I live in a 2,000 sqft ranch where I've been slowly improving my house's thermal performance through air sealing and insulation, along with a recent 2-ton geo installation. My attic is around R-60, walls are R-13. I have around 200 sqft of glass which are 1970s double pane casements. This is winter #3 in the house, that last 2 of which were with the new Geo system installed. Of course, both winters seem to be colder than average but overall the system is doing well. I'm realizing I'm not quite there for a true 2-ton system, as I have leakier than expected exterior doors and windows. I also haven't completed my insulation work in the basement which is what prompts this question (more below). I have been logging the electricity usage and some EWT and LWT data, which is why I feel I'm not quite at the 2-ton load yet. I've found that when temps are in the teens, the system needs to run 24/7 to keep the house at 68F. The design temp here is 5F. The EWT is in the low 30s so the system can't quite hit two tons under those conditions (Climatemaster) which probably explains the long run times. I have my aux heat lockout at 12F so backup can kick in when it's cold. This winter (starting Dec 20) I've used 1691 kWh on the compressor and 211 kWh of aux heat against 2,586 heating degree days This leads me to my question...

As it stands now I have 2" of foil faced polyiso glued to my foundation walls, for which I plan to add another inch. The basement is running 57F today, and probably won't get any lower. A couple of mistakes that remain un-addressed are 1) no sealing around the edges with spray foam, and 2) the seams aren't taped. I will do that once the third layer gets up. I've read a bit about the low vapor permeability (or should I say non-existent) on foil faced polyiso, so I'm curios if I should be worried at all about those walls not drying to the interior? About half of my foundation perimeter has a foot to 4' above grade, the other half has less than a foot. My band joist is insulated with 2" of the same polyiso with foam around the edges. I realize I should probably add another 1" to that as well. In my load calculations, taking the foundation walls to R-19 had a big effect on the load, so I'm hopeful completing that will take the house closer to 2-ton. I plan to replace doors and windows this summer, along with adding at least 2" of Roxul comfortboard to the exterior so that should help as well. Any input is appreciated!

A foot of above-grade exposure to the exterior is sufficient as long as it isn't chronically wetted by splash-back from the roof's drip edge, covered with snow, or poor grading saturating the ground next to foundation with every passing storm.


Less than a foot can be an issue. If it's possible to jack up the house 1/8" on that side and slip in EPDM (roofing membrane) or copper flashing as a capillary break between the concrete & foundation sill you'll be in good shape. Without either good drying or a capillary break you are likely to run into rot issues in the foundation sill over the long term. (A year or two probably isn't going to make it or break it unless there are exceptional circumstances.)

The economics of replacing the ~U-0.5-U0.7 casement windows isn't really great, but there is probably reasonable financial "payback" on a set of low-E interior storm windows, and a comfort payback too. If you set up the interior storms with the hard-coat low-E coating facing the interior it reflects your body heat back at you when standing next to the window. The Larson Insider (handled by the big box-store chains) are pretty reasonable, and I THINK there is a low-E glazing option (check on that) the most important thing is to measure carefully to get an air-tight fit, or you'll end up with condensation on the window behind the low-E storm. There are others, but you may have to order them online to get the low-E option. With the low-E coating the U-factor would drop to about U0.30, and the window would have a net energy gain. A clear glass interior storm would only drop the performance to about U0.4, and it would be a net energy loser (though better than what you have.)