Looking at
Weatherspark.com data, Ottsville's low temps track those of Allentown's so using
Allentown's 99% outside design temp of +10F would be the right number in any load calc. (Yes, it get's down to 0F sometimes, but that's less than 1% of the time, and the thermal mass of the buildings carry you through those relatively brief dips if if the heat load calc and boiler had ZERO margin.)
The greenhouse may have the largest heat load at +10F, since it has the highest U-factor.
A 3" closed cell 2lb density polyurethane foam is about R18. At 3" of open cell half-pound density polyurethane foam is about R11. The air-tightness is pretty good, but these are below current code-min R values for residential buildings in PA.
Rigid board insulation is rarely made of polyurethane. Polyisocyanurate (iso) is an off-white slightly yellowish look and always has either a fiber/papery or bright foil facer. Extruded polystyrene (XPS) is uniform in color throughout with no facers, and it's usually pink,blue, or green from the most common vendors. Expanded polystrene bead (EPS) is usually white, with obvious bead/grainular structure boundaries (it's the same stuff as cheap coolers and coffee cups, but higher density), and sometimes comes with plastic or foil facers. Iso is ~R6/inch of thickness, XPS R5/inch, and EPS is ~R4/inch. At 3" only iso would meet current code min for wall R-values.
Thermopane varies in U-factor from ~U0.22 (low-E coatings on at least 3 of the window surfaces) which is about R4, to about U0.6 (clear glass, no coatings) which is less than R2, so it really matters what you have. If it's pre 1985 assume it's no better than U0.6.
U-factor is BTU per square foot per degree-F difference. The U-factor of an insulated wood framed building depends on both the insulation and the thermal bridging of the framing. A studwall with 3" of R11-R18 of foam between the studs is between R9-R11 after the thermal bridging of the wood is factored in, so for rough purposes call it R10. The U-factor of the wall is 1/R, or 0.1 BTU per square foot per degree-F difference. If it's rigid foam on the EXTERIOR of the sheathing (or roof deck) that is continuous, with no bridging rafters are studs, the R-value and U-factor of the assembly is the same as that of the foam. So 3" of continuous iso (R18) would have a U-factor of 1/18= 0.056 BTU per square foot per degree-F.
So, start measuring the square feet of each assembly & construction type, and add it all up. Subtract out the window areas from your wall dimensions, and add up the window areas separately, using a U-factor of 0.6.
eg: Say your green house has 200 square feet of U0.1 wall, and 400 square feet of U0.6 window, no roof. You're keeping it at 45F, the design temp is 10F, so the delta-T is (45F -10F =) 35F.
The heat loss from the walls is then:
200ft x U0.1 x 35F= 700 BTU/hr
The heat loss from the glazing is:
400ft x U0.6 x 35F= 8400 BTU/hr
The total heat load for the green house is then something like (700 + 8400 =) 9100 BTU/hr.
For solid wood exterior doors, use U0.5 for the door U-factor. For panelized, use U0.75.
For the section of roof that has the fiberglass insulation between rafters, use U= 1.3/(fiberglass thickness x 3.2)
eg: If it's 5.5" thick batts the U-factor is about 1.3/(5.5 x 3.2)= 0.074.
Run the numbers on the whole place, using your standard thermostat settings (not the "guests only" values) for the delta-Ts. Add it all up, and you'll have a pretty good idea what the lower-bound is. The true heat load will be higher than that due to air leakage and possible gaps in the insulation, but it won't be 2x, it won't even be 1.5x. If the place is pretty air tight and draft free you can just run with it, but if the buildings have known drafts, add maybe 15-25% and you should be good. Don't upsize from your calculated number by more than 50% or you risk running at below the rated AFUE on the boiler.
If you hire somebody to do the calc using a standard heat load software package, be sure to look over their shoulder to get the right indoor and outdoor design temps. If they enter 72F indoor temp and 2F outdoor temp and you're actually running 45F/10F, their calculation would be twice your actual load. They need to understand very clearly the temperatures you intend to operate it.
Most of the time your temps are well above the outside design temp, and raising the guest quarters to 65F or higher won't be a problem. Design the system to run efficiently for the average load and still meets your min-indoor temp numbers at the 99% outside design temp. Don't buy an oversized boiler just to meet the <1% condition that have house guests on the very coldest day of the year. Odds are it would keep up anyway even at 15% upsizing from your spreadsheet number, and if it doesn't, a $50 electric radiator type 1500W space heater would usually make up the difference. The fact that much of the space is maintained at 45-50F takes a HUGE chunk out of what would be a typical residential heat load.
BTW: Many 1.5 ton mini-split air source heat pumps can put out about 22,000BTU/hr at +10F with a 70F room temp (and much more at lower room temps.) The average efficiency is quite good- your average coefficient of performance would be over 3.0 if you're only keeping the place 50F, or even 60F. And the cost of the power used would be less than half what it would take at recent propane & electricity pricing. The installed cost of a high-efficiency 1.5 ton mini-split is ~$4-4.5K, but it's also possible to do a DIY install for less than $3K. A 2-ton mini-split is only about $500 more, and could put out 30,000BTU/hr @ +10F. These things modulate output with load and run most efficiently in a "set and forget" rather than a setback strategy. It may be the "right" way to go for some of your spaces, since the reduced operating costs might pay for the thing in short years compared to propane pricing. Think about it, after running the room by room, or building-by-building heat load numbers. Since some mini-splits don't have settings below 60F, run a separate sheet using 60F as the indoor design temp. Greenhouse growers use these things a lot in lieu of propane burners, since it's often just 1/3 the operating cost.