Thanks for any feedback on: I'm in Detroit and working on brick/block warehouse building. I'm considering gluing 2.5" iso foam to brick and block exterior walls, sealing edges, furring, then drywalling. OR 2x4 framing in front of 1" iso and blowing in cellulose. I've read lots of debates. It seems that blowing in cellulose onto brick cavity is not recommended. I don't have money for spray foam... Is it best practice to leave a gap btwn foam board and wall?

Also consider eps foam. Possibly a better $/R, especially when poly iso is de-rated for cold weather.

Typically you want to hold that foam board onto the masonry with something better than glue. Look at these connectors. They work very well. http://rodenhouse-inc.com/

If I was considering framing and then adding either foam or cellulose between the framing - I'd choose cutting foam and pushing it between the studs hands down.

It's still best practice to leave a gap between the exterior wythes and the foam, and have both weep holes at the bottom and vents at the top to aid in convection drying of the brick into the gap. Brick soaks up a lot of rain/dew moisture, and in Detroit you have enough freeze/thaw cycles to potentially end up with spalling brick if it doesn't have adequate drying capacity. The brick will be a lot colder and more likely to freeze once you have insulation between it and the conditioned interior.

If the foam is completely on the interior side of the CMU wall and the block cores are empty it's somewhat less of an issue. You can strap th the foam to the CMU with 1x4 furring through-screwed to the masonry, and hang the wallboard on the furring, or install a 2x4 wall tight to the foam, trapping the foam to the wall. For commercial buildings code-min is usually R10 continuous insulation, which can be had with 2" of polyiso strapped to the wall. That is also a common thickness for roofing polyiso, which is often reclaimed/recycled for a fraction of the cost of virgin stock goods, much cheaper than brand-new 1" polyiso + studwall + fiberglass/cellulose at any R-performance.

Wood rafters/trusses, or steel?

Posted By Dana1 on 09 Dec 2016 05:25 PM
It's still best practice to leave a gap between the exterior wythes and the foam, and have both weep holes at the bottom and vents at the top to aid in convection drying of the brick into the gap.

Notice how the size of the weep holes an vents are not being specified here? The reason for the that is this is incorrect and is not "best practice". Seek a local pro that can show by analysis what the cfm/sf and ACH is based on materials properties and alot of other factors that need to be known in ventilation cavities. A mistake here could end up costing lots of $$$. The National Brick Center at Clemson U may be a good source. They have instrumented and tested builds.