Rainscreen with Brick Veneer
New home construction in Climate zone 5
Suburb of Detroit, MI – ave 133 days of rain per year, about 32″ precipitation per year.
2×6 studs. Our insulation contractor wants to use Owens Corning Fiberglass BIBS – we can get R24 in the stud cavity. At that amount in our zone, we would need at least R8.8 outside the studs (or more) – 27% or more. Or use less BIBs for the ratio.
We have about 2-2 1/2” available behind brick veneer for rigid insulation and a rainscreen. I was hoping to have a minimum of 2” of rigid insulation – our contractor prefers XPS.
We have generous roof overhangs – most are 48” from the sheathing, some are as little as 24”. This is a ranch.
There will be weeps.
I have read Martin’s “All About Rainscreens”
How can I maximize my R-value of the external rigid insulation and have a big enough rainscreen?
What is the smallest acceptable/advisable size for a rainscreen? I was under the impression that I needed 1” for brick veneer.
A secondary question is – Would using Rigid Mineral wool change this calculation? (I don’t think we would beat R8.8 but I seem to remember reading that mineral wool affects the assembly differently than rigid foam) or affect the size of the rainscreen? Affect the choice of sheathing?
Thanks in advance for any responses.
Alan
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Replies
Hi Alan -
I have never heard the term rainscreen applied to a brick veneer cladding. When most people use the term rainscreen, they actually mean ventilated rainscreen. A ventilated rainscreen has the free-draining space between the wall cladding and the rest of the wall AS WELL AS openings top and bottom of the vent space to get actual air flow between the cladding and the rest of the wall assembly.
The space behind brick veneer is considered a free-draining space, but not really a ventilated space. I have heard--and it seems to make sense--that the space behind the brick veneer is 1 inch because that is the space needed for a mason's fingers to easily set the brick.
The most important detail in a brick veneer assembly is making sure that the 1-inch space stays free of mortar (because mortar spanning the space defeats the space's purpose: free-drainage and a capillary break between the cladding and the rest of the wall assembly).
Peter
Alan,
The absolute minimum size of the air gap behind brick veneer is 1 inch. For more information, see this article: "Flashing Brick Veneer."
That leaves room for 1.5 inch of polyiso. Call it R-7.5 -- that's a conservative analysis, accounting for some cold-weather drop-off in polyiso performance.
In Zone 5, a house with 2x6 studs and exterior rigid foam needs rigid foam with a minimum R-value of 27% percent of the total R-value of the wall. (For more information on this, see "Combining Exterior Rigid Foam With Fluffy Insulation.")
So the maximum R-value for the fluffy insulation between your studs is R-20. Choose a fluffy insulation with that R-value, or less, and the wall will be safe.
R7.5 is 37% if stud insulation is R20
Thank you. I appreciate both of your responses.
The space limitation behind the brick veneer of 2-2 1/2" is at the brick ledge below the top of the foundation wall. Can you push the sill plate, rim joist, and wall studs/sheathing in from that to have more room for external rigid insulation above foundation wall? Maybe another 1" -- I'd rather have R10 or more of external insulation.
The minimum space behind the bricks is 1". There needs to be weepholes on every 3rd mortar joint along the bottom course of bricks and top courses (at foundation and wall head )as well as above and below windows and above doors. Generally little spastic screens are pressed into the weepholes to keep insects out.
The free space behind the bricks must not be allowed to be blocked by mortar falling during construction.
Flashing details need to be done right.
This is fairly standard construction in Montreal.
The other response regarding making sure the mortar does not block the weeps are correct, but there is NO WAY that you can check/monitor this. Masons are just so used to letting mortar fall behind the brick.
The solution to this is that you need to require that they install mortar net at the bottom course behind the weeps. This will go much further in ensuring you don't have blocked weeps than anything else...