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Rowhouse Insulation Retrofit

Charles Chiampou | Posted in Energy Efficiency and Durability on

I am looking to start a discussion on retrofit insulation projects for rowhouses. In my city (Baltimore, mixed humid climate) majority of homes are rowhouses built between 1880-1950. Typical construction is brick with floor joists set into beam pockets. Double wythe or single wythe masonry walls divide the homes. Exterior walls usually consist of double wythe brick with interior plaster applied directly to it. Majority of roofs are flat “hot roofs” (most are not vented) with one or two skylights. The only original insulation usually found in these home is loose insulation in the rafter cavities. Current standard procedure is to frame in the interior walls and install kraft faced fiberglass batts. But I know from experience in living in un-rehabbed and “rehabbed” homes that performance with these methods is lacking.

So I would like to look at different ways to address the poor performance of these homes in two scenarios: a full-gut rehab and a less invasive remodel. I have been doing my own research on insulating/ building science for some time now and seems that the first step to improving performance is to eliminate air leakage and then insulate. It also seems that the most efficient places to start are at the bottom of the home (foundation, basement) and at the top (attic). I would like to address the air barrier / insulation issue in a single step with rigid foam. Here are some rough ideas I am playing with, starting with the roof (any feedback would be appreciated):

1.) Existing roof = flat, with torch down rubber, 15×55. No ventilation considerations are in place. Previous owner installed kraft faced batts of r-19 and sheetrocked right to the rafters. Roof has a skylight (the old school kind: pyramind shape with a vent flap at the top) and several mechanical penetrations. I would like to install 4-6″ of XPS rigid foam, or as much as I can afford with reasonable payback time, taped and sealed around all penetrations. Can I go directly over the rubber roof? I have seen commercial details like this where the waterproof membrane is below the insualtion. The insulation is then held down by stone. I like this detail but am not willing to add all that stone given my existing framing. Is there another way to hold down the foam without compromising my roof surface with a lot of penetrations? Would this plan provide considerable results / improved performance?

Do I need to strip my roof, add foam / sheathing and reroof? A complete tearoff would provide the best opportunity to air seal, blow-in insualtion and then resheath and foam but I don’t necesarily have the time/resources for it. I also need to consider how I would transition from roof to wall foam. I am thinking I would rigid foam the interior side of the brick at the front of the house (to preserve the facade) and the frame and batt insualte with unfaced fiberglass.

I know I’m getting into a lot here but any feedback would be appreciated. For example:

1. Most efficient starting point (whether it is the roof or not)

2. Other ways to address the roof project

3. Experiences in similar projects (ideally some with tested results)

Thanks All

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Replies

  1. James Morgan | | #1

    Tops and bottoms are always the first place to look for the lowest hanging fruit in energy upgrades. I've seen those B'more flat roofs and always imagined the solar gain would be terrific. Your best bet on the roof would likely be a new (light-colored) membrane above the new rigid foam. This would probably mean raising the curb on the rooflights (which would be a good opportunity to replace with an insulated glass unit). Leaving the drainage plane below the new insulation would negate much of its value, even if you were to resolve the ballast issue.
    Are the beam pockets in the exterior walls or in the partitions between the townhomes? If exterior I would be concerned about the potential for new insulation leading to condensation and rot around the beam ends. Others more familiar with the local climate might advise.

  2. James Morgan | | #2

    Oh, and when I say 'terrific' I don't mean it in a good way.

  3. Charles Chiampou | | #3

    Floor joist pockets are in interior party walls. Not exterior.

    I would need to re-sheath over the rigid foam on the roof before the new membrane right?

  4. James Morgan | | #4

    I'd be very surprised if the party walls, if structural, are less than double and more likely triple wythe, at least up to 2nd floor level. Single wythe would have stability issues over two stories and even double, with beam pockets, would be a fire spread concern. Just a single wythe parapet might be left to project as fire separation through the roof covering over a more substantial wall below.

    I expect an insulation board pre-faced with sheathing on one side would be best. Talk to your roofer.

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