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How can I improve the insulation and air sealing of my walk-in attic?

ericbarker | Posted in Energy Efficiency and Durability on

I recently decided to tackle the air sealing and insulating of my attic. The house was built in the 40s and I’m in climate zone 5 (NJ). It is a walk-in attic with a combination of vermiculite and fiberglass batt insulation on the floor and what looks like mineral wool covered (mostly) with plywood on the wall separating the finished portion of the attic with the unconditioned. In addition, there’s a “knee wall” (I may be using the wrong term here) along a portion of the attic which divides the unconditioned side from the cathedral ceiling in the living room. I’ve done a lot of research on the best approach for all of this but I could still use help in some areas.

I’m going to use blown-in cellulose on the attic floor after performing air sealing. Ideally I’ll be adding 2×10’s on top of the original joists to allow for additional depth, attempting to get to around R-48, while retaining the walk-in functionality. Orienting these new joists perpendicular to the existing 2×6’s would reduce thermal bridging but could create structural problems (existing max span is 12’ at 16” OC). As an alternative, I may do the cellulose to fill the current joist bays this year (which will still be a huge improvement) and sort out the raised joists in the spring. Thoughts?

The “knee wall” currently has the standard fiberglass batts which are falling off. I plan to install new batts in the bays which are 3.5” deep and then cover the whole thing with rigid polyiso. Does this sound reasonable? Any moisture concerns using faced batt/board insulation? My plan would be the same for the entryway wall as well. Would the R-20 wood frame wall requirement be an appropriate target for both of these cases?

Also, I do have soffit vents and a ridge vent, but most of them would not appear to have a path to the attic due to various obstructions (chimney, skylights). I’m going to use Accuvent baffles to keep the functioning ones clear. I suppose it doesn’t hurt, but how much ventilation is “worth it” given that most of the pathways do not appear to be open? I’ve read the suggestion that a ventilated attic should be REALLY vented.

Or should I give up on all of this, pull the current insulation and just insulate the attic roof? There isn’t any ductwork in there to consider.

Edit: I was just realizing that converting to a conditioned attic would be challenging, because the house and garage attics are currently joined, which I guess would mean creating an insulated wall between the two.

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  1. GBA Editor
    Martin Holladay | | #1

    You have asked some very basic questions. I suggest that you read some introductory articles on GBA to get started.

    First, though, a safety warning: because it is very common for vermiculite insulation to contain dangerous levels of asbestos, you should not do any insulation work in your attic until you have called an asbestos abatement contractor to assess (and probably remove) the vermiculite. Asbestos is dangerous.

    While that work is being done, you may want to read these articles:

    Air Sealing an Attic

    All About Attic Venting

    Two ways to insulate attic kneewalls

    Insulating Attic Stairs

    Insulating Roofs, Walls, and Floors

    Insulation Choices

    Blown-In or Loose-Fill Insulation

  2. ericbarker | | #2

    Thanks Martin, appreciate the links. I think I've read all of those previously, but I will definitely review. Understood regarding the vermiculite.

    Mainly looking to get a sanity check on the plan. I guess my two specific questions were relating to the structural considerations of adding joists on top of the existing structure and the specifics of my kneewall. I came across another post here that discusses the structure:

    In this you discuss adding joists perpendicular to the existing joists, but isn't this likely to cause structural problems (sagging ceiling)?

    Regarding the kneewalls, if I look at the image in the Fine Homebuilding article (which I've read and is very helpful), my house would not have the floor under the triangular area, the triangular area would be conditioned, and the area to the right of it (my attic) is unconditioned. It seems to be the opposite of most structures that I've seen described as "knee walls". I had planned to follow the "good" example in that article, but was wondering if there were any issues applying that approach to my situation.

  3. GBA Editor
    Martin Holladay | | #3

    If you are unsure of the ability of your floor joists to handle the weight of additional framing and insulation, you should consult an engineer. In general, if you aren't planning to install a subfloor and use the space for storage, you probably won't have a problem with your plan. But every house is different, and you should certainly have your joists evaluated by an engineer -- especially since you are worried enough to raise the question.

    It sounds like you will be fine with your plan to insulate your kneewalls. Just pay attention to air tightness, and install a continuous layer of rigid foam on the cold side of the kneewal to address thermal bridging through the studs. Tape the seams between the rigid foam sheets. Don't forget to install airtight blocking between the joists directly under the kneewall bottom plate.

    You should probably check with your local building department to see if your local code requires rigid foam to be covered with a layer of gypsum drywall.

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