Ceiling insulation - point of diminishing returns

I have what used to be a 12' x 16' carport that I've double-glazed on three sides. The fourth side is against the house. I'm right in the center of USDA Zone 6, and I intend to use the carport to overwinter potted citrus trees, hoping to keeping the room at least 40 degrees Fahrenheit with passive solar and electric heat backup. This will be a step up from the basement, where the plants have overwintered the past two years. In the longer term, I hope to build a walipini to take over this function.

The ceiling is currently solid plank roof sheathing on exposed 2x4 rafters, and I want to insulate it. Is there a rule of thumb I can apply to determine what R-value I should insulate the ceiling to? I've seen the charts showing the diminishing returns of added inches of insulation, but I have a feeling that heat will be pushed out the R-2 glazing of the walls long before reaching the spot on the charts that assume conventional R-19 walls and R-30 ceilings (but I certainly could be wrong!). The rafters are puny, so another factor is weight, so foam board is probably what I'll use, so cost becomes an issue at conventional ceiling R-values.

I'm not necessarily looking for citations to academic studies. I'd just like to hear the opinions of those who have experience considering these kinds of insulation tradeoffs in design situations that are a bit off the charts. Thank you.

When I added more insulation to my house, I used unfaced rolls of R-25 insulation. This choice was not made based on R-25 being the ideal, but because rolls of R-25 were by far the least expensive at the time in my location.

If you googled heat loss calculator

there's a ton of free resources to answer that question.


this one for example:

http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm


Then you can run 3 or 4 scenarios and get a pretty good idea of the optimum insulation level.


I suspect, like you, that the glass will run the show, no matter what you do to the ceiling.

Adding another layer of glazing (greenhouse plastic perhaps) would have more effect than raising the ceiling from r-7 to r-30, I suspect.



hth,

troy

Troy Rhodes, the builditsolar calculator link you provided is exactly what I needed. Thank you! I just used it to run a quick analysis and I think a single layer of 1/2" polyiso foam board is the right way to go for now. Also, your idea regarding an additional layer of glazing is well-taken. I do intend to do rudimentary movable insulation, specifically blankets over the windows at night. It might make sense to run some plastic up as well.

John Wolfram, thank you for another idea that I hadn't considered. I'll check the economics of the other options before I buy the polyiso.

If that foam insulation were used to create good quality, interior window shutters, it would make a huge difference on heat loss.

 Blown cellulose is the only product I would put in an attic like that. It's superior to fiberglass in many ways.  Gives a better air seal, higher R value and it's cheap. Vermin love to nest in fiberglass.  They don't like blown cellulose, probably because of the borax.

Another handy technique is to insulate the plants themselves.

If you're going to get a really cold period of a few days or weeks, and it's uneconomical or impossible to keep the room above the bare minimum, you can huddle all the plants together and put them under a light sheet of polyethylene and put a couple incandescent light bulbs or a tiny electric heater under the plastic.

Post-project follow up. The existing roof rafters couldn't bear any additional load, so I added a set of ceiling rafters below and structurally separate from the roof. That gave me space to use cheap pink fiberglass batts as well as 5/8" drywall. So far I'm pleased with how it turned out. I'll probably be adding foamboard exterior shutters later this winter to replace the blankets that get hung up over the windows every cold night.

I am in zone 6 and have R38 in my ceilings.   I noticed a significant difference when I doubled the insulation I had. I am curious as to the R value others in zone 6 have in their attic.

John F. Dean,.  I'm in Zone 6 Near Tahoe.  My Smart  thermostat is set to 68 degrees.  My condo ceiling was R19.  I added a layer of R30 fiberglass roll on Jan 1, 2021.  Same degree days that year .. 20% less LNG usage over 2020.  On Jan 1, 2022 I added another layer of R30 (total now up to R78).  Results 30% less LNG useage than year 2021- again, no significant difference in degree days.  And Smart thermostat confirmed same figures, in terms of "Furness in use" time    I don't think adding a third layer will make as much a difference.
Also, I used the swamp cooler much less than last year. This year's project will be insulating the heat supply ducts in the crawl space.  Cheers. ( I only used fiberglass rolls due to having a 60% off clearance price  on the product.  If I were to do it again, I would blow in.....)