Is ZIP really all that great?
We’re going to start digging in a few days on our new house in climate zone 5. I’ve been set on the following wall assembly (from interior to exterior):
Drywall
2×6 framing with rockwool batts
ZIP for the air and weather barrier
comfortboard exterior insullation (probably 3″)
maybe some sort of wind screen material?
1 x 3 or so strapping
some sort of cladding (probably mostly hardi)
I have thought for a long time that ZIP is just the ultimate solution for air sealing especially.
But I’m beginning to have my doubts. I think we’ll get the tape installed correctly. The people who are building for us are true craftsman and although they haven’t built this way in the past, they really want to do a good job.
But I’m literally depending on tape for my water and air seal. Granted, with external insulation and a wind screen outside of the ZIP hopefully the house won’t have to deal with a lot of either, but still, arguably the most important layer of my build relies heavily on tape. It just sounds dumb.
Not only do I want this home to be energy efficient, but I want it to be durable. Like I want my grandkids to be able to live in it and not have to do anything with the bones and the insulation and so on if they don’t want to.
Heck, the house I’ve lived in for most of my life is horribly built and it’s probably 100 years old. If all the expense I’m putting into this new house can’t make it last twice as long as the house I’m living in now, then why am I doing all of this? Would be better to just do a crappy low quality build that’s more affordable.
Are there economical ways to make my airtightness detailing be likely to last a LONG time?
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Replies
dpilot83,
My own preference for sheathing is still plywood with a separate WRB for two main reasons:
- Higher (and variable) permeability, which makes the wall more resilient to moisture.
- Relies on lapped joints rather than tape at flashings and other intersections.
As for longevity: The one thing modern high performance houses sometimes sacrifice is resilience. Old, poorly insulated houses leaked air like sieves but lasted a long time, often because you had to throw heat at them to stay warm, and that heat dried out the damp parts of the structure. Very well insulated assemblies can be unforgiving of air and water leaks.
Broadening out the discussion a bit, what affects longevity is not necessarily the quality of the construction, but rather two other things:
- A house that is comfortable, beautiful, and meets the needs of its occupants will be maintained by them. Houses that get maintained last, those that get neglected, no matter now well built, don't.
- Demographics and location. Houses last when they are located in regions close to good economic drivers. If that disappears it doesn't matter how nice the house is it will have a short life. That's always been true and you can see it recently in Detroit.
A long-winded way of saying that most of the things influencing the chances of your new house lasting two centuries are out of your control.
Is there a specific WRB that you'd recommend?
Tom_K,
My choice is influenced by common practice up here in Canada where the primary air barrier is usually still interior poly - which means I don't worry about an airtight sheathing layer. In those circumstances I like Tyvek Commercial. It has a good perm rating, and more importantly for me, is really hard to tear.
If I was locating the air-barrier at the sheathing I'd be tempted to use a self-adhered WR. If like the OP you are a bit skeptical about the longevity of tapes, that would provide a belt and suspenders approach.
I have watched some Christine Williamson stuff (Build Science Fight Club). She’s a Canadian who seems to have a remarkable grasp on building science topics.
She has a module (you have to pay for her content) called, “Quit Worrying about Dew Point!”
At about 37:47 into this module she discusses how it’s common in Canada to use poly as an interior air barrier. She says it works (barely) in Canada for the following reasons:
- There are quite a few homes in Canada that simply don’t have air conditioning
- On the homes that do have air conditioning, it’s possible that the home owners don’t set their thermostats as low as people who live further South
- Summers are shorter further to the North. Less time for problems to develop and more time for them to have an opportunity to naturally dry out and be fixed
- Canadian’s are pretty religious about building a gap between cladding and the wall structure. This allows more ventilation to get into the wall assembly and possibly solve some of the issues poly can create
She says the thing that makes it a near failure point even in Canada is that in the summertime when you’re in an air conditioned home with a poly interior air and vapor barrier, you have cool drywall and cool poly. She says, “what’s the likelihood that warm moist air will leak through the rest of the wall assembly in at least some places and end up touching the cool poly? Pretty likely”
She basically says that while this works barely in Canada (and not always in Canada as can be seen by the Vancouver condo crisis in the 90’s) it becomes less and less reliable the further South you go.
dpilot83,
I hesitate to disagree with Christine Williamson, but I've read the official report and attended seminars on the causes of the condo crisis. The envelope failures were overwhelmingly due to inappropriate design for the climate, and poor exterior detailing.
The presence of poly slowed drying, but wasn't a contributor to the moisture problems. In the intervening 20 years, both better design and detailing (including rain-screens) have almost entirely eliminated envelope failures here, and poly is still being used as the primary air/vapour barrier on almost all buildings.
Now all that said, poly may well become more of a worry as climate change progresses.
Thanks Malcolm. I guess this site only allows so many levels deep of replies so I’m unable to reply to your comment directly but that’s good insight.
dpilot83,
It brings up interesting issues around how much we should build for the conditions we experience today, or what may occur in the near future. Canada's reluctance to shuck interior poly and use more progressive air-sealing methods may well come back and bite us in the ass.
Zip is going to outperform plywood and house wrap for air sealing. The tape will last a good long time exposed to weather, and pretty much forever once fully covered by the siding.
But as far as making the house last forever, there are more important things you can do like having a generous roof overhang and eliminating common failure points like complicated roof intersections.
According to JL at Bld SC if you want zip to last 100 years you need to have an external insulation layer to protect the tape (probably freeze thaw cycles play a role here) . That implies zip R systems are less durable that zip system plus ext insulation. Also if you want a really tight house (1.5 ACH or less) tyvek or other non adhered membranes don't get you there most of the time
I’ve been googling for JL at Bld SC trying to figure out how to read more about what you’re saying and I’ve been unsuccessful.
He's saying Joe Lstiburek at Building Science Corp. I believe Joe's perspective is more along the lines of: "if you want to be SURE that the Zip system will last 100 years, put it behind exterior insulation.
I tend to agree; I think that behind enough continuous exterior insulation to keep the sheathing above the dewpoint temperature, the Zip system is pretty ideal. When used in more conventional applications, I prefer to use a separate WRB to ensure long-term performance, and prefer to use sheathing that will hold up to 100 years of wetting and drying--either plywood or board sheathing, preferably the latter.
The Zip System, with a little attention to detail, has resulted in many of my projects coming in at or below 1.0 ACH50.
Thanks Michael. When you say board sheathing, are you just talking about 1x10 lumber type stuff or are you referring to something else? I guess I’m not familiar with any board type products that are devoted to sheathing.
If it is like a 1x10 or something, I assume you want something other than pine to make it last awhile?
I’m regards to a WRB for such a structure, is regular housewrap going to last 100 years or more? Wonder if it might deteriorate as well?
Reply to dpilot83, #13: yes I mean softwood boards; around here that's usually rough-sawn eastern white pine. It's affordable, goes up quickly (though not as quickly as sheet goods), provides code-compliaint lateral resistance in most places, the scraps can be safely burned or buried, and the boards hold up to repeated wetting cycles (eventually resulting in rot) better than any engineered wood. My house was built in 1830 with board sheathing.
WRBs are plastic, and if protected from UV (or other) degradation, it will last indefinitely. I don't use open-joint rainscreens because it puts too much pressure on the WRB.
Great information, thanks.
If you were in a seismic area, would you prefer plywood to board sheathing?
Have you used a self-adhered WRB on top of board sheathing, and does it work well?
andyfrog,
if you are in high seismic area your code probably doesn't give you the choice. Diagonally oriented board sheathing still doesn't have the shear strength necessary to resist racking.
Andy,
"Zip is going to outperform plywood and house wrap for air sealing."
Sure, but that's not really the choice we are talking about. If the sheathing is the primary air-barrier, both OSB and plywood can also be taped or sealed just like Zip. Then the question is whether Zip is a better WRB that those other sheathing materials with a separate one?
Now this isn’t a controlled experiment…just an observation but as far as durability goes, ZIP can last a long time. My neighbor built his super insulated shop with zip sheathing & zip tape with 20” of spray foam inside about 10 years ago. He started to put siding up and for whatever reason he stopped and just left the zip exposed. 10 years later the tape still has perfect adhesion and the zip is intact. He did tell me that he rolled the tape religiously during install so I am sure that is a contributing factor.
Interesting. What climate zone?
4A (central NC)
Also, to the OP, I also had trouble trusting just zip to do both the air and water barrier duties so with lots of advice from this forum I ended up going with taped OSB, sealed bottom plate and band joist and Benjamin Obdyke Hydrogap. This utilizes the OSB for the air barrier and the house wrap for the water barrier. I don’t know if it will last forever but I think it will do better than code minimum / standard building practices for my area.