Wall system
kp_newbuild
| Posted in General Questions on
We are in the process of building a new home in Wisconsin and I have come across a builder who is building 2×6 walls insulated with batted fiberglass or spray in cellulose next is OSB sheathing followed by 1/2” exterior foam and over that housewrap. I have read the article that calculates for our climate that we would need at least 3” of exterior foam so I guess I am a little bewildered to what to think of this wall system and why is a good or bad. It seems to be common to hear more companies putting on 1” foam, but is that enough?
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Replies
KP-Newbuild,
I do hope you are not very far into the process of actually building at this moment. Aside from the weather in Wisconsin at the moment, the idea of 1/2" of foam over OSB is almost criminally bad in your climate zone and humidity profile. I won't even get into the apparent position of the house wrap just in case you misunderstood what you were told.
I can't cite the articles you most urgently need to read on GBA, but perhaps the monitors will step in and suggest a few of the most critical ones. If you have several months before actually starting framing (which I would suggest as a minimum) then you might have time to develop a bit more understanding of the better level of house design GBA promotes. If you don't have the time, I would have to suggest the builder you mention would not be a wise choice.
I would hazard a guess that you will find it hard to find a builder familiar with GBA best practices. This is not peculiar to Wisconsin, just builders in general. Too frequently they say, "I've been doing it this way for years". You are correct in thinking that 3" of foam is much closer to what you need in near to CZ 7 conditions. However, there are many more details regarding air sealing, window quality, cladding choices, and much more; that all need to be considered and integrated with an understanding of moisture control in your particular environment. Simply sticking foam on the outside of a house and pretending to be energy smart doesn't cut it.
Sorry to sound so harsh, but you are about to put a significant amount of money in play and there is much to think about regardless of whether you do it or you find someone knowledgable that can guide the build details. GBA will be a great asset for you either way.
Thank you for your response unfortunately we are father in the process as the house is framed with 2x6 and I was asking about thermal bridging. I am upset I didn’t ask sooner to have better building practices. What type of insulation would you suggest in our current situation?
What are you trying to achieve? There are basically two or three (depending on how you look at it) methods to determine how much exterior foam to use:
1- code minimum for your climate zone. Most of the people on GBA will probably want more than this.
2- enough to keep your sheathing from getting wet in your climate zone. There are tables for this.
3- enough to either allow you to safely run higher humidity inside or enough to hit some particular overall R value for your wall.
Which are you trying to achieve? Which type of foam you want to use plays into this too since you’re really trying to hit an R value, not just some number of inches of foam. 3” of exterior foam is a lot, and is probably because you’re trying for reason #3.
Bill
I was asking about continuous insulation in order combat thermal bridging. Do you have any suggestion as we already have the structured framed and will probably not be going with the needed amount of exterior eps.
Code allows you to build with no external foam at all. IMO, less than the recommended amounts of external insulation should only be used with unfaced EPS or mineral wool - combined with other things that reduce wall moisture risk. Ie, 1/2" can work, but more is much better.
Specifically why is it not as good? - because 1/2" foam leads to colder sheathing, more condensation and probably more moisture accumulation than 3".
Thanks for the response. How can 1/2” work? As to the responses above it seems like a very bad idea.
Walls with little or no external insulation work (in Winter) by having more external drying than internal wetting (both from air movement and/or diffusion). And sometimes more moisture resistant sheathing (or none at all). Most walls have been built with little or no external insulation and work fine.
The recommended amount of external insulation produces a good (not best) wall. More is even better (because it can prevent all condensation on the sheathing). If your are going to do less, external mineral wool is the most conservative (less Winter wetting, doesn't reduce outward drying). You can also put foam on the interior side (also no effect on outward drying).
K.P.,
Start by reading this article: "Calculating the Minimum Thickness of Rigid Foam Sheathing."
Wisconsin has two climate zones. You are either in Zone 6 or Zone 7. Here is a link to the climate zone map.
If you are in Zone 6, the minimum R-value for exterior foam over a 2x6 wall is R-11.25.
If you are in Zone 7, the minimum R-value for exterior foam over a 2x6 wall is R-15.
The problem with 1/2-inch foam is that it reduces the ability of your sheathing to dry to the exterior in the spring -- but it isn't thick enough to keep your sheathing dry.
Thanks for the response. We are in climate 6 and the county above us is climate 7. My builder would like to use closed cell spray on the interior walls and when I suggested doing something about thermal bridging he suggested adding 1/2” foam on the outside (not sure on be type for sure). We are already framed up so I know it’s late, but I am hoping to make an informed decision.
At just 1/2" even extruded polystyrene (XPS) will have a vapor permeance much higher than OSB or CDX siding, half inch unfaced EPS would be more vapor open than standard latex paint.
As long as the foam doesn't have a foil or plastic facer it's vapor permeance isn't low enough to cause a problem, but at that low an R-value an interior side vapor retarder tighter than 1 perm would be necessary.
So, what is it- pink/blue/green XPS, faced/unfaced EPS, foil faced polyiso or... ???
Do you have OSB sheathing between interior and exterior?