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Insulating finished attic

Shoeman | Posted in Energy, Heating & Insulation on August 22, 2008 03:45am

Looking at trying to improve the insulation in a finished attic. 

The house is in Minneapolis, MN – so cold weather. 

The space is being used as a master bedroom. 

All existing drywall will be torn out – everything down to the studs and rafters.

There are knee walls that are about 4′ high and about 5′ of flat ceiling down the center.

The space behind the knee walls has a floor and is used for storage.

My thought was to cut 1′ rigid foam into 1.5″ strips and attach to the sides of the rafters tight against the roof all the way up from floor to above the ceiling height to allow for ventilation from soffet to ridge vent.  Then cut 2″ rigid foam to fit between the rafters and to fill from the 1″ strips to the bottom edge of the rafters – 2×4 rafters.  Then maybe another 1″ or 1.5″ of foam right across the bottom of the rafters.

Would there be an issue with also insulating the knee walls and ceiling?

Is there a better way to address this – short of sprayed foam.

 

Open to all opinions.

 

Thanks in advance,

Shoeman

 

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Replies

  1. DanH | Aug 22, 2008 06:00am | #1

    I don't understand where exactly you'd be putting the rigid foam. You said "tight against the roof", but if you do that there will be no room for ventillation.

    Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
    1. Shoeman | Aug 22, 2008 07:43am | #2

      I would use the 1.5" strips of 1" on the sides of the rafters then the 2" filling the space between the joists.  The strips of 1" would hold the 2" foam 1.5" off the roof - leaving a 1.5" air gap between the roof and the 2" foam.

      Hope that is understandable.  Best I can explain right now.

      1. DanH | Aug 22, 2008 01:13pm | #3

        OK, that makes sense. Not sure why you'd want to use foam for the strips rather than wood or something else, though.A 1.5" air space is about right.
        Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes

  2. CeltsFan | Aug 22, 2008 03:41pm | #4

    The rafters are only 2X4's?

    I second the question about using rigid foam to create your air baffle. You could use OSB or firring strips or something easier to work with (and likely less expensive).

    Since you only have 2-3" to work with, rigid foam insulation "welded" into place with DIY spray foam seems like your best bet. You can also go across the rafters with another layer of rigid foam to give yourself a thermal break with the rafters.

    Normally, you wouldn't insulate the kneewalls, but since you don't have the greatest insulation in your slopes, you could consider it. Not sure what you'd do with vapor barriers, though - you should look into that.

    1. Piffin | Aug 22, 2008 05:56pm | #5

      Using narrow srrips of foam for the spacers makes perfect sense to me. That is just about the amt of waste product he will have when he slices the foam sheets on the tablesaw and it will be very easy to work with. 

       

      Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

      1. Shoeman | Aug 22, 2008 06:52pm | #6

        I also happen to have a couple sheets of 1" foam laying around.

        I am planing on using foamular 150

        Do you see issues with insulating the knee wall and ceiling as well? 

        1. Piffin | Aug 22, 2008 07:08pm | #8

          I think you definitely want to add it across the bottoms of rafters too so you end up with a thermal break and the vapour barrier.. Use spray foam to seal the joint between that kneewall and the ceiling insulation to keep from having moisture sucked into the space where it can condense.. 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

    2. Shoeman | Aug 22, 2008 06:54pm | #7

      The vapor barrier issue is what I was hoping might get some discussion here.

      I am planing on using Foamular 150 across the bottom of the rafters, which would act as a vapor barrier.  Not sure if I should add insulation in the knee walls with a vapor barrier on the warm side.

      1. FHB Editor
        JFink | Aug 22, 2008 09:39pm | #9

        If you're going to insulate the underside of the roof and it will be airtight, I see no reason to insulate the knee walls as well. As Piffin said, just make sure to pay close attention to sealing the transition from roof to wall. Don't just insulate with a piece of foam, but air seal the edges with can foam to make tight.

        If you're set on insulating the knee walls too, use fiberglass batts with no vapor barrier. No need for two vapor barriers.  

        ** Edit - I assumed when reading your posts that you would insulate the roof rafters all the way down to the plates. In other words, down behind the kneewalls. If this isn't possible, then you would want to insulate the kneewalls and make them a vapor barrier...but I would do anything possible to insulate behind those kneewalls, that's an area that may lead to ice dams.

        Justin Fink - FHB Editorial

        Edited 8/22/2008 2:41 pm ET by JFink

        1. DanH | Aug 22, 2008 10:22pm | #10

          Yeah, generally insulate either the lower part of the rafters or the knee walls, not both. Ventillate the area outside of the insulation envelope. If the knee walls are insulated, don't forget to insulate the ceiling below as well.A major area of concern in any insulation scheme is transitions -- from ceiling to wall, rafter to wall, rafter to knee wall, etc -- wherever there's a break in the continuity of the insulation. You need to plan ahead so that these areas are tightly air sealed and as well insulated as one can manage, given the frequent presence of "excess" wood in these areas.
          Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes

  3. User avater
    basswood | Aug 22, 2008 10:41pm | #11

    --"Then maybe another 1" or 1.5" of foam right across the bottom of the rafters."

    For that step, see if any supplier in you area stocks the "InsulPink" basement finishing product by Owens Corning. I like it just as well for above grade applications. 2x8 sheets pre-formed to accept 1x3's integrated into channels (takes less room space than strapping over foam).

    On attic finishing, I like to cove the kneewall to rafter and rafter to ceiling transitions with a curved rubber drywall knife (from a drywall supply). It is a good look and easy to do.

    1. User avater
      BillHartmann | Aug 22, 2008 11:33pm | #12

      Dow walmate (or is it wallmate) is similar product..
      .
      A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

      1. Shoeman | Aug 23, 2008 12:49am | #13

        Thanks all for the input

        Feeling better about the plan

        Was intending to seal things up tight with can foam as well.

        Will have to look for the foam with the grooves for straping - space is at a premium.

         

        Thanks again,

        Shoe

        1. User avater
          basswood | Aug 23, 2008 12:58am | #15

          Get some extra 3/4 XPS for a starter course, so you can start with a full 1x3 furring strip at the floor to nail base into. Knee walls and sloped ceilings will be odd heights, so you will have to use some rips and cut in some of your own channels for furring.More tips available, if you go that route.

          1. Shoeman | Aug 23, 2008 02:08am | #16

            You may be hearing from me

          2. User avater
            basswood | Aug 23, 2008 02:31am | #17

            I'm game.Here is another one (maybe obvious):After your first course of foam board has been run accross the studs, run a bead of PL300 or gun foam along the top edge and press the next coarse into the bead of goo (like setting courses of brick in mortar).I also run a continuous bead on all framing to isolate stud spaces from each other (a moot point on a knee wall though).

          3. FHB Editor
            JFink | Aug 24, 2008 02:19am | #18

            all good tips...thanks for sharing them here basswood.Justin Fink - FHB Editorial

          4. User avater
            basswood | Aug 24, 2008 02:52am | #19

            Hi Justin, It is always good to share something others can make good use of.If you followed the logic on the "starter coarse" to allow for a base height nailer... I should add that if the wall is an even 8' or 10'--it makes some extra work (XPS ripped and new furring channel needed at the top of wall). For odd height walls (like many basements & attics) the ripping and channeling step has to happen anyway.That starter strip of 3/4" XPS is just 1-1/2" tall, but it also makes the center of furring at the joint between drywall courses at 49-1/2"--No big deal--I just hold the drywall up off the floor about 3/4"-1" (so I can still hit the furring with the drywall joints) and the base covers the gap (the gap is a good idea in basement applications, especially).Now I probably giving out too much advice. ;o)Cheers,Brian

      2. User avater
        basswood | Aug 23, 2008 12:49am | #14

        If you prefer blue to pink.The Dow is also available in both 1-1/2" thick and 2" (R 7.5 & R 10) vs. just 1.5" in the pink version.

  4. Jay20 | Aug 24, 2008 06:48am | #20

    I just completed a similar but smaller room similar to yours. You need the air space on top as you indicate constructing. This will allow the area outside the knee wall to ventilate. I used alum faced material for my top layer. Two reasons, will get some reflective value in the summer and if the roof happens to leak it can run down the alum foil and not harm the ceiling. I caulked and or foamed the edges of the upper layer real well to cut down on air infiltration. I installed short pieces of furring on the bottom of the ceiling rafters then added 2 X 2 Material. This gave me  some thermal break and allowed some more insulation in the ceiling.

    Caulk the bottom of the knee wall to prevent air infiltration. I also installed building wrap and 2" thermax that I also taped on the outside of the knee wall.

    The area below the knee wall needs to be air sealed. If not heated ceiling air will come up to the 2ND floor subfloor and migrate out to the unheated attic area outside the knee wall. Seal the access door to that area real well also.

    1. Shoeman | Aug 24, 2008 05:42pm | #21

      Thanks for the input

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