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Exterior foam with stone and 10 mm rainscreen

I am planning to build a new house and I have a couple of questions.

The house will be in climate zone 2 - hot-humid (near Houston, TX). The walls will be 2x6 with 3.5" of open cell spray foam. The walls will be sheathed with OSB and we are planning to add 0.75" of foam board (R4) on the exterior and use Tyvek drainwrap and a 10 mm rainscreen (i.e. MTI Surecavity).

Some of the walls will have a full stone veneer (3"-5" bed). Some will be half stone and half Hardie and some will be all Hardie. My questions are:

1. Should the walls be constructed: OSB -> Tyvek -> Foam -> Surecavity -> Stone or OSB -> Foam -> Tyvek -> Surecavity -> Stone?

2. How much space is needed between the stone and the Surecavity?

3. Is a metal flashing needed on the full siding wall that will protect the foam at the bottom of the wall?

4. For the siding, I planned to use during strips to attach the siding to the wall. Where the wall transitions from siding to stone, how would the transition be made from turning strips to Surecavity?

Asked by E247
Posted Dec 6, 2017 12:39 AM ET
Edited Dec 6, 2017 7:47 AM ET


6 Answers

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First of all, can you tell us your name?

If you plan to use MTI Sure Cavity, I suggest that you follow the instructions provided by the manufacturer of MTI Sure Cavity. Below is an example of one of the details that are available on the MTI web site.

Concerning your question, "Where does the Tyvek go?", the answer depends on your window details and window location (because window flashing needs to be integrated with your WRB, and I'm assuming that the Tyvek will be your WRB). For more information on this issue, see this article: Where Does the Housewrap Go?


Stone veneer detail 3.jpg
Answered by Martin Holladay
Posted Dec 6, 2017 7:46 AM ET


The order of the foam and wrb is your preference. Martin has written about using foam as a WRB and using both together (see http://www.greenbuildingadvisor.com/blogs/dept/musings/where-does-housew...)

Because the 10mm sure cavity will be creating the rainscreen air gap, there does not need to be space between the stone and sure cavity from a building science perspective. In fact, MTI generally recommends slushing the cavity full. However, it is always a good idea to check with your codes official.

As to the transition, MTI suggests using a flashing to drain the siding at the transition (see attached detail).

Sure-cavity-full-stone.png sure-cavity-siding-to-stone.png
Answered by Tyler LeClear Vachta
Posted Dec 7, 2017 12:50 PM ET


Thanks for the information. If the WRB is installed behind the foam, will the flashing at the siding/stone transition need to be installed before the foam so it can be installed under the WRB? Also, are two layers of WRB needed if I'm using Tyvek Drainwrap?

Answered by E247
Posted Dec 7, 2017 1:42 PM ET


You want to flash to your water control layer/water resistive barrier, so if the Tyvek is your WRB flash to the Tyvek.

Check with your codes official, but one layer of Tyvek should suffice for WRB.

Answered by Tyler LeClear Vachta
Posted Dec 8, 2017 10:52 AM ET


I contacted MTI and they provided the attached drawing that details how to install the flashing in case anyone else has the same question in the future.

Also, when installing the foam board, should it extend all the way up to the top of the wall behind the eave?

30CR06WD014_image.pdf 129.42 KB
Answered by E247
Posted Dec 12, 2017 10:50 AM ET
Edited Dec 12, 2017 10:51 AM ET.


You have to give MTI credit -- the company provides good details.

Ideally, any exterior rigid foam on the walls will extend at least to the top plate of the exterior walls. Even better, it's good to extend the rigid foam further up, between the rafters, to form a wind-washing dam, as shown in the detail below.


Exterior rigid foam - Fine Homebuilding 6.jpg
Answered by Martin Holladay
Posted Dec 12, 2017 12:20 PM ET
Edited Dec 12, 2017 12:20 PM ET.

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