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Insulating a Finished Attic on Cape House

entomodonata | Posted in Energy Efficiency and Durability on

I’m embarking on a project to redrywall and reinsulate the finished attic in my 1.5 story 1945 built Cape Cod style home. I’m in climate zone 5A. My gable walls are 2×4 construction, and the sloped ceilings are 2×6 rafters. House/roof are sheathed with 1x boards, and the house is brick veneer, so exterior insulation is not possible.

My roof venting approach is a classic vented roof, continuous soffit vent with a ridge vent at top.

The attic space has 48″ high kneewalls, and the area behind the kneewalls was reinsulated through a subsidized program through my electric company when I first moved in years ago, bringing the behind-knee wall area into the conditioned space. Baffle vents were installed against the roof sheathing, the 2×6 cavity filled with fiberglass batts, and then 2″ of foil faced polyiso was installed continuous across the rafters, seam taped and spray foamed where necessary for air sealing. No other insulation work was done at that time.

I have about 150 sqft of 2×4 gable wall and 350 sqft of sloped ceiling 2×6 rafter area to reinsulate and am trying to come up with the best approach to improve whole wall R value and air seal. Currently the insulation in those cavities is OLD 3.5″ mineral wool paper faced batts.

For the 2×4 gable walls, I am open to furring out the walls to increase cavity space and add some thermal bridging blocks. Current thought is the Bonfiglioli method of 1″ foam sandwiched between the studs and a 1×3 strapping, which would increase my cavity depth from 3.5″ to 5.25″. For the cavities themselves, I’m between 5″ of closed cell, a flash coat of closed cell for air sealing and the remainder open cell, or a flash coat of closed cell and the remainder Roxul batts. Since the total area is quite small, the cost differences between the approaches are not extreme and all are within budget, so my focus is on ease of install and best overall insulation/air sealing.

For the 2×6 sloped ceiling area, I need to maintain a vent cavity against the sheathing. My plan was to use 1-1.5″ blocks against the sheathing as spacers, and install unfaced polyiso sheets (1″?) against those to create the cavity. I’m having a hard time finding consensus on the gap needed for the air cavity, so to be conservative, let’s assume 1.5″.

After, the polyiso, this would leave me with 3″ of cavity depth.I would then closed cell spray foam on top of the polyiso to air seal, and at that point, it would probably make sense to do as much of the cavity with closed cell as possible (~2.5-2.75″) to maximize R per inch. This would still leave me with rather poor center R value for the sloped ceilings (R-25, maybe?) but certainly better than what I have today (R-10 at best with poor air sealing?).

With this approach, I would need to be creative in tying in the 2″ polyiso in the behind knee wall area, with the air sealing efforts in the exposed sloped ceiling area, but I think I could manage that with foam.

It would be nice to fur out the rafters to address thermal bridging and add some R value, but doing so would reduce the space of the finished attic considerably since the sloped ceilings are such a large portion of the walls. I could consider continuous 1″ polyiso across the rafters. If I were to go that route, I could go foil faced, and instead of air sealing with closed cell against the baffles, fill the rafter cavities with Roxul or ‘cut and cobble’ unfaced polyiso and air seal by seam taping/spot foaming the foil faced poly-iso. The issue with this approach is the considerable addition of labor in cutting down all the polyiso and fitting it between the rafters.

At this point, you may be curious about the attic ceiling. This area was formerly covered in acoustic ceiling tiles, with the same measly 3.5″ mineral wool batts as the only insulation. As such, when my utility subsidized insulation work was completed, they were unable to blow cellulose up there because of concerns of supporting the weight with the existing ceiling. So to add some much needed insulation, I added two perpendicular layers of unfaced R30 fiberglass batts (R60 total) in between the ceiling joists (2×6), and then on top of them. It made a noticeable improvement in the attics ability to retain heat when I initially did it many years ago.

We’ve removed the ceiling tiles and the paper faces of the existing batts are in great shape. I’m considering leaving the ceiling as-is, and putting new gypsum up, however I do have some concerns about odors absorbed by the old mineral wool batts lingering after finishing the ceiling. Also, again, it would be difficult to tie in the continuous air sealing from the sloped ceilings to the flat ceilings in this case.

One approach to improve this I’ve considered is removing the 3.5″ mineral wool batts from below, and then inserting foil faced polyiso (thickness to be determined – cut to the width of the ceiling joist spacing) in their place, foamed on the edges. This should address the odor concerns, air seal quite a bit better, but would involve considerable labor.

If you’re still reading at this point, thank you. I’d love to hear your thoughts on the approaches outlined above, see if I’m missing any solutions that may be better both from an effectiveness and labor to install perspective, and continue to learn from all the experts in this community.

Let me know of any questions, and thank you for your time. Cheers!

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  1. entomodonata | | #1

    Hi folks. ‘Bumping’ my own post here. I understand it’s long, so I’ll pair it down and repost if necessary. Thanks for your time.

  2. Expert Member


    You plans sounds logical to me. A good mix of improvement without undue disruption. I think I'd stick with your simpler ceiling plan.

    1. entomodonata | | #3

      Hi Malcom,

      Thanks for the reply. Which approach for the 2x4 gable walls do you prefer?

      Closed cell
      Closed cell/open cell combo
      Closed cell/roxul combo

      Any by simpler approach to the ceiling, you just mean leaving the existing mineral wool batts in place?

      1. Expert Member
        MALCOLM TAYLOR | | #4


        Sorry my answer was a bit brief.

        Of the alternatives you list for the gable wall I'd probably opt for closed cell on it's own. But I'd also consider filling the cavities with batt insulation and covering the wall with a layer of taped EPS. Before deciding, check what your code requires as an ignition barrier for the various options.

        Yes I'd be inclined to leave the batts. Drywall should block the smells, as it is a good air-barrier.

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