2021 IRC Insulation

[Glenn]

I ask again if the full 14" of uncompressed R49 has to be fully above the top plate of the wall OR can some or all of it be below the top plate as long as it extends to the exterior wall sheathing? Something that might look like balloon framing or a dropped ceiling or soffit at the walls both could allow a full uncompressed insulation membrane from wall to wall.

My house has a 4:12 pitch with 2x8 rafters, full hips, so I've actually thought about what you are suggesting. A sort of "large crown molding" around the exterior walls, but then I thought about the retrofit cost of that at an existing home vs the energy savings and realized I was drunk. But in new construction, that's a different story. Of course at that point, why not just get energy trusses. It seems to me that would be a reasonable alternative if you can provide the R-value and the envelope equivalent to conventional.

 

[bill1952]

My house has a 4:12 pitch with 2x8 rafters, full hips, so I've actually thought about what you are suggesting. A sort of "large crown molding" around the exterior walls, but then I thought about the retrofit cost of that at an existing home vs the energy savings and realized I was drunk. But in new construction, that's a different story. Of course at that point, why not just get energy trusses. It seems to me that would be a reasonable alternative if you can provide the R-value and the envelope equivalent to conventional.

In the instance I have in mind, insulating an existing building with trusses. Actually looking at a vaulted (arched) ceiling. Not sure "plate" wording permits it.

Agree not many instances where "uncovential framing" proves practical.

 

[steveray]

I have allowed an "insulated interior soffit"....

I ask again if the full 14" of uncompressed R49 has to be fully above the top plate of the wall OR can some or all of it be below the top plate as long as it extends to the exterior wall sheathing? Something that might look like balloon framing or a dropped ceiling or soffit at the walls both could allow a full uncompressed insulation membrane from wall to wall.

 

[steveray]

In the instance I have in mind, insulating an existing building with trusses. Actually looking at a vaulted (arched) ceiling. Not sure "plate" wording permits it.

Agree not many instances where "uncovential framing" proves practical.

It's really a killer in additions to match roof lines and wall thicknesses now...

 

[SDS]

Ignorant me here,

Understanding it's not a specifically conventional material, wouldn't the use of a TJI rafter (+14" or better) address the insulated heel issue, while still providing all of the features conventional material does, without going to a energy heel truss system?

Should the IRC address the 'conventional' use of engineered materials in light of the insulation issues now inherent to true conventional framing?

Perhaps it does already...

 

[bill1952]

Ignorant me here,

Understanding it's not a specifically conventional material, wouldn't the use of a TJI rafter (+14" or better) address the insulated heel issue, while still providing all of the features conventional material does, without going to a energy heel truss system?

Should the IRC address the 'conventional' use of engineered materials in light of the insulation issues now inherent to true conventional framing?

Perhaps it does already...

So I think you're suggesting that because the IRC doesnt include TJIs in rafter span tables, they are not prescriptively allowed? I'd guess quite a few officials would accept it without a sealed drawing. I've wondered if they'd be accepted as adequate for rafter tie purpose, used for an attic floor?

 

[SDS]

So I think you're suggesting that because the IRC doesnt include TJIs in rafter span tables, they are not prescriptively allowed? I'd guess quite a few officials would accept it without a sealed drawing. I've wondered if they'd be accepted as adequate for rafter tie purpose, used for an attic floor?

No, I realize the mfg's span tables are an acceptable alt to the IRC's prescriptive tables. The impression I've gotten from following the thread is that there are only the two options available....2x12 conventional lumber, or a raised heel truss, prescriptively, but that's not correct.

I'm only trying to follow along and maybe learn something and wanted some clarification on using engineered alternatives prescriptively.

 

[bill1952]

As a fan of rafter roof construction, I get it. Trying to imagine downsides of setting a plate on top of rafter ties/ceiling joists. Besides a band joist and another plate or double plate, seems simple.

Interesting to think about.

 

[steveray]

Ignorant me here,

Understanding it's not a specifically conventional material, wouldn't the use of a TJI rafter (+14" or better) address the insulated heel issue, while still providing all of the features conventional material does, without going to a energy heel truss system?

Should the IRC address the 'conventional' use of engineered materials in light of the insulation issues now inherent to true conventional framing?

Perhaps it does already...

Yes....they just don't have a good detail for a thrust connection to the ceiling joist last time i looked....And getting guys to cut rafters well is a challenge, TJI's will be impossible....

 

[bill1952]

I *think* TJIs can only be used as rafters with a ridge beam. Probably reason for lack of thrust connection details.

 

[Wooly]

In a word, NO. R-60 does not make sense As you can see above the math has been done, only conductive transfer, but the point remains. At what point will time and money be considered resources that also need conserving?