Hi everyone,
I work for a custom home builder in Southwest Virginia and for the past five years or so our go to wall assembly is as follows:
-2×6 framing
– 7/16 OSB
– Tyvek housewrap (not always used)
– 1/2″ XPS foam insulation (sometimes used as wrb)
– Hardie lapped siding
As a newcomer to the company, I thought it would be worth exploring using Zip R-3 instead of our standard detail. It’s always met with the Zip R being too costly. I think part of it is the unwillingness to try something new but that’s a whole different conversation. We ran the numbers on a two story home at about 2400 square feet and the cost difference was the Zip R-3 being $5,000 more in just material. It’s tricky to calculate the labor savings because our framers pretty much bid on a rule of thumb basis. This is partly because of the good ole boy network and relationships that have been established and not questioned. I’m getting to the point, I swear.
My question is – what are arguments for and against each wall assembly detail (our normal vs ZipR-3). How can I convince folks who are hesitant to changes like these? It cost more, I get that, but is the improvement in quality worth it? Are other builders having this kind of dilemma? To me, installing one product that accomplishes three different goals is kind of a no brainer (structural sheathing, exterior insulation and wrb). Also, I’m wary of the exterior insulation shrinking and deteriorating with how we currently install it. What are some other considerations? I’d love to get feedback from folks and hear arguments for each.
– James
Replies
Hey James.
This is a great question and I'd like to offer some things to consider regarding the "quality" of the assemblies. First, the walls aren't exactly an apples to apples comparison and I can think of at least two reasons why.
The first reason is because the walls you are building now do not have a decent air barrier (unless you are doing something on the interior that you did not mention). Mechanically fastened housewraps like Tyvek are very difficult to detail as a good air barrier. Because you have to tape the seams of ZIP sheathing, you're making a big improvement on air sealing just by using it. That said, if you tape the seams of the OSB in the wall you are building now with a decent air sealing tape, you have achieved the same amount of air sealing and the walls become a bit more comparable.
Another difference is the location of the continuous insulation. With ZIP R, the insulation is inside the sheathing. On the wall you are building now, the insulation is outside the sheathing. Some experts would argue that having the insulation outside the sheathing, where it acts to keep the sheathing warm, makes for a more durable wall (Google "Joe Lstiburek's Perfect Wall"). For this benefit, you have to use an appropriate R-value of continuous exterior insulation for your climate zone. Some builders use table R702.7(3) in the IRC, which specifies when a class III vapor retarder can be used as a proxy guide to what R-value is needed to keep sheathing warm (and prevent cold sheathing issues like moisture accumulation).
There is one more debate to consider. When it comes to water management, no one would install flashing, housewrap, roofing, siding, or anything else that is meant to shed water in any other way but "shingle style," meaning the material above covers the material below. But for some reason, many builders (I just used ZIP on my own house) give ZIP and similar products a pass and rely on taped seams, which create a "reverse lap." There are some builders who don't use it for just this reason.
Putting the location of the insulation and reliance on tape aside, I would say that if you considered taping the seams of the OSB on the wall you are building now, you can compare the walls a bit more evenly. And in my opinion, both walls are just fine. The details have to be meticulous, of course. If a window is not flashed and installed properly, and it leaks, the quality of the materials used doesn't mean a thing. But beyond that, it's up to each builder to determine what works for them when it comes to cost and process.
BP makes very good points in his comment. As a Construction Administrator for an Architect firm, I've seen my fair share of installations of both assemblies. I'll breakdown your questions below in separate paragraphs.
"If it isn't broken, then why fix it" mentality is tough to overcome in any subject. I would start by breakdown the installation of the layers for each system. You will start to realize the cost of labor between the two systems.
"Quality over quantity" or in this case cost should be reviewed closely, but neither system is worth anything if the installation isn't held to the same quality of standards each product has been tested. The most common thing I see with both systems is the lack of use of a j-roller over the flashing tape to get proper adhesion. On larger commercial jobs, we have found the Zip sheathing to be more costs prohibitive than the other system, but this has to do with the labor side and multiple trades involved to complete one system versus the other.
Owners, Builders and Designers are having this same dilemma on every project, you're not alone. I believe there are ways to reduce overall project costs elsewhere than the building envelope. We found that the cost of Zip sheathing changed very little compared to the costs of OSB over the past year or two. There were many factors, of course, but each project may have been different.
I agree with you on the all-in-one versus individual layers. This is where the labor rates come into play for the cost savings. Using the insulation as a WRB is nice, but I'm hearing more and more manufacturers are not making the seam tapes anymore. If you're concerned about the longevity of the foam, then this really doesn't become a viable option as a WRB anymore, which is a point to make when comparing the two systems.
PB mentions "shingling" and "reverse lap" in their comment and both systems have both these issues when it comes to flashing penetrations, metal flashing or other components. These are only a problem if the installer is pay attention to the details or installing the flashing tape poorly.
In conclusion, more and more companies are coming out with similar all-in-one systems similar to Zip. Labor and equipment use is where you typically see the savings between the two systems. System 1: 7/16" OSB + Labor, Tyvek + Labor, Insulation + Labor. Each layer also has to use equipment multiple times or be on site longer. System 2: Zip sheathing + Labor and done. The taping of the seams gets completed as they install the system, which allows the seam tape to be properly "shingled" as you move up the wall. Similar breakdown can be said if you are building walls on the ground and lifting in place too.
Hope this helps, but as you can see there is really no short response to your inquiry.