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Musings of an Energy Nerd

Three Code-Approved Tricks for Reducing Insulation Thickness

Savvy builders know that there are several legal ways to install less insulation than the minimum requirements in the prescriptive code table

Building codes allow builders (in some circumstances) to install less insulation in a cathedral ceiling than is required by the prescriptive code table. This option is only available if the area of the cathedral ceiling is small, and if the insulation fully covers the top plate of the exterior wall near the eaves.
Image Credit: Fine Homebuilding

How much insulation should you install in a ceiling or a roof? When the question comes up on GBA, I usually advise builders to install at least as much insulation as is required in the prescriptive table found in the International Residential Code (IRC) or the International Energy Conservation Code (IECC).

This prescriptive table is known as Table N1102.1.1 in the IRC (see Image #2 at the bottom of the page). In the IECC, the identical table is known as Table R402.1.2 (Image #3).

The minimum prescriptive requirements for ceiling (roof) R-value haven’t changed recently; the requirements in the 2018 code are the same as those in the 2013 and 2012 code. These requirements are:

Builders sometimes complain that these requirements are too stringent, and they ask, “Is there any way that I can get away with less insulation and still meet the code?” As it turns out, there are at least three ways to do that.

Trick #1: In a vented attic, make sure that your insulation covers the top plates of the perimeter walls

GBA has always pointed out that the insulation installed in a vented unconditioned attic must cover the top plates of perimeter walls. Although energy experts have been giving this advice consistently for at least 30 or 40 years, sloppy builders often screw this detail up.

As an incentive to do the right thing, the code allows builders who install insulation properly to get away with less insulation than required in the prescriptive table. The loophole I’m talking about is found in section N1102.2.1 of the various editions of the IRC, and in section R202.2.1 of the various editions of the IECC (see Image #4, below).

The relevant section reads: “Where Section R402.1.2 [or Section N1102.1.1] requires R-38 insulation in the ceiling, installing R-30 over 100 percent of…

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4 Comments

  1. User avater
    Armando Cobo | | #1

    Sad system
    Your blog talks about ventilated attics, but the most common way to cheat on insulation in an attic is to follow the performance code on unvented attics, where builders install 5.5” R21 open cell foam under the roof decking and call it good. No air supply or no ridge venting (a new thing with BSC).
    The sad part of it is that it appears that ICC couldn’t care less about the laws of physics and potential for condensation issues, just so builders can build “cheaper”… who cares about the homeowners if they have problems later on, especially when builders only provide a 1-2 year warranty. ICC is caving to industry pressures.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Armando Cobo
    Armando,
    Building codes establish certain regulations, but no one (as far as I know) believes that building codes tell you how to build a good house.

    To learn how to build a good house, you have to use resources like Lakesideca Advisor.

  3. User avater
    Dana Dorsett | | #3

    A 20% framing fraction for a roof would be pretty unusual.
    "Let’s assume that the roof has a framing factor of 20%."

    Even for 16" o.c. rafters that would be high by ~2x. California code has long assumed an average 10% framing fraction for 16" o.c., 7% for 24" o.c. attic or roof framing. Unlike walls, roofs don't usually have a lot of window & door headers, jack-studs/rafters etc.

    See the assumptions at the bottom of the last page of this document:

    "Batt insulation is assumed between the framing members and no insulation over the framing. The framing percentage is assumed to be 10% for 16 in. OC and 7% for 24 in. OC "

    Of course complex roof lines, hips dormers and skylights can really mess that up, but if I recall correctly the CA framing factor assumptions that make it into code are based on framing design reviews of a large sample of existing house designs in that state. I'm sure homes with 20% attic/roof framing fractions exist, but they're probably more than 2 sigma out from the median, making the example used in this article's analysis a worst (rather than typical) case.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Dana Dorsett
    Dana,
    The point of my example was to illustrate the math. But I appreciate your comment nonetheless.

    I have edited my article so that the math demonstration uses a framing factor of 10%.

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