I could install the R38 Roxul in the joist bays without a vapor retarder and find a safe way to hook up a space heater with a thermostat to kick in if the temperature dropped below 45°.

I have the following information, but I'm not sure what to do with it: 

HDD 4888, 5/12 roof planes facing SW and NE, average wind speed 13.3 MPH (16 MPH in January), annual low (16°F in January), percent possible sunshine 56. The house is heated using a mini split heat pump on the main floor. The temperature is maintained at about 70°F. The house footprint is about 630 ft² with an eight foot ceiling (5040 ft³). The volume of the attic will be approximately 1015 ft³.


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