Dan Morrison | July 18, 2016


details // Concrete & Masonry, WRB & Flashing, Insulation & Air Sealing

Warm and Dry Monolithic Slab with Brick Ledge

 

Adding brick to the outside a house complicates insulating outside a slab. So Insulate inside.

 

One cold spot on a house is where the edge of a slab is exposed to the outdoors. Because outdoor air is a lot colder than the ground in winter, and hotter in summer, this thermal bridge is really more like a thermal runway.

We have covered two ways to Insulate monolithic slabs—for hot places and cold ones— but when you add brick the picture, things get a little more complicated.

Basically, you need a shelf to support the brick. This can be either a piece of angle iron bolted to the foundation, or it can be a cast in place step.

 

But let’s start from the beginning.

  • A bed of compacted gravel caps the ground under the slab as a capillary break and solid base for the concrete.
  • Lay a plastic vapor barrier atop the gravel and Wrap the edge of the plastic up to grade as a barrier to ground moisture.
  • Pour that slab, adding a shelf to the outside. It is best to slope the shelf away from the house. Also, make sure it is deep enough to support brick, at least an inch of drainage space, and room for ANY exterior insulation you will add. 
  • Install a drip edge flashing, preferably stainless steel or copper. It is best to tuck it behind the wall sheathing if possible, but it at least should be lapped under the house wrap.
  • Install the exterior insulation. If you are not adding exterior insulation, add another layer of building paper. 
  • Call in your brick layers. As they lay that first course of brick, make sure that they add weep holes every couple of feet along the bottom course. Otherwise, water that gets behind the brick will not be able to escape. Weep holes and flashing with a kicked out edge give it a place to go.

This should be all you need for a dry slab, but before putting down a floor, it makes sense to use a waterproofing membrane or masonry paint. 

 

With the water management system in place, turn to slowing heat flow. Lay sleepers on the slab every 24 inches with strips of rigid insulation between them. Cover it up with subfloor panels, and you are on your way to a slab assembly that ain’t scared of the weather.

 

—Details based on The International Masonry Institute details with additional technical assistance from Harrison McCampbell, AIA, in Nashville, TN

 

Comments

Good article.  One additional component, consistent with best practices, is to add a mortar collection device at the base of the wall to prevent the formation of mortar dams that block weep holes and prevent proper drainage and ventilation.  Also, regarding the weep holes, best practice indicates they should neither be unprotected or closed (as is the case of sash cord is used).  They should be protected with a device that allows water and air flow.  Both drainage and ventilation are critical to building high performance brick cavity walls.

Daniel Morrison's picture

Thanks for the comment, Gary.

Yes, I agree about having some sort of mortar net or mesh at the bottom to ensure airflow. 

Thanks for contributing,

—Dan

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