Manual J Questions

[Cr0ntab]

Sorry that I didn't flag this earlier, but IRC code requires either a 1" air gap between the insulation and roof deck, or (in your climate) R5 on the exterior of the roof deck with the batts snugged up to the under side of the roof deck. It looks like you have 3.5" deep top chords to the trusses and 3.5" batts- no air gap. Re-roofing soon? (Didn't think so.)

Rock wool batts are air retardent enough that they don't really need a top side air barrier. The gap issue can be fixed by installing 2x2s (1.5" deep) on the underside of the truss chords to staple the perforated RB onto, leaving 1.5" of air between the roof deck and batt.

Hey Dana,

I'm glad you brought this up for other readers!

Specifically for California, Title 24 allows me to put the insulation right up against the roof deck.

https://title24energyreports.com/images/attics-OptionB.jpg

I have a concrete tile roof that has an air gap/airspace on the top side of the sheathing that will allow things to dry out.

I actually reached out to the fine folks at https://energycodeace.com/ which is a state utility sponsored program.

They helped me decipher the Title24 requirements and I sent them a plan of the roof assembly I was planning.

They agreed that it conformed to the requirements for an Option B high performance roof.

I will stress this though for other readers. You NEED to look into your local codes to see if this is allowed or not.

In general, this isn't recommended for most climates. I just happen to have a special exception for where I live. (I'm considered a California climate zone of 10)

 

[Dana]

That's good to know!

Parts of Title 24 sometimes read like the Quran translated into Sanskrit, vaguely familiar yet still opaque, taking the village pandit ( पण्डित ) to interpret it for just us folks.

It had crossed my mind that though a code violation under the IRC it probably wasn't hazardous in your local climate, but wanted to flag it in case an inspector was going to make you rip it all out and start over.