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Insulation help

webbj010 | Posted in Energy Efficiency and Durability on

I have a 1926 cape code style home. It has amazing charm including lath and plaster walls but also some draft, uneven hvac flow and an upstairs that is fridged in the winter and hot in the summer.

Through my own investigation and a few contractors that have come out with IR guns here is what I know to be the situation.

1. We have knee walls and most is insulated with r11 fiberglass batting along the roof. Much of the batting is coming out of the cavities and is dirty and appears wind washed.

2. We have ~3ft of sloped cathedral ceilings which most are insulated using the same batting. The rafters are 2×4 and so there isn’t a lot of space in there. Using the IR gun there are some trouble areas and some bays that appear to have been skipped entirely. There are no soffit vents and there for no vents heading to the attic. 

3. The attac has decent space to move and do work. Two large gable vents for ventilation and 3 inches of cellulose probably original or super old. 

4, 4here is no insulation in the walls or basement (half of which is finished). 

Based on my research I would like to use Mineral-Wool batting followed by rigid foam insulation for the knee walls. Thus keeping the knee wall as part of the conditioned space for the duct work and plumbing.  Thoughts?

for the cathedral ceiling one contractor said leave it and the other said they could try and pull out whatever they can and then dense pack cellulose. They mentioned this could damage the walls. How should I proceed on this issue.l? What about loos fill Mineral-Wool?

for the attack I’m thinking adding on top of the cellulose to R50?

any recommendations on the walls?

thanks for any help. 

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  1. GBA Editor
    Martin Holladay | | #1

    You can't install mineral wool between your unvented rafters unless either (a) you first install an adequately thick layer of rigid foam on the exterior side of your roof sheathing (an approach that requires a second layer of roof sheathing and new roofing) or (b) you first install an adequately thick layer of closed-cell spray foam on the interior side of your roof sheathing.

    If you can manage to install soffit-to-ridge venting (establishing an uninterrupted air channel with ventilation baffles), you could go ahead with your plan.

    For more information, see these two articles:

    "How to Build an Insulated Cathedral Ceiling"

    "Insulating a Cape Cod House"

  2. webbj010 | | #2

    Thanks Martin,

    Those articles are very helpful. In my situation where there is no work planned for the roof (it’s less than a year old) and demoing the interior walls is not an option what should I do?

    Should just leave the fiberglass batting that has no ventilation and has debatable effectiveness?

    Should I have some try to pull out the batting through the Kneewalls and attic and the do the vents from soffit to attic and fill whatever space is left with cellulose?

    Is there another option that you would recommend?

    Thanks! JW

  3. GBA Editor
    Martin Holladay | | #3

    Q. "In my situation where there is no work planned for the roof (it’s less than a year old) and demoing the interior walls is not an option what should I do?"

    A. Sometimes, retrofitting insulation in an older house is disruptive and expensive. If a homeowner says, "I don't have the money and I don't want the disruption," that doesn't mean there is an easy way to do the work -- just that you can't do the work you want to do.

    Since you have 2x4 rafters, your options are very limited. Here's a link to another article you might want to read -- but I'm not recommending the technique for a house like yours with such shallow rafters: "Sliding Insulation Between Rafters From Above."

  4. Expert Member
    Dana Dorsett | | #4

    What is your location (DOE climate zone)?

  5. webbj010 | | #5

    I’m in 4c, Portland Oregon

    1. Expert Member
      Dana Dorsett | | #6

      There are a lot of nicely appointed 1910-1940 vintage kneewalled bungalows & capes in Portland- sounds like one of those!

      Are the 2x4s full-dimension 2" x 4", or milled 1.5" x 3.5" ?

      Air sealing the kneewalls and the joist bays that cross under them adequately in a retrofit is often a fools errand (having played the starring role of "fool" multiple times in the past.) It's usually cheaper and more reliable air seal and to insulate at the roof deck. In zone 4C the current IRC code minimum is R49, and at least 20% of the total R-value has to be on the exterior of any air permeable insulation for dew point control. Clearly you won't get to R49 in a 2x4 (milled or full dimension), but if you will be re-roofing at some point it's possible to do this in steps.

      An inch of (greener) HFO blown closed cell foam is R7, and about 1.5 US perm vapor permeance. An inch of (climate damaging) HFC blown foam is cheaper, about R6 / 1 perm. Either one would be adequate for air sealing which would leave you 2.5-3" for fiber insulation. Compressing a high density rock wool (preferred) or fiberglass R15 batt to 3" would deliver a bit more than R13, so with either R6 or R7 foam you would still have more than 30% of the R-value as air impermeable low (but not too low) vapor permeance foam protecting the roof deck, with very low risk of moisture accumulation in the fiber insulation.

      That's still only an R20-ish cavity fill, but if it's air tight that's not terrible, and LOT better than R13 in the kneewalls with an air-leaky mini-attic space with thermal by passes under the floors.

      When it's time to re-roof, installing 3.5-4" of polyiso (R20-R24-ish) above the roof deck would bring it up to code compliance on a U-factor basis, since there would be R20 or more of thermal break over the more conductive rafters. Code max U-factor for roofs in zone 4C is U0.026, which is R38.5 "whole-assembly-R", with the R-values and thermal bridging of the all elements factored in (rafters, roof deck, ceiling gypsum, insulation, shingles, interior & exterior air films, etc.).

      If it's not a crazy-dormered out exterior foam on a cape isn't too hard to do. Quite a bit of money can be saved by using reclaimed roofing polyiso (about 1/4-1/3 the cost of virgin stock), which can be safely assumed to perform at R20 or better at 4".

      This outfit in Salem seems to have stock in a few thicknesses at the right price:

      These surplus nailbase panels might be the way to go for the top layer, if you would be doing it soon, but it's only 544 square feet:

      There are other reclaimers in Portland who often advertise here:

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