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Old 03-11-2012, 05:46 PM   #1
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Insulation for rafters


We have an attic that was converted into an apartment in a two story house in Seattle, Washington (the attic/apartment is the second floor). The ceilings are vaulted and the studs (or joist or whatever they're called) are 24" on center and 3 1/2" deep. What R-type of insulation do I need?

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Old 03-11-2012, 05:51 PM   #2
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Insulation for rafters


Quote:
Originally Posted by anuvanoob
We have an attic that was converted into an apartment in a two story house in Seattle, Washington (the attic/apartment is the second floor). The ceilings are vaulted and the studs (or joist or whatever they're called) are 24" on center and 3 1/2" deep. What R-type of insulation do I need?
Your plans don't have that information?

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Old 03-11-2012, 06:30 PM   #3
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Insulation for rafters


I'm not sure what plans you're talking about - we recently bought this house and we, the homeowners, are tackling this renovation project ourselves. We basically gutted this apartment because the walls and ceiling were bowing and bulging from the 10" and 6" inch insulation that was installed (again, the studs & joists are only 3 1/2").

We just need to know the right insulation to install in the vaulted ceiling/rafters for our depth (3 1/2") and zone (Seattle, Washington).
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Old 03-11-2012, 06:30 PM   #4
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Insulation for rafters


http://www.energystar.gov/index.cfm?...sulation_table

If this is already a finshed apartment how do you plan on insulating it?
There called rafters if it's holding up the roof. There not even close to enough room to install the needed insulation and needed venting in that roof system as it stands now.
There needs to be soffit vents, and a ridge vent to allow air flow.
Another way would be to insulate the roof it's self and add spray foam inside between the rafters.
But that would involve removing all the shingles.
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Old 03-11-2012, 06:48 PM   #5
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Insulation for rafters


Well it was finished but it was badly finished by the previous owners so we ripped everything down to the frames. Yes it's supporting the roof and yes there's a ridge vent.
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Old 03-11-2012, 06:54 PM   #6
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Insulation for rafters


Can you add framing below the rafters to increase the thickness of the insulation?
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Old 03-12-2012, 01:03 AM   #7
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Insulation for rafters


Under our own WA`State Energy Code, pp. 55, for your location you require R-38: http://www.energy.wsu.edu/Documents/...ers%201-10.pdf

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Old 03-12-2012, 01:41 AM   #8
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Insulation for rafters


Well, the depth is only 3 1/2" and after some googling it seems like R-11, 13, 15 and 19 insulation are only 3 1/2". Increasing the thickness of the rafters most likely won't be an option. We're just working with what we got here for this project and would prefer to not change anymore structural stuff unless absolutely positively necessary.
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Old 03-12-2012, 04:23 AM   #9
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Insulation for rafters


Quote:
Originally Posted by anuvanoob
Well, the depth is only 3 1/2" and after some googling it seems like R-11, 13, 15 and 19 insulation are only 3 1/2". Increasing the thickness of the rafters most likely won't be an option. We're just working with what we got here for this project and would prefer to not change anymore structural stuff unless absolutely positively necessary.
Furring down the rafters is not structural. It will only carry the Drywall. Sometimes it is not advisable as it does lower the ceiling.
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Old 03-12-2012, 05:39 PM   #10
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Insulation for rafters


If you are committed to work with what you have.... is the attic behind the knee wall vented at the soffits?

You can either: isolate the upper attic (separate venting) from the knee wall attic (separate venting) and not vent the sloped ceiling (rafters) but treat them as a "Hot roof"; or furr down the rafters for an air space (1-1/2") to tie the attics together.
Simply using f.g. in the 3-1/2" depth is not wise: http://www.buildingscience.com/docum...d-roof-systems

Air seal the attic is of utmost importance:http://www.google.com/url?sa=t&sourc...wWATQw&cad=rja

Even the drywall, as any hole left uncovered is about 100 times more dangerous than diffusion: http://www.buildingscience.com/docum...researchreport

http://www.buildingscience.com/docum...wall-approach/

Block the joist cavities from the knee wall attic and install a housewrap on the attic side of the wall to prevent wind-washing there.

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Old 03-13-2012, 01:13 AM   #11
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Insulation for rafters


well, there's no soffit vents but looking from inside the attic/apartment itself I can see about 8" to 12" slivers of daylight about every 8 feet or so in the general area where soffit vents should/would be. I'm assuming they're acting as some kind of venting solution.

A little more info: the house was built in 1918 and basically everything is original (including "old timber" for the all framing, beams, etc.). The attic/apartment's exterior walls and the floor joist behind the knee wall are insulated with blown insulation.

Here's a photo of the rafters we're trying to insulation: https://picasaweb.google.com/lh/phot...eat=directlink

Here's the best shot I can find of what behind the knee wall looks like (most of what you see is cleaned up, there's just the mound of blown insulation now): https://picasaweb.google.com/lh/phot...eat=directlink
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Old 03-13-2012, 01:28 AM   #12
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Insulation for rafters


You may want to look into closed cell spray foam, but you'll still have the 2x4's bridging heat outward. Open-cell will work, too, but it needs a vapor barrier-type paint. Or, you could tear off the roof and install rigid foam on the outside. If you use enough, you could put Roxul in the open bays inside. Be aware that a vapor barrier on the inside is even less of a good idea w/ foam on the roof, as it is a bit of a vapor barrier, ie, pretty serious retarder, though I keep seeing different perm rating numbers for it. (In general, 2/3 of the R value should be outside to keep the roof sheathing warm, I believe. If that is wrong, Gary will correct me. )
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Old 03-13-2012, 09:55 PM   #13
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Insulation for rafters


I see your new ceiling joists/collar ties and wiring, I suggest you get a permit to finish it per minimum Code.

Gary

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