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Old 12-23-2009, 07:06 PM   #1
 
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Walk up Attic


I have a walk up attic in a new contstruction in Attleboro, MA. I am looking to finish the attic but am wondering the best way to insulate. The attic has an insulated plywood floor as this is the ceiling for the second floor. Currently there is the second floor furnace laying flat on top of the plywood in the attic. The roof rafters are exposed all the way down to the soffit vents. There are soffit vents on both sides of the "A" frame roof with one window on each end. My plan is to build knee walls on both sides and insulate those as well as build a ceiling. Am I supposed to leave an "empty space" between my framing and the soffit vents for airflow? There is no ridge vent at the top as far as I can tell. Just the soffit vents. The furnace and ducting would be outside of the finished space.
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Old 12-23-2009, 07:53 PM   #2
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Do you have gable vents or some other type of vent in the attic ?



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Old 12-23-2009, 08:16 PM   #3
 
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Scuba,
I was just checking out your pics. I figured I would post some pics rather than possibly confuse you. I am not 100% sure there is not a ridge vent. There are soffit vents on both sides of the attic though. I took a pic of the inside top of the rafters. It may be a ridge vent as well but not what I am used to seeing from the outside at least. Thanks for the quick response by the way!
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Old 12-23-2009, 08:43 PM   #4
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Those cuts should mean you have a ridge vent
Unless the house was re-roofed & they went the roof & did not install a new one
With a ridge vent you will see a raised shingle area at the peak
There are different ridge vents...so may look a little different



Rafter vents get installed from soffit to the peak
This allows moisture to exit the decking & keeps the decking cooler
And the attic will stay cooler as a result
Insulation is then installed over the vents



Up here R30 to R38 or better is needed for insulation
One problem you will have is the vents going across the rafters
Not sure what to do about those
Possibly you could put fiberglass in the rafter areas
Then put rigid foam insulation across everything



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Old 12-23-2009, 08:57 PM   #5
 
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Could be that I cannot see the vent at the top looking from the driveway. The cross section you see in the pictures is a good 3-4feet beneath the peak so the cross section is pretty spacious. Those are just rough 1x10s put in to nail the supply and return air wireflex to. Can that area be insulated similar to the sides? Can I have a 3 foot gap to the peak as long as I have it insulated to the correct R spec for the northeast? The rafters are 2x10 do I would make the cross section 2x10 as well and insulate accordingly.
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Old 12-23-2009, 09:25 PM   #6
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With 2x10's you can use R30C - for cathedral ceilings to fi tit in w/the rafter vents
The top part - ceiling - you can use probably 2x6's since the distance isn't that far
You might be able to use R38 at the top flat part of the ceiling

Just an example:





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Old 12-23-2009, 09:33 PM   #7
 
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Looks exactly like I want to do. This is a great drawing to use as a reference! My only question would be do I run the rafter vent past the "box" to the open space at the top triangle?
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Old 12-23-2009, 10:45 PM   #8
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Yes the rafter vents runs up past the insulation at the triangular top box
That leaves a clear path for venting from soffit to ridge vent
Even if they got rid of the ridge vent you would still of this
That way when the roof is eventually re-roofed a new ridge vent coudl be installed

If the knee wall are is going to be used for storage many people will insulate the roof rafter space instead of the knee walls
On my house the knee walls & the rafter space is insulated
Keeps the Master bedroom warmer & also keeps the storage space fairly warm



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Old 12-24-2009, 07:37 AM   #9
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I just done the same thing to my attic (also live in MA) that you are planning to do...everyone on this board was very helpful.

If you plan on having an inspection, the inspector is going to want to see R38 on your rafters. I also had 2x10 rafters and used the R38C insulation from Owens..

Dave
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Old 12-24-2009, 09:11 AM   #10
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Quote:
Originally Posted by ddave23 View Post
I just done the same thing to my attic (also live in MA) that you are planning to do...everyone on this board was very helpful.

If you plan on having an inspection, the inspector is going to want to see R38 on your rafters. I also had 2x10 rafters and used the R38C insulation from Owens..

Dave
Did you fur out the 2x10's to fit the R38C ?

One reason I suggest using R30C in the rafter space & then rigid foam over top
1" rigid foam will be R10 (I think) & insulate against cold transfer from the 2x's
A much better solution then just fiberglass



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Old 12-24-2009, 03:42 PM   #11
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http://www.ornl.gov/cgi-bin/cgiwrap?...le/ins_fact.pl Requires R-38 for sloped ceilings and R-49 in the top attic space, so run the baffles all the way up to the ridge vents. Leave a 1 space between baffle ends as per manufacturer: http://www.adoproducts.com/duro.html

Add 2x4s on edge to the rafters to get required R-value and give some room to install unfaced rigid foam board (R-5 per inch) only between the heating ducts between rafters. (Board goes at baffle/duct, thickness = space available). After cutting supporting strapping to install foam boards, install string a in zig-zag pattern across rafter faces to temporarily hold up ducts until drywall install. (No crimps or turbulence spots created).

Insulate the ceiling at furnace area behind knee wall to help the furnace heat semi-conditioned air. Supply outside air source to furnace, if required. Unfaced batts in this area. Your air leak/cold area will be at the wall below/rafter area if not insulated: http://www.homeenergy.org/archive/he...95/950309.html

2x4 ceiling joists 16o.c. span 89 for the top attic. If used as a bedroom, install an egress window: http://www.taunton.com/finehomebuild...s-windows.aspx

Be safe, Gary
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