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Insulation in a 100-year-old brick rowhouse

Jacob Fenston | Posted in Energy Efficiency and Durability on

We’re fixing up a 100 year old rowhouse in Washington, DC, and wondering how / whether to insulate the two exterior walls. The walls are 2 layers of brick with lyme mortar, and originally had plaster directly on the brick (it was removed in a previous renovation). I was planning to stud out 2×4 walls, and thinking of using rigid foam insulation between the studs and the brick, and using spray foam to seal the gaps between sheets of foam.

Should I worry about moisture damage to the walls? Is there a better solution? I don’t want to have to hire someone to do spray insulation.

Any thoughts would be welcome. Thanks!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The safest/best thing is to insulate the exterior of the brick, but for now I'll assume that's out of the question.

    If the eave & rake overhangs are a foot or more you build a studwall with the studs set back 2" from the brick and dense-pack it with cellulose. But this has some risk if there is frequent and direct rain-wetting of the brick.

    Otherwise, putting 1.5-2" of unfaced rigid EPS foam on the interior face of the brick and trapping it there with a 2x4 fiber-insulated studwall snug up against the foam yields pretty-good performance with minimal moisture risks.

    Since the interior studwall is not structural, you could use 1" of EPS and 2x2 plates and 2x4 studs turned sideways set back 3-3.5" from the foam. Then install an unbroken continuous R15 rock wool or fiberglass batts between the foam &in that studwall, with split R13 fiberglass or split R15 rock wool compressed between the sideways studs. That way you have quite a bit less thermal bridging, and get full benefit of the R15 batts. In your climate the 1" of EPS is sufficient to avoid accumulating condensation in the studwall, and in that stackup you'd hit about R20 for a "whole wall" R value, after the thermal bridging of the subfloor/joists/framing/sills is factored in. (That's about the same as 2x6 construction with 1.5" of continuous foam sheathing.)

  2. Jacob Fenston | | #2

    Dana, thanks so much for your answer.

    Should I use EPS rather than XPS?

    Thanks again.

  3. Expert Member
    Dana Dorsett | | #3

    EPS is both greener (due to far less damaging blowing agents used) and more vapor-permeable than XPS. At 1" thickness 1.5lb density "Type-II" EPS has about the same vapor permeance as standard latex paint on wallboard (3-5 perms). It's semi-permeable, but not so permeable that you'll suffer summertime moisture issues inside the studwall. At 2" Type-II EPS runs about 1.5 perms, which is still more permeable than 1" of XPS.

    With both the interior & exterior at the same vapor permeance the studwall cavities can dry in either direction at about the rate as seasons change, yet at a combined permeance of about 1.5 perms between the brick and the interior air, not a whole lot of moisture can move quickly between the masonry and conditioned space- the air conditioning can easily keep up with the "latent" cooling load. (There will be far more moisture coming in on your ventilation air in summer than through the walls.)

    As long as you have sufficient R value to the EPS that it doesn't accumulate frost/condensation inside the wall over the course of a winter it's a GOOD thing. For R15 of fiber insulation even R2.5 is enough to prevent that in your climate, but if you're doing R15 plus 1.5" of additional fiber between your turned 2x4s it's better to go with at least 1" (R4.2).

  4. Floris Keverling Buisman | | #4

    Jacob,
    You will get the biggest improvement by making the assembly airtight. We at 475 like to do that without foam, but as Dana points out, it is important to protect the brick against humidity in winter, while allowing it to dry inward in summer.

    At our company, foursevenfive.com, we have a lot of experience with masonry renovations and like to optimize their performance with the use ofProClima's INTELLO, a smart vapor retarder. It Is applied on the inside of your studs as the air barrier. And can be used as dense pack netting for cellulose (it is reinforced) or other fibrous insulation. We recently supplied a project in DC and have done many successful airtight renovation projects in NYC,that have used the complete ProClima airtight system (tapes, adhesives, pipe gaskets) with success. See for example this masonry renovation case study with a number of tips and details how to get these projects as tight as possible (below 1ACH50 or better), while allowing the brick to stay dry.

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