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Musings of an Energy Nerd

Exterior Insulation over Masonry Cladding

The best way to insulate an old brick building is with a continuous exterior layer of rigid foam

Image Credit: Laz Scangas

How do you insulate an old building with exterior walls made of structural brick? The best approach, according to building science professor John Straube, is to install a continuous layer of exterior insulation. Straube told me, “It’s a great solution for ugly buildings.”

This approach was used a few years ago in a rehabilitation project in Brandon, Vermont, by a nonprofit developer of affordable housing, the Housing Trust of Rutland County. The developers converted a remarkably ugly three-story brick office building into attractive, energy-efficient apartments for low-income Vermonters. To insulate the walls, the project team decided to install 4 inches of polyisocyanurate on the exterior side of the existing brick walls. They also transformed the existing flat roof by installing new roof trusses with generous roof overhangs.

The building had been abandoned

The building acquired by the Housing Trust was the former administration building at the long-closed Brandon Training School, which operated for years as an institution for developmentally disabled Vermonters. When the rehab project began, the administration building had been empty and unused for over ten years.

Using a variety of funding sources — including funds from the federal low-income housing tax credit program, the Vermont Housing and Conservation Board, Vermont Housing Finance Agency, Department of Housing and Urban Development, and USDA Rural Development — the Housing Trust converted the building in 2013 to 18 apartments.

The project architect, Laz Scangas of Arnold and Scangas Architects in St. Albans, Vermont, shared details of the rehab project at a presentation he gave at the Better Buildings By Design conference in Burlington, Vermont, on February 8, 2018.

The existing building was nothing special

The existing 24,393-square-foot brick building was built in 1956. “It was a a basic box with a flat membrane roof,” Scangas told attendees at the Burlington conference. “It was used for offices. There was…

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  1. Expert Member

    Nice project!
    In every respect. A quite simple renovation that has yielded a really welcoming, efficient building. I know it isn't the remit of this site, but I'd like to have seen floor plans of the units.

  2. charlie_sullivan | | #2

    Almost every respect
    It's a shame to see otherwise excellent projects using XPS insulation. Fortunately, if we stay on schedule for the phaseout of HFC blowing agents in XPS the outsized global warming impact of XPS will go away soon--it seems that that will happen sooner than we will succeed in persuading all "green" builders (much less all builders) to avoid it.

  3. dbaerg | | #3

    Charlie, what do you suggest as an alternative to XPS for below grade applications? I haven't done a lot of research on this, but here's an interesting blog post on the topic

  4. dbaerg | | #4

    I'm wondering why the article expresses air leakage in cfm@50 Pa. Whenever authors do this, it sends me reaching for my calculator. Why not just use ACH50?

    I get an ACH50 of about 0.6. Have I misplaced a decimal point? That's impressive! Of course, it does benefit from a low surface area to volume. In that respect ELA per square foot of surface area might be useful too. But I don't think most of us have a good feel for that metric.

  5. GBA Editor
    Martin Holladay | | #5

    Response to David Baerg
    Green builders who install rigid foam below grade usually specify EPS.

    Either XPS or EPS loses R-value when saturated, so good drainage details around a foundation are always essential (and code-mandated). You don't want your insulation to sit in a puddle. Good drainage details prevent that.

    Check with the EPS manufacturer to determine whether a particular type of EPS is rated for ground contact. In general, you want to avoid EPS with very low density -- the denser types are better when used below grade.

    If you want to convert cfm at 50 Pascals (cfm50) to air changes per hour at 50 Pascals (ach50), multiply cfm50 by 60 minutes per hour and divide the product by the building volume, including the basement, measured in cubic feet.

  6. dbaerg | | #6

    Right. I'm just wondering why so many authors use cfm50 when it's really meaningless if you don't have the volume. And, even if they do give you the volume, why not just use ACH50? In this case, I guessed at the ceiling heights and calculated a volume to come up with an ACH50 of 0.6. Is there a good reason why I see it so frequently? Am I just being crusty and contrarian? It wouldn't be the first time.

  7. GBA Editor
    Martin Holladay | | #7

    Response to David Baerg
    If you are a blower-door operator, cfm50 is what you read on your digital manometer when conducting your test. So blower-door operators tend to think in terms of cfm50.

    Passive house builders tend to think in terms of ach50, because they are aiming for a goal expressed in terms of ach50.

    Both metrics (cfm50 and ach50) are imperfect, however. Many experts argue in favor of an entirely different metric -- for example, the Envelope Leakage Ratio, or ELR. To calculate the Enclosure Leakage Ratio, divide cfm50 by the surface area of the building enclosure.

    For more information on these issues, see:

    Air Leaks Happen at the Surface, Not in the Volume

    Blower Door Basics

  8. JohnRockwell | | #8

    Exhaust only bath fans at 0.6ACH50?
    Not accounting for common areas, 2,264cfm divided by 18 apartments is 126cfm. If the bath exhaust fans are running at 60cfm, is there not a substantial negative pressure (approx. 24 Pa) forever being exerted on this nice retrofit?

    PS The GBA website will not let me enter my name, John Rockwell.

  9. GBA Editor
    Martin Holladay | | #9

    Response to John Rockwell (Comment #8)
    Continuously operating bathroom exhaust fans do, indeed, create negative pressures in the apartments. As long as the apartments don't have any atmospherically vented combustion appliances, however, and as long as there are enough cracks in the building and air pathways to provide makeup air, the depressurization is not necessarily problematic.

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