Can you build a "green" house in the Adirondacks?

[John H Swanson]

John - let me know the name of the reference you are using. I'm going to try to pick up a copy, even if it's somewhat archaic. I have my college heat transfer book but that's highly generalized mechanical engineering, and I haven't even picked it up yet.

https://www.ashrae.org/technical-resources/ashrae-handbook

New latest edition is $200+
Used older edition on Thriftbooks less than $20.
They are updated every 4 years - last was 2021. IMO, No need to get latest.

There used to be a metric version and american version. With the metric version showing traditional units in parenthesis. Thats the one I have. Not sure now.

[montcalm]

https://www.ashrae.org/technical-res...shrae-handbook

New latest edition is $200+
Used older edition on Thriftbooks less than $20.
They are updated every 4 years - last was 2021. IMO, No need to get latest.

There used to be a metric version and american version. With the metric version showing traditional units in parenthesis. Thats the one I have. Not sure now.

Thanks again. Yeah, I can usually find used text or reference books for almost nothing.

I prefer metric, honestly but I can convert any.

[montcalm]

Here's a chart of my baseline heat transfer.

I wish I had some more data to see how much of the "scatter" is caused by my uninsulated basement and how much is just the earth.

But the trend is such the data points below the trendline are in the fall, and the others in spring. Winter and summer at the ends. Summer has AC loads and I should probably just force 0 delta T to 0 energy. But either way it's close enough.

I don't really have enough info to know how energy usage correlates to day vs night temps, so I simply used the average for my delta T.

I'll do a calculation based on my home dimensions, etc and see how it correlates.

[montcalm]

Maybe a little explanation on how I came up with that chart. Not hard to do.

It's simply the NG energy (converted from therms to kWh) I'm using each month, then multiplied by my furnace efficiency. I've corrected slightly for my hot water usage by subtracting off the average summer usage.

What it tells me: A rough, overall system efficiency. I converted to heat flux and calculated an equivalent thermal resistance for the whole house. It's on the order of R9 (RSI = 1.6 in Jan). That's all infiltration, windows, in and outs, etc. related to an average temp. Rough, but it's where the rubber meets the road.

Note that even though there's some hysteresis, either the basement controlling the structure or the overall effect of the earth (probably some of both), it shows to be very conservative, that is, what gets stored comes back.

I think it would be best for me to take 30 days of winter data, collect my energy usage and see what I get. Compare that to a theoretical model of heat loss based on actual layout.

[montcalm]

This is the same data, just slightly adjusted via a weighted average of temps based on median monthly daylight hours and converted to heat flux via number of hours in a month and home surface area.

It's also converted to English units and flipped so the slope equal to R value.

[montcalm]

So, results!

Using John's analysis I was able to figure in some points of compare:

- If I transported my house to OF, I could net zero it with 10.3kW solar system (31 panels).

- The single story design super-insulated, provided I had enough storage could go off-grid with 16kW system (48 panels). Net zero at 8kW (24 panels).

This assumes:

- Heat pump managing an average COP of 4. I'd probably drop that to 3.5 or so for closed loop.

- Negligible AC. Loads are so small if at all. We never used AC at our camp and it was never hot. It was insulated, but not real well.

- Heat pump hot water. Adjusted based per ave COP based on my current usage.

- My current electrical usage, base of 415kWh/month (note less than my earlier number of 600kWh/month which included AC). This include electric dryer, refrigerator, oven and range, microwave, LED lighting, misc. charging and entertainment.

- No wood in either scenario. Backup only.

- Solar gain neglected.

- 1200 sq. ft. in both cases. Base is 2 story 20x30'. Super-insulated is single story 20x60'.

- Ave high temp applied to day hours, ave low to night hours weighted average based on median day hours per month.

[montcalm]

A lot to digest here...

This has been a bit revealing to me. I mean I kind of had some thoughts way back, but now I'm pretty convinced: I don't want to do an off-grid home. If I do end up buying an off-grid piece of land, it'll be a camp. 4 season, with some solar for summer but no way do I want to go that deep into solar + storage + expansion for vehicles in the future. I really think it will be far beyond my budget as well, and perhaps with a payback beyond its lifecycle and mine.

Net zero on this footprint I'd do 110%. I'd also plan to add additional panels for vehicles if not initially.

Next step is to evaluate the min requirements for the northern zone, see where that puts the design in terms of solar load, then look at a cost analysis for whether it's best to stick with that or try to add more insulation. There's a 20% plus improvement in efficiency over my current home (base) in the same environment, so it's not insignificant but perhaps the current regulations meet a good balance between cost and efficiency.

I'd also like to look at solar gain again and try to see about damping that. I was initially thinking a brick wall veneer on the north and west walls of the living and dining room could be enough to damp some solar gain without being too much to lag the rest of the system. Typical solar passives are using a 4" concreted slab floor to damp their gains, but I won't need that much mass as I'll have much less glazing. I think brick could be a good compromise.

[montcalm]

27x44' (golden ratio) single story structure, with 12:12 roof pitch gives about 19' on the hypotenuse without considering overhangs.

Using a 65x43" 330 watt panel, you could fit as many as 40 panels for a 13.2kW system.

I didn't do the math on this to see what the ideal footprint is for max panels, but this is pretty ideal. I think maybe using the same W/L ratio as the panel might yield better packaging. That's a weird ratio (for today's architecture) and more likely you'd see 30x40' or something like that.

By my calcs 40 panels is enough to max out the State tax credit without considering the NYSERDA rebates, which exists, but aren't readily available info unless you contact an installer, and they are a grant, so the money may or may not be used up by the time you get there.

40 panels is also enough to net zero (I mean true net zero: no wood, fuel oil, NG, etc. except as backup) this home and charge at least one vehicle. I get around 12 panels per 12k mi/yr, but maybe that's optimistic. I'm not sure how much charging efficiency one might lose but 12 is based on solar data for the region and average electric vehicle mileage per kWh currently.

[John H Swanson]

A lot to digest here...

I'd also like to look at solar gain again and try to see about damping that. I was initially thinking a brick wall veneer on the north and west walls of the living and dining room could be enough to damp some solar gain without being too much to lag the rest of the system. Typical solar passives are using a 4" concreted slab floor to damp their gains, but I won't need that much mass as I'll have much less glazing. I think brick could be a good compromise.

Funny in my class, my first project used the brick wall on the north side of the room. The second design used the slab floor with an air circulation system that pulled the hot upper air and ciculated it across the slab. And the grand finale design (full passive solar) used a water wall between the greenhouse and the house. You can calculate thermal mass storage using m c deltaT. I would do that first for just the sheetrock on the wall ceiling; plus something for the floor. Compare temperature rise for peak solar gain between say 10 and 2. It might be a choice between add thermal mass or remove 1 window.

[John H Swanson]

It's simply the NG energy (converted from therms to kWh) I'm using each month, then multiplied by my furnace efficiency. I've corrected slightly for my hot water usage by subtracting off the average summer usage.

What it tells me: A rough, overall system efficiency. I converted to heat flux and calculated an equivalent thermal resistance for the whole house. It's on the order of R9 (RSI = 1.6 in Jan). That's all infiltration, windows, in and outs, etc. related to an average temp. Rough, but it's where the rubber meets the road.

Hmm. When you balance the heat gain/loss for the whole house, I see you're leaving out the solar gain, but have you considered the intrinsic gains? There is a heat gain component from lost heat from motors (frig, etc,) lighting and people. When lighting was incandescent, this was more significant. It was noted to be a significant contributorin my past analyses. If you are doing the analysis for comparison than you might say it will be the same in the two applications, however if you are looking to see if your new application will be net zero than I would look at these other gains. If it is a part time residence then leaving them out could be considered a cautious case but not an average situation for a full time residence. And an uninhabited place could be left at 50 to 55F. For losses one could argue that infiltration is partially impacted by people coming and going. Infiltration losses have two components and one is delta t related so you could add that into a heat loss evaluation that is based on delta t....

[John H Swanson]

I didn't do the math on this to see what the ideal footprint is for max panels, but this is pretty ideal. I think maybe using the same W/L ratio as the panel might yield better packaging. That's a weird ratio (for today's architecture) and more likely you'd see 30x40' or something like that.

By my calcs 40 panels is enough to max out the State tax credit without considering the NYSERDA rebates, which exists, but aren't readily available info unless you contact an installer, and they are a grant, so the money may or may not be used up by the time you get there.

40 panels is also enough to net zero (I mean true net zero: no wood, fuel oil, NG, etc. except as backup) this home and charge at least one vehicle. I get around 12 panels per 12k mi/yr, but maybe that's optimistic. I'm not sure how much charging efficiency one might lose but 12 is based on solar data for the region and average electric vehicle mileage per kWh currently.

What's the installation configuation of the panels. Meaning how many rows? Are you still concerned with brushing off snow? 19 foot is quite high for a brush even if you're on a catwalk. I remember the video about that guy that lived in CO off grid and he recorded the weather for decades and that is being used as a baseline for climate change study...but the point I want to mention is that his daily routine included: the first thing was to record the weather and brush off the solar panels. Every day.

I've told that the magic angle for avalanches is 40- 45 degree. Less than that and the snow usually stays put. Greater than 60 and it sloughs off immediately. At that angle (45) is stays until it slides. Im not saying you should be avalanche concerned but rather a 12:12 roof might not have enough pitch to slough. I'm thinking of those steeper roof chalets in Europe. You pay more for roof to do less brushing. I would consider other roof designs: maybe saltbox or leanto style roof with overhangs.

[montcalm]

All interesting points. I'll see if I can clear up some things.

12:12 was chosen mainly as an assumption at this point. I know snow will slide if the surface is slippery enough. I'm not sure how that might work with panels, but I'd guess tightly packed would be a better bet than edges to get hung up on. My hope was heavy snow would slide off and light snow would need to be brushed. It's also the minimum I could tolerate in terms of added height and wasted space. I'm still not convinced I wouldn't want to vault over the "great room" i.e. dinging and living area and add a small loft. But the hassles of insulating this way are not my favorite. A truss roof would be by far the easiest to build and insulate and still keep an open floor plan.

I did look at the panel optimization.

For 1200 sq. ft. one can fit 45 panels with a 25x48 footprint and 12:12 pitch. It's few inches over actually, but with overhangs it can fit.

This is with panels arranged in 5 rows of 9 long, long side along the 48' dimension.


I think thermal gain from electric motors will be minimal - lights, non-issue these days. Solar gain will be real, but I'm also not convinced one needs to insulate as much as we had. My guess is that current regulations are sufficient except perhaps walls - maybe up to R30 there would be better. I tend to wonder how much gain there would be going standard R13+10* wall investing extra money into higher efficiency windows - going from a standard U=0.3 to U=0.2. That would be an interesting analysis to look at.

*2020 Regulation is R13+10 or R20+5 vs R13+5 for 2009 in zone 6.

[montcalm]

Also I should add - this whole "net zero" thing is a game of averages. It's the game I'm playing now. But that's not to say you might have a year you're in the hole a few hundred kWh, and another you're surplus. I think the idea is really just to generate as much as you can IF the cost of panels outweighs the cost of electricity. Since our state wants to boost our solar output, this is the way it is now. And they want to increase storage as well - that'll be next. They have incentives for Long Island, but not upstate yet. Once there's enough solar generation, then I'm sure the focus will be batteries.

This is also why I've abandoned the idea of "off-grid". It just doesn't make sense. I mean mooregm outlined this before, and it's not that I didn't believe him, I just really had to weigh all the options. The issue is you have to meet your demands in the worst times, so that means you wind up having HUGE surplus every other time, and nowhere to put it.

Another side note, I saw that Panasonic now has a 410W panel (22% efficiency) - so things like this will be significant for small footprints. I already want this for my current home as it would boost my potential output up an entire kW.

[montcalm]

And John - I was thinking about one other thing...

My current home has 7(!) north facing windows. And 2 west facing. I feel like just reducing that could be a significant gain with no other changes to insulation. Recall my effective resistance (simply dT/Q) is on the order of R9. I calculated your design to be effectively R17. That gave a heat energy consumption reduction of 43% for the year.

I wonder how much of that is simply reducing windows?

[montcalm]

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