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Building Science

The Value and Limits of Manual J

These basic energy models can teach us more about our plans than what heating and cooling loads to expect

An early SketchUp drawing of the house to be modeled with a Manual J shows the large south-facing patio door and fixed window.

When my wife and I decided to build our next home, my first email was to Martin Holladay. I was familiar with Martin’s 2014 Pretty Good House Manifesto, but wondered if any of his advice had changed. Martin replied to my email, and then updated his manifesto with a new blog post on the topic. We will soon be applying for permits, and hope to be building early in the spring, a house that shares many, but not all of Martin’s suggestions. I worked with my friend, architect, and long-time Fine Homebuilding author Paul DeGroot on the floor plan and elevations, and drew performance and construction details from many projects published at FHB and GBA over the years.  

The house is in Climate Zone 5. It’s smaller than average with a 38 ft. by 34 ft. footprint—nearly square in shape. It will have a basement foundation with an “R-3 or better” insulated slab (more on that to come) and walls insulated on the interior to around R-20. There will be with some minimally-finished multi-use space in the basement. It’s a single-story house and, at this point, I’m planning to build 10-in.-thick double-stud walls insulated with dense-pack cellulose.  

The roof is a clerestory-style gable framed with raised-heel trusses. There will be no clerestory windows though—the attic will be vented with about 16 in. of blown-in cellulose on the floor. The low-pitch roof isn’t optimized for solar, but there is a large south-facing expanse that I hope someday will be covered with panels. I’m hoping for 1ACH50 or better airtightness and currently plan to air-seal the lid using a variation of a technique recently shared in FHB, with the roof trusses sitting above a sheathed ceiling structure. 

There will be a very big, south-facing patio door and…

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3 Comments

  1. dackdotcom | | #1

    So much great info here; thanks for sharing. I'm sold on energy modeling. Brian, please post more on your design/floorplan (the floorplan is not "enlargable"), and of course the foundation as promised.

    1. Expert Member
      Malcolm Taylor | | #2

      dackdotcom,

      You can enlarge the plans by holding down the Control (or Command) key and then pressing the + sign.

      1. dackdotcom | | #3

        Well that's browser functionality that works best for changing font size. GBA should open these up in a modal or new tab or inline a la the NYT. At the very least please hyperlink them to the source URL so users don't have to. Anyhow, the intrinsic floorplan image is huge and very readable: https://www.greenbuildingadvisor.com/app/uploads/2021/12/FP.png.

        I know this is primarily an energy modeling post, but wondering if there was any thought given to a galley kitchen in-between the living and dining rooms. This area's flow seems quite awkward, to always bend around a rarely-used dining room table to go back and forth between kitchen and living room.

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