R10 Ci under metal deck

[Bigwood]

We are installing insulation under a metal deck. The code calls out R10 Ci. We are not installing Ci, but rather up to the metal I-Joists with batt insulation. What is the equivalent R-value with non-continuous insulation. Is there a conversion rate calculation?

We are being told that it would automatically revert to the "joist/framing" R value which is R30 for climate zone 4. Any ideas?

 

[RLGA]

Per the 2018 IECC, if you are commercial, it would be R-38 for "Attic and other." If you are residential, then it is R-49 for "Ceiling."

I don't see an R-10ci required anywhere in the IECC for insulation above the roof deck. Are you working with another code, or maybe the IECC with local amendments?

 

[Bigwood]

It's a garage ceiling, not the roof level.

 

[Bigwood]

So it is listed on table C402.1.3 under "floors"

 

[RLGA]

What is preventing you from putting the insulation between the joists?

 

[Bigwood]

We have the insulation between the joists, but they are saying it needs to be R30 if it's not R10.4 Ci. It doesn't seem correct. There must be some calculation that shows what the equivalent Rvalue is, when you install a non-continuous product.

 

[RLGA]

The reason for requiring R-30 is that the joists are thermal bridges and it reduces the overall thermal performance of the floor system.

To illustrate this, look at IECC Table C402.1.4.1, which is for metal studs in walls. Your floor system is very similar. According to the table, if you put R-25 in a metal stud wall, 8-inches deep, at 24 inches o.c., the effective R-value is 9.5. Applying the same correction factor to your floor assembly, the effective R-value with R-30 insulation between the joists is R-11.4--very close to the R-10ci.

If you put R-10 between the joists, you are probably only getting an R-4 or 5.