• Welcome to the new and improved Building Code Forum. We appreciate you being here and hope that you are getting the information that you need concerning all codes of the building trades. This is a free forum to the public due to the generosity of the Sawhorses, Corporate Supporters and Supporters who have upgraded their accounts. If you would like to have improved access to the forum please upgrade to Sawhorse by first logging in then clicking here: Upgrades

R10 Ci under metal deck

Bigwood

Registered User
Joined
Dec 28, 2020
Messages
13
Location
New Kensington, PA
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

Sawhorse
Joined
Oct 18, 2009
Messages
2,636
Location
Phoenix, AZ
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

Registered User
Joined
Dec 28, 2020
Messages
13
Location
New Kensington, PA
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

Sawhorse
Joined
Oct 18, 2009
Messages
2,636
Location
Phoenix, AZ
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.
 
Top