Polyiso Foam for Exterior Insulation / Controlling Interior Humidity
Vpachmayer
| Posted in General Questions on
We are possibly looking into/still researching doing a 2 inch exterior rigid foam then tyvek on perimeter for our new build. The plans include 2x6s. Then plan to do blown cellulose on inside of the walls. Building in NE Ohio.
My father, who has retired from construction, is questioning how that won’t make the house too tight and cause condensation on windows. What steps do we need to take to make sure the R value is high….but not overly done.
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Replies
Excessive R value doesn't lead to condensation. Excessive indoor humidity does. It's not possible to "over insulate" a house, aside from spending too much money for the efficiency improvements you get at some point. You can't make a house "too tight" either, that's the old "houses need to breath" stuff that isn't really accurate.
2" exterior rigid foam in your climate zone is pretty good, and you can seal it up as a good air barrier (I'd do air tight drywall on the interior of the exterior walls too). You want to avoid using an indoor humidifier, and you should consider an HRV if you are concerned indoor humidity levels might get too high.
Better insulating windows will help to limit condensation too, but keeping their interior surfaces warmer. At the extreme of this is would be full-thickness (1-3/8") triple pane windows in insulated fiberglass frames, but you can get good double pane windows too for less money that can perform pretty well too, although not as well as a good triple pane window.
Bill
what would the benefit be of doing the poly iso over just adding a thicker amount of blown cellulose?
Assuming you're not doing a double stud wall, the exterior polyiso will be what is known as "continuous insulation", which means it's not broken by the studs so there aren't any thermal bridges. This means you get the full R value of the rigid foam. Any insulation added to stud bays doesn't do anything to address the thermal bridging of the studs, so the studs reduce the overall performance of the entire wall.
Bill
The insulation on the outside keeps the sheathing from getting cold enough to hit the dew point and accumulating moisture on the inside which could lead to rot.
> keeps the sheathing from getting cold enough to hit the dew point
A common myth. In most cases, the sheathing will often hit the dew point. But follow the recommendations and the wall works anyway.
Agreed, the primary value of exterior foam is warmer wood sheathing - not reduction of thermal bridging.
Note that for the same R value, unfaced EPS will produce a dryer wall than foil faced polyiso (because it will dry outward).
If you decide to go the polyiso route, I recommend you read this article by Martin Holladay: How to Install Rigid Foam Sheathing. In terms of controlling interior humidity, Alex Wilson offers these tips.
There is such a thing as a house being too tight resulting in condensation on the windows. The tightness in question is not the R-value, but the air leakage. Taped foil faced polyiso can be a very effective air barrier, there's a plan to have the air barrier continuous around the whole envelope. That's a good thing, and a new high performance building can be and should be made nearly perfectly air tight.
But then you do need to plan for ventilation to avoid buildup of excessive humidity, excessive CO2, and other forms of indoor air pollution. That would be true whether the walls were R-14 or R-44.
Here's what I think is the main GBA article on that--it says 2009 but has been updated much more recently:
https://www.greenbuildingadvisor.com/article/designing-a-good-ventilation-system
Just so you don't think that building less tight will save the cost of powered ventilation: a naturally vented house will be under-ventilated in mild, windless weather and over-ventilated when it's windy. You don't want this - so seal it up tight, test the air sealing to verify and then ventilate it correctly under all conditions with powered ventilation.