My husband and I are currently renovating our 1951 modern home in Connecticut. This is my big opportunity to learn correct building envelope details. I have a question about the section of insulation added to the outside of a brick wall. I'm hoping you'll have time to clarify the following:

The east wall of our house is two wythes of brick, with a 2" air space in between, (the east, north, west walls are all brick - the 75' south wall is primarily glazing). We are insulating the exterior so as to cover the slab end - and leave a painted brick finish on the interior. The sketch you drew at Affordable Comfort Kansas City can be interpreted as:

the exterior brick cover by
housewrap (is there a better product than Tyvek?)
insulation (Dow board R7.5 for 1.5")
horizontal furring strips (3/4" thick x 2.5" wide)
long ties to the brick (3.5" masonry anchors)
siding (vertical - which matches other exterior surfaces of the original house)
There is a drip edge shown on the vertical siding.

We have these questions:

1) What type of material is best as a base, horizontally, partially buried, which the vertical siding overlaps? (perhaps cement board?)

2) How far below grade should this base material placed, and how high up the Dow pink insulation should the base be before the vertical siding overlap, (we're thinking ~3"-5" below grade, and 8" - 12" above?)

3) The sketch you drew shows flashing over this base material that extends through the insulation. Does the housewrap overlap the back of this thru-insulation flashing? This flashing would cut through the insulation at the height of the slab end, which is the area of the wall most needing insulation. Do we really need that flashing to go through the Dow board?

...

John Straube replies:

1) I would suggest cement board (or Hardie board), OR, you can use a cement stucco with glavanzued lath over extruded polytyrene.

2) The depth below grade is usually at least 6" or so, since this is how deep people dig flower beds. The height above is based on splash back and aesthetics. 6" is a bare minimum. I prefer 8" and it can increase to whatever looks OK. A 4 ft wide sheet ripped to 16" widths works well.

3) The flashing should dump the water out, but it is just as important (more actually) to provide a drip edge along the bottom of the vertical siding. You could likely avoid it in your siutation, but you will need to have a detail that transitions from the below grade cement board to the siding. In your case, an overlap might work perfectly well (leaving about a 1" gap between the back of the vetrtical siding and the front of the cement baord.

If you use a flashing, it will not cause a thermal bridge through the slab since it is very thin. It is critical that you lap the housewrap over the vertical leg of the flashing to control water. The foundation wall should have some bitumen painted on it until it is at least 6" above grade. I would recommend running the housewrap down over this bitumen in the non-flashing scenario. Tyvek is a good material to use.

JS