EIFS RAINSCREEN EXTERIOR or SIMPLY STUCCO THREAD

Subject: EIFS Rainscreen Exterior or Simply Stucco

Richard, Nina, Rik, Ned Jeff, Timothy, Mark, David B., Jeffrey, David A., and Wagdy:

We are hoping to join your BEC group here shortly with a BEC - Minnesota (Our state AIA is the organizing AIA group over the local city chapters here.)  I am in the process of organizing it now with AIA Minnesota.

Coincidentally, I have an exterior building envelope question that has arisen in the past few weeks that is right up the alley for the BEC Councils.  Could I politely request your opinions on the question? And can you respond immediately since we have a meeting this afternoon to make a decision on the issue?  Thank you very much. 

The Owner, Architect, Contractor, and various consultants on the project in question are wrestling with the choice of using either EIFS (with the most current rainscreen drainage features) or straight stucco as an exterior finish on a large hospital addition.  The existing hospital has both stucco and EIFS, and there have been leakage problems with both systems. Most of the leakage problems were due to flawed flashings at window penetrations and joints.  However, the General Contractor and the consultants (including myself) are very leery of using EIFS because of the historical failures associated with it and its thin, relatively vulnerable veneer. Another negative factor associated with the EIFS systems is that insurance companies are pressuring both the Contractor and many architects to avoid using EIFS because of it failure record and litigation risk.

The Architect insists that the most current rainscreen type of EIFS has addressed its previous causes of failure, and is a viable system.  They would like to use it.

The system will be fastened to 6² steel stud wall framing, with the insulation applied to the exterior.

Finally, besides which system do you recommend, what do you see as the respective layers of material from the studs outward?  Where and what is the vapor barrier?  How many layers of sheathing would you use, how thick, and in what order?

The Architect is contemplating using 2' of rigid insulation.

Thank you for any input you may have about this.

I look forward to sharing recommendations about building exteriors in the future through BEC.

Judd Peterson, AIA

Subject: RE: EIFS Rainscreen Exterior or Simply Stucco

Judd;  Glad to hear about the Minnesota BEC.  We could use more representation from cold climates.

Regarding EIFS or stucco, I would pick the stucco with a significant caveat. It is possible to use the stucco as part of a drained cavity system, assembly as follows:

• Cold formed steel stud framing with no insulation in the stud space.

• Exterior sheathing, probably Dens Glass

• A continuous air and vapor barrier membrane, by Grace or Henry for example.

• Vertical galv steel zee furring with all of the fasteners sealed.

It may help to space away from the wall 1/4 inch for drainage and a better thermal break.  The depth should be around 1/2 inch larger than the thickness of the insulation to provide an continuous drainage plane.

• Semi rigid mineral wool insulation, thickness to comply with energy code or ASHRAE 90.1.

• Paper backed self-furring metal lath.  The paper does not provide weather resistance but keeps the stucco from going through the lath into the air space.

• 7/8 inch three coat stucco with plenty of control joints.

• Stainless steel flashing to drain the cavity.

After all of that, I would then change the stucco layer to Alucobond dryjoint metal panels anyway.  Linda Brock has a book that shows the stucco system in some detail.  I can find the title if you need it.

Regarding the EIFS, assuming that the drained systems do fix the weathering problems (i am not totally convinced), you still end up with a building that

relies extensively on sealant for performance but the sealant cannot be reasonably maintained over the life of the building.  In 15 or 20 years when the sealants need to be redone the building will need to have a network of silicone "band aids" applied to the face of each sealant joint because it is impossible to remove the old sealant without damaging the EIFS.  Despite what Dow says, the silicone strips are ugly.  I don't think it is prudent to select a system for a significant institutional project that cannot be reasonably maintained for the life of the building and is so easy to damage.

I'm interested to here other opinions form you guys.

Dave

Subject: RE: EIFS Rainscreen Exterior or Simply Stucco

While I agree for the most part with David's comments, I shall offer a few of my own thoughts:

*  Insulation in the cavity should be evaluated through a computer simulation tool for due point and thermal  performance.

*  The exterior sheathing is not as critical with the  installation of a water membrane, but when we tested that product it  performed only slightly better than paper-faced gypsum sheathings before delamination. There are several cement-based sheathings available as well as other non-paper faced gypsum panels which work much better!

* The use of a breathable air/water barrier may work better here moving the Vapor barrier further interior in cold climates if additional insulation is required. If the 2" rigid is acceptable then David’s assembly is great.

* I am not a fan of conventional (7/8" 3-coat exterior) Stucco in cold climates! Too many cracks.

Later
Rik Master, AIA

Subject: RE: EIFS Rainscreen Exterior or Simply Stucco

Rik; Good points.  All of these are trade offs and sometimes personal preferences.  Many qualified architects can come to different solutions.

* I agree that it may be possible to place batt insulation within the stud spaces if proven acceptable by computer modeling.  However, if more than around 50% of the total R value is in the stud space, then the vapor barrier may need to be located on the interior face and the air barrier may need to be vapor permeable.  I would rather keep all of the insulation in the air cavity which also provides for an open and continuous surface of the sheathing for installation of a combined air and vapor barrier membrane.

* I agree that there are several cement-based sheathing products that would be excellent choices.  The reason I don't like paper faced products is not there performance once covered, but the problems that might arise from being exposed to rain and snow during construction.

* See the first comment about the batt insulation and air/vapor barrier.  I would bet that actual heat loss throught the opaque portions of the wall is a relatively minor percentage.  I would much rather have a system that has a higher probability of long-term performance regarding the control of air, vapor and water over one that has a higher R value.

* Agree with the last comment wholeheartedly, try those metal panels instead.

Dave

Subject: RE: EIFS Rainscreen Exterior or Simply Stucco

To All:

Thank you all so much for responding to my questions so thoughtfully.  This is everything that I¹ve thought that BEC should beŠa resource for great advice on exterior envelopes.

As for my own opinion on this, I am in agreement with the rest of you here. I see that most of us are preferring true stucco, assembled with the ability to drain and weep, with subflashing membranes that act as vapor barriers / weather barriers simultaneously, and plenty of expansion joints.

A couple of other things I recommend:  I actually prefer using the EIFS acrylic finish coat on my 7/8² stucco because it seems to hold together against those hairline cracks better with its flexibility.  If you really want to seal off the water penetration on the face of either stucco or EIFS or exposed concrete, consider using Sika 550W elastomeric coating.  It’s super flexible, even in extreme cold, can span cracks, and comes in over 240 standard colors.  It sheds water and still breathes vapor.  The downside is that the color becomes very uniform and it has the slightest rubbery sheen to it when you¹re looking at it within a few feet of distance.  That may dull with age.  We¹ve used it on several buildings with exposed concrete frames, which we coated, and the results are very nice.  Really cleans up the buildings.  Protects rebar in the exposed concrete too.  It’s a little expensive, but very effective.

Getting back to the stucco vs EIFS situation, our group here in Mpls got a little hung up on whether there should be Z clip furring to carry the stucco beyond the rigid insulation.  I think the consensus here was that it would be fastened and sealed against the vapor barriered sheathing, between the insulation boards.  The sheathing would act as a thermal break.  So I guess we agree again with Dave here.

Then there was a discussion about whether the stucco would need another sheathing layer.  I like the paper-backed wire mesh which eliminates that thickness. 

Finally, my first choice for exterior materials here was metal panel cladding, like Alucobond, so again, we seem to be in agreement.  That choice was eliminated for budget, unfortunately.

Regarding the possibility of trading advice in the future, we might consider developing a ³List-serve² function on the internet.  I don¹t know if any of you belong to a list on the internet, but it allows you to voluntarily subscribe to a single location which ties you to the entire list of subscribers with a similar interest.  You can ask questions of the list (of all subscribers), and they go out to everyone¹s email address on the list. Those other subscribers can either simply read the info, or reply with comments or answers.  The replies go out to everyone on the list.  You get threads of topics, some of interest to you and some not.  You reply, save, delete the emails as you see fit.

(Personally, I belong to the Fiddle-L list where, for years, I’ve asked fiddling technique, theory, style, general info questions, and have gotten responses and advice from knowledgeable, great fiddlers from all over the world.  Occasionally, there are the personality and topical taste clashes, but in general, it¹s a valuable connection.)

Judd Peterson, AIA 

Subject: RE: EIFS Rainscreen Exterior or Simply Stucco

To all:

Great conversation.  Since as architects I think we are still graphically inclined, I took the liberty to sketch what I understood to be the direction of the conversation and have attached it.

One additional question for all regarding the assembly:  In David’s initial description he noted to provide stainless steel flashing to drain cavity.  I concur that stainless is appropriate but I¹m equally concerned that there is a significant potential for the galvanized zee furring to rust and stain the stucco.  Undoubtedly, the zee furring will be field cut and scratched compromising the galvanized coating.  Does this concern anyone else?

Regards,
Ned S. Kirschbaum, AIA, CSI, CCCA, LEED AP

Subject: RE: EIFS Rainscreen Exterior or Simply Stucco

Yes, that it might rust and that the staining will migrate down and out on the flashing more than through the stucco.

Rusting of the lath could also occur if too much moisture and free water is in the air cavity. This has more potential to migrate through conventional stucco in my findings since it is already in that layer and travel the hairline cracks common in this material.

Rust, water, air, etc. take the easiest path of resistance.

PS I agree!  We need a Forum for this kind of GREAT discussion! We are looking into various options for BEC Chicago, since we get into the same kind of discussions. So would love any suggestions!

Later
Rik Master, AIA

Subject: Re: EIFS Rainscreen Exterior or Simply Stucco

Hi everyone, again!

One, hopefully last, question. I was thinking of using pressure-treated wood furring to get the drainage cavity behind the stucco. Any comments on that?

Subject: RE: EIFS Rainscreen Exterior or Simply Stucco

Richard,

At MnSCU (Minnesota State Colleges and Universities), we have found, over the hundreds of buildings that they¹ve reroofed, that pressure-treated lumber has a tendency to warp after installation, even if kept dry.  Early on, MnSCU believed that they were being conservative by installing pressure-treated lumber for parapet blocking and cant construction around the roofing.  What they found was that the lumber warped so much that it was tearing up the sheet metal flashings and roof base flashings.  Since the lumber was being covered by subflashing membranes and sheet metal, they decided that the lumber was intended to be dry and that they would not use pressure-treated lumber, just plain, oven-dried lumber.  They haven¹t had any problems since.

I only use pressure treated for decking or sleepers where I can’t avoid moisture and the materials are expected to be replaced in the relatively short term.

How about using something like Korolath PVC shims to space the stucco lath off the sheathing, except with some continuous length?  Maybe something like PVC piping ripped in half?

Judd  Peterson, AIA

Subject: RE: EIFS Rainscreen Exterior or Simply Stucco

There may need to be some disclaimer that these are individual opinions without benefit of full knowledge of the project and cannot be used as professional advice nor do they represent the views of BETEC or NIBS, otherwise, I think it would be great.  Maybe we all have some other topics to cover (such as; success getting the window and curtainwall manufacturers to allow flashing the air barrier into their glazing pocket, how to get a double seal to storefront sections, how to integrate a hollow metal door frame into a drained cavity wall, how to deal with very low allowable accessible operating forces on doors while still keeping them closed against the wind, etc. etc.)

Regarding Richard's last suggestion, we have been avoiding PT wood because we worry it will warp over time in a wet environment that goes through significant temperature swings, no basis in history for that assumption though.  Anyone with actual project experiences?

Dave