I think there should be a drainage plane somewhere in this wall assembly.

The Tyvek (made for stucco) wouldn't be sufficient? I'm going off what I was told at the local lumber yard, just wanted to check to make sure. Drainage was a major concern of mine with this set up as well. Thoughts to get proper drainage?

Recommend 2 layers- tyvec stucco wrap, then 15# felt, then stucco for best performance

I haven't heard of that combination of underlayments but they seem reasonable.

There should always be a "drainage plane" behind the metal lath and it can consist of wrinkled Tyvek Stucco Wrap covered with a "sacrificial" layer of cheap ordinary house wrap (surfactants in cementitious stucco will eventually damage plastic house wrap) so another layer of building paper (asphalt saturated felt) seems reasonable.

I am a little concerned that the two layers of wrinkled Stucco Wrap will be under building paper that will also become wrinkled but maybe that much wrinkling is OK. I don't get a chance to do much real stucco in New England.

Be sure to allow the drainage plane to weep at the bottom of the wall and over openings.

Source: Building Science Corp.

Here is a link that might be useful: Understanding Drainage Planes

If you are going with a three coat stucco system (scratch, brown and finish coats) then foam is not a part of the "traditional" system. It consists of 2 ply paper (coated in an asphalt type of resin), lath (wire), scratch coat, brown coat and a finish coat (acrylic or traditional type stucco mix).

You can use tyvek (or housewrap) if you want to, but the paper that is used is a lot cheaper. If money is not a major concern, then you could wrap the house in tyvek first, then apply the paper over it. Most contractors will say this is way overkill, but it's your house, so do what makes you most comfortable.

In a one coat stucco system, foam is used to replace the "scratch coat" and provides additional R value to your overall rating. The foam used in this application is 1" in thickness and really does not add a whole lot of R value itself.

There really is no way to incorporate the foam in a three coat system (and pass inspections), especially a foam that is 2" thick.

I was actually wondering where you live and why are you wanting such a high R value. A value of 21 is quite good and will insulate quite well, plus the cement on the walls itself will actually add to that value too, so this is something to keep in mind as well.

The pic by worthy is fantastic and explains a proper water drainage system in a stucco system.

Hope this helps,

Ryan C.

Dense pack cellulose would be better than fiberglass in the stud cavity.

The 2" foam board will raise issues at the openings so figure those details out carefully and draw them well.

if you are in a zone 5 or higher, do not listen to rcclark. Sorry. r 21 (batts) in a heating environment is not "good" and will insulate poorly. Sure, it may hit the current code minimum, but thats it. The MIN. allowed by code. And these values are changing. Even a 1" layer bump (r5) will boost the whole wall r value some. If XPS is used, the additional boost in air sealing is also a plus. An extra R10 on the exterior of an "r21" batt wall gives you real world values of about an r22 all said and done. A blown product such as fiberglass or cellulose will increase this slightly and help insulate more fully. I have never seen batts installed correctly on a job. The biggest advantage the foam gives is the thermal break in the shell, thus working to eliminating cold radiation through areas without a break, such as every stud location.

1" foam is fairly easy to detail. 1.5" foam is not much different, and allows for window openings to be furred out with 2x material easily. 2" foam changes the game. I applaud your desire to use it, but along with it comes other detailing and attachment issues you need to account for. The attachment of tyvek and mesh if used needs to be thought about. You can not really attach through 2" of foam. You will turn the foam into a pin cushion. The tpyical detail I use in this situation is furring strips. simple 1x material would be installed outside of the foam and screwed through the foam into the stud, roughly 2' oc vertically. A SIPs screw or similar is used for the attachment. This would also serve as your drainage plan required for the system. I have also used 3" cap nails to attach tyvek. See the image for both.

If 2" of foam is used, chances are your drying ability of the wall will be inward due to the low perm of the foam, so that has to be detailed accordingly (if EPS is used, should be fine). Depending on your zone, the plastic air barrier on the inside should be eliminated and a high perm paint should be used (standard acrylic is fine, do not use epoxys or wallpaper).

Not sure if it's good to piggy back on this..but I agree with the self furring paper backed lath over Tyvek stucco wrap. It appears to do a lot less cracking using the Tyvek stucco paper vs felt. Does anyone know methods for placing t random rock in irregular pattern along with the stucco. Just as accents. Have some really pretty moss rock that look like they would really look nice incorporated in the stucco...adding a bit of interest I think to an otherwise boring expanse of wall.

Another question along these same lines -

I live in Michigan, building code says basements need to be R-10 minimum. I am installing 2" rigid foam against the 8' exterior poured cement basement walls. This foam will be buried by backfill except for the top 8"-18". I would like to stucco this exposure for lack of any better ideas. My problems are several;

1) Does anything go over the exposed foam before stucco like tar paper?

2) I plan on using wire stucco lath but how is it attached to the cement walls through the foam?

3) Should there be a gap between the foam and wire lath to bond the stucco to?

4) How is this gap controlled?

5) What is going to keep the foam from flexing and cracking the stucco? I can glue the tops of each foam sheet to the cement but will this be enough? Foam glue is not the most impressive stuff.

I have considered attaching treated 2"x2"x2' strips in slots cut in the foam 16" apart using cement screws then attaching the wire lath to them - best idea to date. I am an experienced builder but I have never stucco'ed before.

I welcome all comments from small to large to a completely different approach.

Thank you in advance

Randy

Instead of using stucco, you might consider cementitious coatings as suggested by Building Science Corp. As well, flashing should be incorporated over the top of the exterior XPS. Because of such detail complications, I've always just insulated from the inside--XPS and fg on the walls and ccspf at the rims, or, where the budget allows, ccspf on the entire wall and the rims.

The only advantage I can see to exterior insulation is it will cost a lot less than inside insulation where the basement is to remain unfinished. In other words, you won't have to spray a fireproof coating or apply drywall to the interior insulation.