Rock ledge and rigid foam insulation
deerefan| Posted inGeneral Questionson
Hi all, I hope to get some advice on a question.
Our structural house plans unfortunately did not take into account 1.5″ of polyiso which we are planning on placing outside the house framework. A significant portion of the house will be covered with 4″ thick stone, we do have a 5 1/2″ rock ledge planned. The foundation contractor and builder think this is good enough and we do not have to extend the rock ledge. Does this seem right? or do we need an 1 1/2″ air space behind the rock which is unobstructed.
Thank you for helping.
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Replies
Deere,
Stone veneer over wood framing has always been a very risky type of cladding. I don't believe in cutting corners on this type of installation.
You absolutely need the vented rainscreen gap, just as you do for stucco. For information on stucco disasters -- similar in many respects to stone veneer disasters -- see this article:"To Install Stucco Right, Include an Air Gap."
A 4" stone veneer will be installed just like full brick. This is an anchored stone veneer, not an adhered stone veneer that generally fails just like stucco. There is typically a 1" (or so) gap between the veneer and the sheathing. The gap is this wide to prevent mortar "bridging" that would allow water to jump across the gap and wet the sheathing. The IRC actually allows the space between the stone and sheathing to be filled with mortar, but that's a bad idea.
You can reduce this gap to 1/4" (3/8" in Canada) if you use one of the tangled mesh drainage plane materials. Obdyke's Home Slicker Max has the rainscreen with a fleece face that keep the mortar from filling the mesh. Make sure to install flashings and weep holes at all horizontal stops per code. The mesh is a bit of a PIA to fit around the brick ties, but a quick slice with a box knife is generally enough to slip the mesh over the ties.
Hanging the stones 1/2" or so over the ledge won't hurt anything.
That really helps, thank you for answering.