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-   -   Help on proper insulation R value in attic (http://www.diychatroom.com/f103/help-proper-insulation-r-value-attic-152476/)

Calc 08-03-2012 04:07 PM

Help on proper insulation R value in attic
 
I have an attic that I would like to finish, it's a very typical attic in a 1.5 story ("Cape Cod" style) house. I live in northern NJ.

It's a typical gable roof. I would like to add knee walls on the sides and a ceiling across the top in the normal fashion. The sloped roof rafters will make up the corners connecting the knee wall to the ceiling.

Here's my issue, what do I need to do to insulate this room correctly? I would like to use fiberglass insulation. If I put 2X4 knee walls up, can I still use thicker fiberglass insulation? Or do I have to install 2X6's or 2X8's? Second, how about the corners where the rafters make up to sloping walls? Do I have to build them out so I could fit thicker insulations? I plan on using baffles to let the air flow, the attic currently has soffit vents and a ridge vent.

What R value is going to be effective without overdoing it for my area? I know that once you get up high in R value the returns are diminished. For what it's worth, I love the cold and I'm sure the room upstairs will be comfortable for me in the Winter, I am just worried about it being hot in the Summer.

Thanks!

Windows on Wash 08-04-2012 10:02 AM

This can be done a bunch of way.

Foam board to the attic side of the kneewalls works as a good thermal break.

1" foam strips across the slope to allow for venting and than fill the cavity with rigid foam and foam across the inside surfaces of the rafters to get you up to the R-Value you need.

Airtight drywall over the foam to the inside.

Vented attic above the space with R-50 and sealed up ceiling/attic floor.

joecaption 08-04-2012 10:13 AM

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

Do you have a ridge vent and soffit vents if not your going to need them now.

Calc 08-04-2012 10:22 AM

Is there a good insulation forum somewhere? A forum in which people read the question? A place where you don't have to wait many days for non-answers?


Quote:

Originally Posted by Calc (Post 980721)
I would like to use fiberglass insulation.

Quote:

Originally Posted by Calc (Post 980721)
I plan on using baffles to let the air flow, the attic currently has soffit vents and a ridge vent.


Windows on Wash 08-04-2012 03:07 PM

Quote:

Originally Posted by Calc (Post 981245)
Is there a good insulation forum somewhere? A forum in which people read the question? A place where you don't have to wait many days for non-answers?

I read and saw quite clearly where you indicated fiberglass insulation.

Doesn't change whether or not it works effectively in this scenario.

Kneewalls are problematic for for fiberglass because it does not address the thermal bridging of the stud wall, the natural convection and resultant R-value loss, or the inadequate R-Value it provides in most cases.

Having fixed a bunch of cape cods, I can tell you that a bunch of clients have all the fiberglass they can shake a stick at and yet the summer temperatures are still quite unbearable.

I personally prefer 2x6 construction for the kneewalls and if you go that route, with an R-19 batt, and a 1" foil faced iso foam, it will make that insulation work very well.

Nothing about using the higher density or thicker batts does anything to address the thermal bridging of the wall studs and the big thermal conduit that they are.

If you are dead set on using fiberglass, run it inside the walls and another layer across the studs and cover it with and air barrier (i.e. Tyvek).

Fiberglass in the ceiling is relatively terrible because by the time you figure in the vent space, the amount of R-Value is minimum and the air wash over it derates it as well.

Gary in WA 08-05-2012 05:01 PM

Find your County/location/Zone and we will go from there...http://resourcecenter.pnl.gov/cocoon...s/full/973.jpg

Gary

Windows on Wash 08-06-2012 07:21 AM

Zone 4 or 5 (4 Marine)

Zone 4:

Ceiling R-value38 Wood Frame Wall R-value 13 Mass Wall R-value i 5/10 Floor R-value 19 Basement Wall R-value c 10/13 Slab R-value d, Depth 10, 2 ft Crawlspace Wall R-value c 10/13 Fenestration U-Factor b 0.35 Skylight U-Factor b 0.60 Glazed fenestration SHGC b, e NR

Zone 5 or 4 Marine:

Ceiling R-value38 Wood Frame Wall R-value 20 or 13+5h Mass Wall R-value i 13/17 Floor R-value 30g Basement Wall R-value c 10/13 Slab R-value d, Depth 10, 2 ft Crawlspace Wall R-value c 10/13 Fenestration U-Factor b 0.35 Skylight U-Factor b 0.60 Glazed fenestration SHGC b, e NR


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