8" exterior CMU with rigid insulation

[Dimtick]

I understand the concept of cavity/continuous insulation but I'm having trouble when it comes to the actual details.

I'm working on the renovation of a big box retail building.

the existing exterior walls are 8" cmu with no insulation, no core fill.

To meet comcheck, our mechanical engineer is proposing that our exterior wall be:

(because of the nature of the project, I am limited to insulating on the interior rather than the exterior)

8" cmu

1 1/2" mtl hat channels @ 16" o.c.

1" rigid insulation on top of the metal framing.

5/8" gyp board on top of the rigid insulation.

he tells me that this is the only way to meet the continuous insulation requirement of comcheck. this wall type bothers me in so many from both a thermal and constructability standpoint that I don't even know where to begin.

my preffered wall type is:

8" cmu

1" rigid insulation glued to cmu w/ 1" z furring @ 24" o.c.

1 1/2" mtl. hat channels @ 16" o.c. horizontal

5/8" gyp boad.

the engineer has rejected this because the hat channels create a thermal bridge, and change the insulation to cavity insulation rather than continuous insulation. I understand his point but don't really agree with it. I've been doing research on comcheck wall types but there is very very very little information out there. the closest thing I could find is a basement wall type that has 1 1/2" rigid insulation glued to 8" cmu w/ 1x2 wood furring at 24" o.c. 1/2" rigid insulation over that, then drywall on that. the 1/2" rigid stops the thermal transfer from the 1x furring and you are able to count the 2" rigid as continuous insulation.

anyone have any experience with this?

hopefully my comments makes sense.

 

[Frank]

Is there a change from unconditioned to conditioned space? If not generally only new walls and roofs have to meet the current energy code.

Hat or Z channels between the insulation do provide thermal bridging making it not continuous. This why a metal stud wall performs poorly compaired to a wood stud wall all else the same.

Might also try total performance energy budget method--Walmart uses this for 2009 code projects to avoid CI

 

[RLGA]

One option is to adhere the rigid insulation to the masonry and install a metal stud partition against the rigid insulation. If the walls are too high, spaced anchors (e.g. short sections of Z-furring) can be attached to the masonry and protrude through the insulation in order to anchor the metal studs. This is not unlike continuous insulation within a cavity wall with masonry ties connecting the veneer to the masonry backup.