Three Code-Approved Tricks for Reducing Insulation Thickness

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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

Posted on Dec 15 2017 by Martin Holladay
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How much insulation should you install in a ceiling or a roof? When the question comes up on GBAGreenBuildingAdvisor.com, I usually advise builders to install at least as much insulation as is required in the prescriptive table found in the International Residential Code (IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.) or the International Energy Conservation Code (IECC International Energy Conservation Code.).

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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. haven’t changed recently; the requirements in the 2018 code are the same as those in the 2013 and 2012 code. These requirements are:

  • In Climate Zone 1, a minimum of R-30;
  • In Climate Zones 2 and 3, a minimum of R-38;
  • In Climate Zones 4 through 8, a minimum of R-49.

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 the ceiling area requiring insulation shall satisfy the requirement for R-38 wherever the full height of uncompressed R-30 insulation extends over the wall top plate at the eaves. Where Section R402.1.2 [or Section N1102.1.1] requires R-49 insulation in the ceiling, installing R-38 over 100 percent of the ceiling area requiring insulation shall satisfy the requirement for R-49 wherever the full height of uncompressed R-30 insulation extends over the wall top plate at the eaves. This reduction shall not apply to the U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. alternative approach in Section R402.1.4 and the Total UA alternative in Section 402.1.5.”

There are two aspects to using this trick: (1) The insulation has to extend over the top plates of the perimeter walls, and (2) The insulation must not be compressed or installed at reduced thickness at the eaves. Needless to say, following these guidelines amounts to common sense and good practice. Surprisingly, though, the code allows builders who follow these common-sense guidelines to reduce the thickness of their attic insulation from R-38 to R-30 (in Climate Zones 2 and 3) or from R-49 to R-38 (in Climate Zones 4 through 8).

Do I think that reducing the thickness of your attic insulation below the minimum values in the prescriptive table is a good thing? No, I do not. If anything, you should increase them. But the fact remains that Section R402.2.1 (also known as Section N1102.2.1) allows you to do this if you want.

Trick #2: In a house with a small area of cathedral ceiling, you can reduce cathedral ceiling insulation thickness

GBA has always advised builders that cathedral ceilings should be insulated to the minimum levels shown in the prescriptive table. But because some builders grumble about the difficulty of doing so, code writers have accommodated the grumblers.

According to a code provision found in Section 402.2.2 of the IECC (and in Section N1102.2.2 of the IRC), you can reduce the R-value of the insulation in a cathedral ceiling below the minimum requirements of the prescriptive table as long as:

  • (1) The insulation extends over the top plates of the perimeter walls, and
  • (2) The insulation near the eaves is not compressed, and
  • (3) The rafters aren’t deep enough to contain insulation meeting the minimum requirements in the prescriptive table, and
  • (4) The insulated cathedral ceiling measures no more than 500 square feet or 20 percent of the total insulated ceiling area of the house, whichever is less. (Note that the code language refers to ceiling area, not floor area; with a sloped ceiling, these measurements will be different.)

If you can comply with these four requirements, the code allows builders in Climate Zones 2 and 3 to downgrade their ceiling insulation from R-38 to R-30, and allows builders in Climate Zones 4 through 8 to downgrade their ceiling insulation from R-49 to R-38 (see Image #4, below).

Do I think that reducing the thickness of your cathedral insulation below the minimum values in the prescriptive table is a good thing? No, I do not. If anything, you should increase them. But the fact remains that Section 402.2.2 (also known as Section N1102.2.2) allows you to do this if you want.

If you frame your roof with 2x12s, you should be able to install fluffy insulation rated at R-38, along with a ventilation channel between the insulation and the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . If you frame your roof with 2x10s, you should be able to install fluffy insulation rated at R-30, along with a ventilation channel between the insulation and the roof sheathing. If you use 2x10s in Climate Zone 2 or 3, you can use this code trick to reduce your cathedral ceiling insulation from R-38 to R-30 — but if you do, I’ll give you the evil eye. I’ll also point out that it would have made more sense to frame your roof with 2x12s than to use the insulation downgrade option provided by IRC Section N1102.2.2.

Trick #3: Adopt the U-factor alternative path

Most builders comply with R-value requirements by following the prescriptive path — that is, the compliance path centered around the minimum R-values listed in Table N1102.1.1 (also known as Table 402.1.2). But the International Codes also offer an alternative compliance path, known as the U-factor alternative (see Image #5, below). The section where this alternative path is found is numbered differently in different code documents:

  • In the 2012 code books, the U-factor alternative can be found in Section N1102.1.3 of the IRC and in Section R402.1.3 of the IECC.
  • In the 2013 and 2018 code books, the U-factor alternative can be found in Section N1102.1.4 of the IRC and in Section 402.1.4 of the IECC.

While the section number of this compliance path has changed over the years, the wording hasn’t. Here’s what the code says: “U-factor alternative. An assembly with a U-factor equal to or less than specified in Table N1102.1.3 [or Table R402.1.3, Table R402.1.4, or Table N1102.1.4, depending on the code version] shall be permitted as an alternative to the R-value in Table N1102.1.1 [or Table 402.1.2].”

For ceilings or roofs, the maximum U-factors listed in the U-factor alternative table are:

  • U-0.035 for Climate Zone 1;
  • U-0.030 for Climate Zones 2 and 3;
  • U-0.026 for Climate Zones 4 through 8.

As most GBA readers know, U=1/R, and R=1/U. So these U-factors are equivalent (more or less) to R-28.6 for Zone 1, R-33.3 for Zones 2 and 3, and R-38.4 for Zones 4 through 8.

At first glance, the U-factor alternative path appears more lenient than the prescriptive path. In fact it is, although it may not be quite as lenient as some people assume, for the following reason: While the prescriptive path is based on the nominal R-value of the insulation rating listed on the insulation packaging, the U-factor path is based on actual U-factors calculated for the entire insulated assembly. The so-called “nominal” R-values shown on insulation labels don’t count.

So how do I calculate the U-factor of my ceiling or roof assembly?

Most roof assemblies have some areas that have no framing (for example, the areas between the joists or rafters, usually filled with insulation, plus the drywall and other layers), and other areas that consist mostly of framing (the joists or rafters, plus the drywall and other layers). The R-values (and U-factors) of these two areas are different. To calculate the U-factor of the assembly, we need to calculate the R-values (or U-factors) of two “parallel paths” of potential heat flow — the path through the framing, and the path through the areas between the framing. Then we need to make a calculation that considers the framing factor (the percentage of the area taken up by framing) to determine the overall average U-factor of the assembly. Be careful: You can’t average R-values and obtain a meaningful result; you can only average U-factors. (After you calculate an average U-factor for a roof assembly, you can convert that U-factor to a meaningful R-value if you want.)

This type of calculation method (called the parallel-path method, or the “two-dimensional assembly U-factor calculation method”) is explained in ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. Fundamentals. It is also described in three GBA articles:

Another resource for information on the parallel-path method of U-factor calculation is .

Here’s an example. Let’s consider a roof assembly with 2x10 rafters installed 16 inches on center. The rafter bays are completely filled with fiberglass batts. The interior finish is 1/2-inch drywall. Above the rafters are the following layers: 1/2-inch plywood, 3 inches of EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. rigid foam, 1/2-inch plywood, and asphalt shingles. What’s the U-factor of this roof assembly?

Let’s assume that the roof has a framing factor of 10%. (That is, 10% of the roof area consists of rafters and headers, while 90% of the roof area consists of fiberglass-insulated spaces between the rafters.)

You’ll need access to a list of R-values of various building components to make this calculation. Such a list can be found in ASHRAE Fundamentals (and many online sources).

In the example we’re considering, the R-value of the area that includes fiberglass batts is, from the top down:

  • Exterior air film, R-0.17
  • Asphalt shingles, R-0.21
  • 1/2-inch plywood, R-0.62
  • 3 inches EPS, R-12
  • 1/2-inch plywood, R-0.62
  • 9.25 inches of fiberglass, R-30
  • 1/2-inch drywall, R-0.45
  • Interior air film, R-0.68

The total R-value for this path is R-44.75. That corresponds to a U-factor of 0.0223.

The R-value of the area that includes the rafters is, from the top down:

  • Exterior air film, R-0.17
  • Asphalt shingles, R-0.21
  • 1/2-inch plywood, R-0.62
  • 3 inches EPS, R-12
  • 1/2-inch plywood, R-0.62
  • 9.25 inches of softwood lumber, R-11.56
  • 1/2-inch drywall, R-0.45
  • Interior air film, R-0.68

The total R-value for this path is R-26.31. That corresponds to a U-factor of 0.0380.

To find the U-factor of the total assembly, we use a formula that accounts for the 10% framing factor:
(0.90 * 0.0223) + (0.10 * 0.0380) = 0.0239. That U-factor corresponds to R-41.9.

If you had made an R-value calculation that just added up the two different types of insulation — R-30 fiberglass and R-12 EPS — you might conclude that the R-value of the assembly was R-42. (This is the type of R-value calculation made when complying with prescriptive paths in the building code.) The R-value of the roof assembly is actually a little less than R-42, however — though not much — because of thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the rafters. The parallel-path calculation method takes into account thermal bridging through the rafters, as well as the contributions of sheathing, drywall, and air films, and is therefore more accurate.

So let’s say that you were considering using this roof assembly in Climate Zone 4. The prescriptive code calls for R-49 insulation, and this roof assembly won’t pass under the prescriptive code (because the value under the prescriptive code calculation method is R-30+R-12=R-42).

But under the U-factor alternative, all you need is a U-factor of 0.030 or less. The assembly under consideration has a U-factor of 0.0239, so it passes.

The U-factor alternative can also be used for walls and floors

Although this article focuses on ceiling and roof insulation, it should be noted that the U-factor alternative can also be used for walls and floors. See Image #5, below, for the relevant U-factors for walls and floors in a variety of climate zones.

The bottom line

Most green builders aim to install a little bit more insulation than minimum code requirements. That’s still a worthy goal. However, designers and builders sometimes find themselves painted into a corner by unusual circumstances or a tight budget. For those circumstances, it’s good to know about alternative compliance paths that allow builders to install less insulation than required by the prescriptive code.

Just don’t make a habit of using these tricks routinely.

Martin Holladay’s previous blog: “Sill Pans for Exterior Doors.”


Tags: , , , , , , , ,

Image Credits:

  1. Fine Homebuilding
  2. International Code Council

1.
Dec 15, 2017 11:10 AM ET

Sad system
by Armando Cobo

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.
Dec 15, 2017 11:25 AM ET

Response to Armando Cobo
by Martin Holladay

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.
Dec 15, 2017 5:08 PM ET

A 20% framing fraction for a roof would be pretty unusual.
by Dana Dorsett

"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.
Dec 15, 2017 5:29 PM ET

Edited Dec 15, 2017 5:40 PM ET.

Response to Dana Dorsett
by Martin Holladay

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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