Question about Passive Houses during Transitional Months

One thing that's notable about super tight, super efficient houses is that how you live in them or "lifestyle" is different (at least to me as I think about these kinds of things). I'm not talking about lifestyle changes like "sorry honey, you have to put on the sweater and wool socks as we're keepin' in at 62 in Dec" or "we'll be 'shower optional' on Thursdays". Its how you experience the house leveraging the external environment to dictate the internal one.

One of the incorrect early approaches to super tight houses was to limit glazing, air flow, and any other way the exterior environment would affect inside. Newer houses like ours not only embrace the external environment, we're somewhat dependent on them to function efficiently. The simplest example of this is in passive solar gain where you're using the lower sun in fall/winter/spring to give you a heat gain. However, there are more subtle ones like having east and west ventilation (where winds tend to blow at most times) for summer cooling. It also brings the outdoors in which is the biggest reason to have a house out in the woods.

This weekend, we had gorgeous weather upstate including Sunday night getting down to 46 degrees. We had the house wide open all day when it was 80 out (house was a comfy 75). Once the sun went down, the temp dropped quickly as there was no humidity. We immediately closed up most doors and windows to preserve our natural heat and that it did.

However, this prompted a question in my mind about Passive Houses. When you're in the transitional fall/spring seasons where sunny daytime temps are in the 60/70s and you want to open some doors and windows to enjoy it, are you restricted from doing that in a passive house given there can't be that level of heat loss at night (say if it drops into the 30/40s)? The house will perform if you keep it closed up but is that the lifestyle in a Passive House? Are you so dependent on heat gain during the day that you can't risk having natural ventilation in the transitional months? Anyone out there with a take on this?

That said, even our house is tricky during the transitional months. In the scenario described above, we'd probably have a few windows and doors open during the day and get nice solar gain on our concrete slab. We'd close them as the sun went down. Most likely the house would stay warm passively but in the case of a strong drop or that we forgot to close something early enough, we have the geo system there to warm us up.

Admittedly, this is a scenario where radiant is not at its best. Periodic and quick warm-ups aren't great with radiant. It takes 1-2 hours just to get the slab up to temp. When that happens for a couple of hours and then turns off for several hours the slab cools. Then it has to warm it all again. I experimented with turning the thermostat down in this scenario which made it run less but some in the household were cool at times. This year, I plan on making liberal use of the fireplace (cut up about a cord this weekend!) and will even try electric space heaters that can warm a 500 sq ft area quickly.

Passing 10Mwh

I'm not that into arbitrary milestones but figured it was of note passing 10Mwh of solar generation this Labor Day weekend. We're about 19 months into our install. Performance has been notably above the estimate provided by the installer (I think he sandbagged so he didn't get calls asking why it was low) and 10-15% below optimal for the area (it's well below optimal for the latitude but that's pretty irrelevant as it doesn't take weather into account).

That 10-15% down was expected based on the solar pathfinder the installer did. We have one big oak that we wanted to save to the west of the house and from Sept through March it gives partial shade for about 60-90 mins each afternoon. Production Oct through mid-Feb was also below expectations as the low sun clipped some of the surrounding trees. I've been doing some thinning of the evergreen hemlocks to the south and west so we'll see how that helps. Fortunately, this summer was hot and dry so Apr-Jul met or slightly exceeded optimal (with July providing a new high point at almost 800kwh).

I have about 1100kwh banked with our utility. My estimate is I need about 1700 to be net zero for the year. Its a Sept+Oct race as Nov will start us back to being net positive for the month. I think my new heating strategy will both allow me to bank more in Oct and use less which I imagine will be the difference.