Cathedral ceiling vs attic

[Inspector Gadget]

NBC allows for less insulation in a Cathedral ceiling (r26) versus an attic (R50 in our zone); but here's an interesting question: at what point does, say, a scissor truss or something else cease being a cathedral ceiling and be treated as an attic?

I'm using "when is an attic hatch required" as a guideline: if it requires a hatch (ie: 60 cm clear space - pulling from 9.19.9.1) then it's an attic. In other words, if it's a cathedral ceiling R26 is OK, and no hatch required.... if a hatch is required, it's an attic.

Thoughts?

 

[tmurray]

Attic is a poor term to use in that section as it is part of a defined term in the code "attic or roof space", which functionally refers to the space between the top of the wall and the bottom of the roof. Obviously if we were to apply this meaning, we would never find a cathedral ceiling, so the code must not intend for us to interpret it this way.

The way we interpreted it, was that anywhere where there was an obvious intent to slope the ceiling (scissor trusses, i-joists on an angle, etc.), it would be considered a cathedral ceiling.

 

[Inspector Gadget]

Attic is a poor term to use in that section as it is part of a defined term in the code "attic or roof space", which functionally refers to the space between the top of the wall and the bottom of the roof. Obviously if we were to apply this meaning, we would never find a cathedral ceiling, so the code must not intend for us to interpret it this way.

The way we interpreted it, was that anywhere where there was an obvious intent to slope the ceiling (scissor trusses, i-joists on an angle, etc.), it would be considered a cathedral ceiling.

Issue at hand is a very tall 1.5:12 slope truss over a large span. It's got a good four feet of open space. Contractor installed two layers of R20 insulation in there and called it good - we have a different view.

 

[tmurray]

The issue is that steeper slopes of like 4 in 12, blown in products are the only economically effective way to get those deeper insulation levels, but they will slide down the slope over time. This should not be an issue with those lower slopes though.

As an aside, my assumption would be that he has full value above the outside wall, because the only way he is able to take advantage of the compression allowances in 9.36.2.6.(3) is if he agrees the area is an attic.

 

[steveray]

We treat it like this: As soon as you need access, (can fit a 30x30 box, or whatever the dimensions are now) then it is an attic....Not sure how your codes read..

 

[ICE]

I try to avoid weighing in on Canadian threads because I am barely proficient in American code and have zero knowledge of Canadian codes. This thread is about semantics.... so I will. The CBC has a definition of an attic:

ATTIC. The space between the ceiling framing of the top story and the underside of the roof.

There is no mention of an access scuttle/hatch. If there is ceiling framing and a space between the ceiling framing and the roof... there is an attic. There is no dimension given for the "space". It can be a inches or feet; all that's required is that there is a space. Some attics do require an access scuttle and some do not but they are attics none the less.


The word cathedral is mentioned once as an enclosed space. Note that it is right after the word attic which is also an enclosed space. Also note that the word cathedral is used in conjunction with the word ceiling. Now we are getting somewhere. So a cathedral ceiling is not an attic and has ceiling framing with no space between it and the roof... yet it is an enclosed space..ie. a finished ceiling on one side and a roof on the other side of the same framing members.

ROOF VENTILATION. The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, attics, cathedral ceilings or other enclosed spaces over which a roof assembly is installed.

The use of the word cathedral in this context is a misnomer in that we tend to associate cathedral with high, pitched ceilings and conflate that with vaulted ceilings. Unfortunately, the CRC has no mention of words cathedral or vaulted and the CBC does not mention the word vaulted. Even with that, it's not difficult to figure out.

 

[steveray]

This is what makes it an attic for me...

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.

 

[ICE]

This is what makes it an attic for me...

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.

If there is less than 30" vertical height or less than 30 square feet, you call it something other than an attic. What is it then?

 

[steveray]

If there is less than 30" vertical height or less than 30 square feet, you call it something other than an attic. What is it then?

Roof cavity?

 

[ICE]

CRC has this definition:
[RB] ATTIC. The unfinished space between the ceiling assembly and the roof assembly.

Is that unique to California?

 

[BN4537]

CRC has this definition:
[RB] ATTIC. The unfinished space between the ceiling assembly and the roof assembly.

Is that unique to California?

That definition is identical to the definition in the standard 2021 IRC. Note that "Roof assembly" is a defined term, and does not include the framing that supports the roof such as trusses or rafters.

 

[ICE]

That definition is identical to the definition in the standard 2021 IRC. Note that "Roof assembly" is a defined term, and does not include the framing that supports the roof such as trusses or rafters.

Then by the strictest interpretation, a ceiling assembly is drywall and paint. I recognize the path but that would expand the possible configurations that could fall under the term attic.

 

[Inspector Gadget]

The issue is that steeper slopes of like 4 in 12, blown in products are the only economically effective way to get those deeper insulation levels, but they will slide down the slope over time. This should not be an issue with those lower slopes though.

As an aside, my assumption would be that he has full value above the outside wall, because the only way he is able to take advantage of the compression allowances in 9.36.2.6.(3) is if he agrees the area is an attic.

I have a 12:12 cathedral ceiling. I insulated it with fibreglass batts, two layers. R44 nominal. Given that I'm *that guy* WRT thermal breaks, I'm pretty confident I've got an effective R40 or thereabouts.

Remember, as building inspectors, we're not concerned about economically effective: there's no phrase in NBC that says "except where it can be proven costly for the owner ...."

 

[Mr. Inspector]

i have a situation were they are adding a second floor with a ceiling without attic space. 2018 IRC N1102.2.2 says:
This reduction of insulation from the requirements of Section
N1102.1.2 shall be limited to 500 square feet (46 m2) or 20
percent of the total insulated ceiling area, whichever is
less.
Does this mean 20% of the new ceiling or 20% of the whole house?

 

[Inspector Gadget]

i have a situation were they are adding a second floor with a ceiling without attic space. 2018 IRC N1102.2.2 says:
This reduction of insulation from the requirements of Section
N1102.1.2 shall be limited to 500 square feet (46 m2) or 20
percent of the total insulated ceiling area, whichever is
less.
Does this mean 20% of the new ceiling or 20% of the whole house?

FYI, you're in the "Canada" section.