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2021 IRC Insulation

IRC Tables N1102.1.2 and N1102.1.4 Insulation and Fenestration Requirements:
Prescriptive wall, ceiling and slab edge insulation levels are increased as follows:

• Wall insulation in Climate Zones 4 and 5 increased from R-20 in the cavity to R-
20 in the cavity +R-5 continuous.

• Slab edges in Climate Zone 3 are required to be insulated with a minimum R-10
at 2-foot depth.

• The depth of the required R-10 slab edge insulation for Climate Zone 4 and 5 is
increased from 2 feet to 4 feet minimum.

• Attic insulation levels are increased in Climate Zones 2 and 3 from R-38 to R-49,
and in Climate Zones 4-8 from R-49 to R-60.
 
From a building science perspective, any only my opinion going from 49 to 60 is worth so little BTU wise...
U x A x Dt =BTU loss per hour. (U = 1/R, Dt= Winter design temp- indoor heating temp)
So not including framing losses, 1000 Sq Ft at an R49 with say a 55 deg temp difference equals 1,122 BTUs in an hour
At R60 its 916 BTUs. 206 Btus saved. Kinda silly in my opinion to consider that a worthwhile endeavor.

Looking at the energy losses of most new homes, exterior walls are typically the highest usage component.
Start revisiting advanced framing, double stud walls, triple pane glass and real honest actual blower door testing...
Losses by mass air transport is significantly more than most could believe.
 
I think the insulating materials and construction details and quality of installation will have some impact on this. If you blow 4" of cellulose on top of fiberglass batts between joists, it might do a lot, including reducing convection loss. If you have 10" of solid, uninterrupted foam, another 2" probably doesn't pay off.
 
The R-60 requirement also interferes with R807.1 Attic Access. This thickness of blown fiberglass can make conformance impossible.
 
Blown fiberglass is about R-2.2/inch. This means R-60 is about 27 inches deep. The code says you must provide access if you have 30" vertical space measured from the top of the CJs to the bottom of the rafters. This leaves just 8.5" for access (with 2x6 CJs). This code section needs to be updated.
 
Energystar said:
Blown fiberglass is about R-2.2/inch. This means R-60 is about 27 inches deep. The code says you must provide access if you have 30" vertical space measured from the top of the CJs to the bottom of the rafters. This leaves just 8.5" for access (with 2x6 CJs). This code section needs to be updated.
Click to expand...
You can't use conventional lumber anymore so this is moot....
 
Energystar said:
Blown fiberglass is about R-2.2/inch. This means R-60 is about 27 inches deep. The code says you must provide access if you have 30" vertical space measured from the top of the CJs to the bottom of the rafters. This leaves just 8.5" for access (with 2x6 CJs). This code section needs to be updated.
Click to expand...
One of the primary intents for attic access is the installation of the blown attic insulation. That's why the requirement for access is measured from the framing members, not from top of insulation to underside of roof framing. It is not really about access after the attic is insulated. It is room for the insulation crew to crawl through the attic and blow the insulation into the space.

2021 IRC - R807.1 Attic Access​
Buildings with combustible ceiling or roof construction shall have an attic access opening to attic areas that have a vertical height of 30 inches (762 mm) or greater over an area of not less than 30 square feet (2.8 m2). The vertical height shall be measured from the top of the ceiling framing members to the underside of the roof framing members.​
​
The rough-framed opening shall be not less than 22 inches by 30 inches (559 mm by 762 mm) and shall be located in a hallway or other location with ready access. Where located in a wall, the opening shall be not less than 22 inches wide by 30 inches high (559 mm wide by 762 mm high). Where the access is located in a ceiling, minimum unobstructed headroom in the attic space shall be 30 inches (762 mm) at some point above the access measured vertically from the bottom of ceiling framing members. See Section M1305.1.2 for access requirements where mechanical equipment is located in attics.​
 
classicT said:
It is room for the insulation crew to crawl through the attic and blow the insulation into the space.
Click to expand...
I thought it was for a firefighter with oxygen bottle all these years, I'll have to stop that lie at once.
Enrgxprt said:
Looking at the energy losses of most new homes, exterior walls are typically the highest usage component.
Click to expand...
I was told by a wise old man, not to many left "That the only good window, is NO window!" I think he's on to something.

I still think theres a point where your wasting your money on additional insulation, maybe blocking the wind is more important? Properly installing the insulation in a wall is a large part of the insiulating issue.

Remember when the government told you to install polyethylene on your interior ceilings? Then you were told not too!
 
Pcinspector1 said:
I thought it was for a firefighter with oxygen bottle all these years, I'll have to stop that lie at once.
Click to expand...
Never once seen a firefighter in an attic, typically I see the ceiling sheetrock pulled down to the floor !
Pcinspector1 said:
I was told by a wise old man, not to many left "That the only good window, is NO window!" I think he's on to something.
Click to expand...
As far a loss goes, your right! One day windows will be a flat screen LCD! with a camera view to outside...

Pcinspector1 said:
I still think theres a point where your wasting your money on additional insulation, maybe blocking the wind is more important? Properly installing the insulation in a wall is a large part of the insiulating issue.
Click to expand...
My gut kick thinking is except for messing with the boundary layer, altering stack effect wind doesnt effect heat loss as much as say thermal bridging....
We perspire to remove heat, this is aided by air flow (wind)... Walls right now, dont sweat so evaporation losses may account for something, but not as much a one might first guess...
Pcinspector1 said:
Remember when the government told you to install polyethylene on your interior ceilings? Then you were told not too!
Click to expand...
Poly on ceilings is smart, depending on your climate location. Think 6500+ degree days where -F is common. You certainly don't want one ounce of interior humidity in any exterior wall. The great sin is creating a wall that has zero vapor profile... The wall (ceiling and floor) needs to dry at least in one direction from that (single) vapor impermeable product. Having poly behind the drywall, AND 2" foam on the outside = catastrophic failure for the framing cavity....
Ideally in a perfect wall ALL of the insulation should be outside. Enough so the dew-point is actually inside the foam... Then if the cavity is air sealed, the cavity will behave just as any other inside portion of the home...


​

​

 
Pcinspector1 said:
I thought it was for a firefighter with oxygen bottle all these years, I'll have to stop that lie at once.
Click to expand...
Modern firefighters just cut a hole in the roof. I can't blame them for not wanting to crawl into an attic that may be on fire with no good way out.
 
Well the 22" x 30" opening requirement has to come from somewhere when it was intruduced into the code. Houses built in the 50s and 60s sometimes have a smaller dimension.

We have issues with the opening requirement when trusses are on 16" OC, the truss designer has to be involved.
 
bill1952 said:
Joe Lstiburek makes a pretty good case for the exterior continuous foam and kraft paper faced insulation inside.
Click to expand...
When I worked on the homebuilder side, we first used foilback insulation board behind masonite siding, but we got a wavy exterior wall.

We changed and applied wall siding over tyvek, on a 2x6 wall with R19 KF insulation and a raised top plate. It worked out so much better.

We tried blow-in in a wall cavity but later found that the insulation would settle at the top plate. Good for reducing sound.

We also tried fiberglas blow-in in the wall cavity and had a little more insulation displacement and probably inconsistant R-values. This was not verified.

We tried the spry foam but only in a attic.

Haven't heard of anyone that has experenced the bluejean insulation.
 
Pcinspector1 said:
Well the 22" x 30" opening requirement has to come from somewhere when it was intruduced into the code. Houses built in the 50s and 60s sometimes have a smaller dimension.

We have issues with the opening requirement when trusses are on 16" OC, the truss designer has to be involved.
Click to expand...
Interesting. All of our trusses are 24" o.c.
 
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