2012 IECC - PEMB

[Jhl]

Working on a small commercial project involving a PEMB used for light manufacturing. Building will be 42' x 48' x 14' eave with a 4:12 pitch roof. We are in IECC Zone 6. Reading the energy.gov guide for a building envelope in zone 6 I believe it requires R25 cavity and R11 continuous on the gross roof area, Walls are required to be R13 cavity R13 ci, foundation is R10 to a depth of 24in (unheated slab on grade).

So I did a test run on comcheck for a building envelope using the above entries (I used a standing seam roof with r5 thermal blocks) with R25 cavity, R11 continuous. I used the PEMB metal wall with R13 cavity and R13 ci, and I used the slab on grade with a perimeter qual to the building perimeter and R10 vertical insulation to a depth of 2'. I used actual square footage of the roof, not the building footprint.

I ran it with and without 2 - 3x7 insulated hollow metal walk doors assuming an R = 2.0 (U =0.5)

Building fails envelope by 5%. I changed the foundation to 4' vertical - still fails (-4%)

I'm not sure how to achieve the insulation requirements with the PEMB the owner wants anyway (they want a standard screw down roof which does not work with thermal blocks) - but using IECC 2012 minimums for our zone fails as an envelope, and we have not even attempted to put sectional doors in yet.

What am I missing here?

 

[RLGA]

Your problem is that you're following the prescriptive requirements and then using an analysis tool to verify the prescriptive requirements. If you provide the prescriptive requirements in the IECC per Table C402.1.2 or C402.2, then the use of COMCheck for evaluating the building envelope is not necessary. COMCheck is best for calculating performance using the ASHRAE trade-off method.

Also, opaque doors are required to have a U-factor of 0.37 per Table C402.2, and roll-up doors will need to have an R-value of 4.75.

 

[Jhl]

Thank you - I thought I needed to validate the envelope as a whole assembly.

 

[JBI]

COMCheck is an alternative method of demonstrating prescriptive compliance that allows limited trade-offs.
Does the PEMB have metal studs and rafters? That will change the base requirements in the IECC also.