Opinions on this Minisplit Layout
Should I go with two or three mini-splits in total for a 2-story, Climate Zone 5A house with a heat load of 37,917 btuh?
Downstairs heat load: 24,077 btuh
Upstairs heat load: 13,840 btuh
My hope is that I can heat the entire downstairs with one 18K btuh Mitsubishi hyper-heat unit, and heat the entire upstairs with one 12K btuh Mitsubishi hyper-heat unit. Or would it be a safer bet to have two units downstairs, say two 9K units, or one 15K and one 6K? By “safer” I’m thinking both of air flow, but also of whether I should oversize the actual heat load by say 1.2 or 1.4 to be more in line with the ASHRAE recommendations. The downstairs is sort of U-shaped where the left side of the U is the living room, the right side is the kitchen and dining room, the bottom is an open-air passageway between the two spaces, and the two points at the top of the U are two entrances to the staircase which face each other (one in the living room, one in the kitchen). Total conditioned downstairs space is ~1,000 sq ft, total conditioned upstairs space is ~ 500 sq ft.
Any help would be appreciated.
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I'm guessing you will get better responses if you include a floor plan.
Good point. Here is a drawing of my first floor layout. I messed up a few things in the drawing because I can't draw: 1.) the opening between the living room and dining room is much bigger than in the drawing, and 2.) the entrance to the staircase from the living room is flush with the front door of the closet (ie: it's not actually tucked behind the closet like it looks in the drawing).
I'd stick with just ACCA Manuals JSTD, which don't recommend over-sizing (but do allow some).
While general advice is that doors and hallways don't provide sufficient heat movement and larger/shorter openings do, I'm not aware of any good way to predict how effectively heating and cooling will self distribute.
Of course properly done ducting distributes well, with shorter runs being easier and better balanced.
Also note that self distribution between floors can sometimes make some sources ineffective. For example, an upstairs unit may not supply much load in Winter - because warm air is rising from the downstairs sources.
Deleted
Let's try it again.
My plan right now:
one 9,000 btuh wall unit in the SW corner of the living room (MSZ-FS09NA)
one 9,000 btuh wall unit in the NW corner of the dining room (MSZ-FS09NA)
one 12,000 btuh floor unit mounted on the ceiling over the staircase, aimed right down the hallway on the second floor (MFZ-KJ12NA)***
All of these would be 1:1 systems so 3 outdoor units in total.
Total downstairs output at 11F design temp: 25,760 btuh (heat load at 11F is 24,062 btuh)
Total upstairs output at 11F design temp: 13,900 btuh (heat load at 11F is 13,855 btuh)*
Total household output at 11F design temp: 39,750 btuh (heat load at 11F is 37,917 btuh)**
*This unit just barely covers the 99% heat load upstairs. Is this cutting it too close? Should I size up to a 15K btuh?
**Whole house system is only oversized by 4.36%. Same question: is this cutting it too close? Should I maybe size up one of the downstairs 9K btuh unit to a 12K unit?
*** The reason for the 12k floor unit mounted on the ceiling over the staircase: the only exterior walls in the upstairs hallway are either directly facing one of the bedrooms or directly below the attic fan. My concern with a unit facing one of the bedrooms is that all the heat would be aimed at that room but the other rooms would suffer. My concern with a unit under the attic fan is that the heat would escape into the attic (even though our attic fan vents have a foamboard cap over them in the attic). And the reason for a floor unit as opposed to a wall unit is because the ceiling over the staircase is only 36 inches wide while the wall units are 36 and 7/16 inches wide and the floor units are only 29 and 17/32 inches wide. Right above this ceiling is a walk-up staircase internal to the attic, so my thinking was that the line set could run along the bottom landing of this staircase to the west wall of the attic which is only 3 feet or so away and from there it would travel down the west wall of the house to the exterior condenser unit right below it at ground level. (All three of the exterior units would be next to each other on the west side of the house, the two 9K exterior units connected to the interior 9k first floor wall heads with 15-20 ft each of line set running along the unfinished basement ceiling).
> *This unit just barely covers the 99% heat load upstairs. Is this cutting it too close?
Warm air rises which shifts supply around - so consider increasing heating capacity downstairs.
Also note that occasional use of some resistance heat is fine.
Good idea. So I could instead heat the downstairs with one 9K wall unit and one 12K wall unit, and heat the upstairs with one 12K floor unit. At 11F the capacity would be:
Downstairs: 28,930 btuh (for a downstairs heat load of 24,062 btuh)
Upstairs: 13,900 btuh (for an upstairs heat load of 13,855 btuh)
Total: 42,830 btuh (for whole-house heat load of 37,917 btuh)
This would make the whole system oversized by 13%.
Alternatively, I could do one 9K wall unit and one 12K wall unit downstairs but decrease the upstairs floor unit to 9K. This would make the upstairs unit undersized but it would reduce the oversizing of the whole house system. At 11F that would be:
Downstairs: 28,930 btuh (for a downstairs heat load of 24,062 btuh)
Upstairs: 12,200 btuh (for an upstairs heat load of 13,855 btuh)
Total: 41,130 btuh (for a whole-house heat load of 37,917 btuh)
This would reduce the oversizing of the whole house system to 8.5%, but the upstairs unit would technically be undersized for the heat load up there. Maybe not a problem though as heat will rise from the first floor.
Which one of these would be most ideal?
If you have an unfinished basement, can you run some short runs of duct there? I think you would be much happier with a ducted mini split at least on the first floor. The cost would probably be similar to 2 ductless units.
Hi Kyle,
I probably could, yes. I hadn't really considered a ducted solution just because I thought they added too much cost, complexity, and inefficiency. But I'm open to it. Why do you think I'd be happier with a ducted system downstairs?
Ducted mini splits are still very efficient, but not as efficient as a ductless mini split 1:1. However, when you start talking about a ducted mini split vs 2 ductless heads then a ducted unit could very well be more efficient depending on the models.
Given your floor plan I think you would achieve a more even temperature distribution with a properly designed ducted system. With an unfinished basement you certainly have the room and could provide conditioned air directly to each room.
Cost is always a major consideration. I would look for Mitsubishi, Fujitsu, and Carrier installers in your area and get bids both with ductless and ducted units and compare.
If your place has a real 37k heat load for 1500sqft of conditioned space, it will never be comfortable with point source heat source. Leaky structures like this need heat around the outside perimeter walls and bellow windows, the only way for that is with ducts.
Since the cost of a ducted mini split is about the same as a wall mount, eliminating one wall mount on the main floor can pay for a lot of ducting. You'll not only end up with something that is more comfortable but also less maintenance. Wall mounts need coil and blower wheel cleaning every 4 to 6 years, not easy and a messy operation.
A single head upstairs can do most of the heating when the doors are open, but I would still keep some baseboard heaters in the bedrooms for night time. The extra cost of running these heaters is not enough to justify more complicated hvac setup.
If your load is 37k, you have a lot of low hanging fruit in terms of energy savings. Spend some of your mini split budget on blower directed air sealing, taking care of some of the uglies in these old houses such as leaky rim joists and single pane windows in the basement and knee walls upstairs. This will not only allow you to get smaller equipment, will be way cheaper to run and will make your place way more comfortable.
OK, you guys are giving me something to think about here. I hadn't really been considering ducted, but you raise good points....
For their ducted air handlers, does Mitsubishi publish the max output at 17F and 5F, the way they do for the ductless units? All I can see on the spec sheets for the Mitsubishi ducted air handlers is the "rated" capacity at 47F. For example: https://www.mitsubishicomfort.com/products/indoor-units/multi-position/compare/results
In terms of the 37.9K btuh heat load in our house, I would be pleasantly surprised to find out this is too high, but the problem is there are some assumptions and approximations that went into the Cool Calc Manual J that I did.
For example, two assumptions: one, the tech who did our most recent blower door test guessed that the walls are insulated to R-5 based on some infrared scanning of the first floor walls, but this hasn't been confirmed by actually opening up any walls; two, I'm assuming R-7 insulation in the ceiling over the first floor addition (bedroom and bathroom), but I actually have no idea what's up there.
And two approximations: one, we insulated the rim joists to R-15 recently, but there's no way to report this in Cool Calc, so the best I could do was to report "floor insulation of R-5 or R-6;" two, the crawl space walls are also insulated to R-15, but the only choices in Cool Calc were R-11 or R-19, so I chose R-11.
In terms of blower door tests and air sealing, we've done several of these as part of free home energy audits and insulation jobs, but some of the work has been kind of sloppy. Our most recent blower door test came back at 3618 CFM (or 201 ELA if I have that right). But we still have 8 failing, super drafty single pane windows on the first floor (I'm replacing these with Energy Star <.03 U-rating vinyl replacement windows in the next few weeks), we still don't have a glass covering on the fireplace, and since you mentioned single pane windows in the basement, Akos, we actually have 5 ancient, rotting single pane windows in our basement, so I know I need to get those replaced too.
One other question about ducted vs ductless while I'm at it: Akos, you recommended putting one ductless unit upstairs along with baseboards in the bedrooms. I agree, but why not also do that on the first floor, rather than ducted? From a total cost perspective, what's likely to be less expensive for the first floor, ducted, or ductless plus baseboards? Ideally, the heat pump(s) would do 95-99% of the heating and only use the baseboards as backup for extremely cold temps at our below design temps. My best estimate of total annual kwh usage for whole house heating using Mitsubishi heat pumps is around 5,500 kwh. Electricity in my area is $.22/kwh, but we have about 5,000 kwh of annual solar production currently not spoken for, so the solar should cover the vast majority of our heating, unless we had to rely on our baseboards for more than we thought we would....
A ducted mini-split sized correctly for the loads will be more efficient than multiple single zone heads OVERSIZED for the room loads.
There are very few <200 square foot rooms that would be right-sized for even a half-ton Mitsubishi head.
>"In terms of the 37.9K btuh heat load in our house, I would be pleasantly surprised to find out this is too high, but the problem is there are some assumptions and approximations that went into the Cool Calc Manual J that I did."
Those numbers are significantly inflated- not by 2x, but WAY over reality.
A reasonably tight 1.5 story 1500' house over a 1000' uninsulated basement (not included) would have a heat load of <30,000 BTU/hr @ 0F, assuming 2x4/R13 and storm windows over single panes. If you have a winter's worth of heating bills for the space, run a fuel use based block load calculation on WINTER fuel bills only (December-January is best):
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
FWIW: I live in a 2x4 framed 1.5 story 2400' house over a 1600' insulated (but not directly heated) basement with maybe 100' of insulated crawlspace, with mostly clear glass storms over antique single panes, and my fuel use numbers come in ~40,000 BTU/hr @ 0F. (My local design temp is +5F.) That's 900' more conditioned space than you're considering, and my house has an extremely inefficient footprint (14 corners!)
Both CoolCalc and LoadCalc will oversize even with aggressive inputs. The dumbed down load calculator built into the (also free) BetterBuiltNW HVAC tool has more appropriate defaults, and was designed SPECIFICALLY for sizing ductless (and ducted) heat pumps appropriately:
http://hvac.betterbuiltnw.com/
There are a few places where the default settings can be overridden, but in the small amount of time I've spent playing with it the numbers have been pretty good. The target audience for that tool was HVAC installers that weren't up to speed on how to get the best performance out of heat pumps. It has a duct design tool too but the heat load tool is dead-easy even for newbies, and a bit harder to screw up with than the other online Manual-J-ish tools.
The room by room load matters, not every 150' room will have the same load, so even with the CoolCalc/LoadCalc tools note the room numbers, and scale them appropriately against the fuel use block load numbers.
You want to start your search here:
https://ashp.neep.org/#!/product_list/
Select the brand you are looking for, you want either single zone centrally ducted (these tend to look pretty close to a standard furnace) or a single zone compact ducted which are the slim cassette style.
Probably something like this is in the ballpark once you seal up your place:
https://ashp.neep.org/#!/product/34564
The reason you can get away with a single hallway unit upstairs plus baseboard heaters is that stack effect is working in your favor. It is moving warm air from the hallway through all the leaks in the rooms out to your attic, not great for efficiency but good for comfort in this case.
On the main floor you have stack effect working against you, the pressure difference is sucking in cold air in through the leaks in the basement and the main floor walls and windows. There is now way for the heat from the single wall mount to make it to the rest of the rooms, the baseboard heaters will be supplying near all the heat.
Assuming your place has 8' ceilings, the 3600CFM @50PA number works out to around 20 ACH, which is super leaky. Never mind energy use, just for comfort, you want to get that number down to around 5.
Replacing the single panes is definitely a good start. Sealing up your knee walls is probably in order as well.
When doing the basement windows make sure they are taking out all the wood down to the bare stone/concrete, not just doing retrofit installs. You want these windows to be sealed solid against the foundation. In my own home replacing just the basement windows and fixing the leaks around the doors took the basement from low 50s to 65F, huge difference on comfort on the main floor above.
Sorry, I'm a little confused. How many of those slim cassettes would I need? One for the entire downstairs? One per room? And where would they go, along the perimeter walls of the first floor? If the best way to heat a leaky first floor is to have heat along the perimeter walls, that sounds like a lot of ductwork running through the basement. Also, just comparing the efficiency numbers of the PEAD-A15AA7 to say the 6,000 Mitsu Hi2 btuh wall unit (MSZ-FS06NA-U1), the ducted unit seems significantly less efficient:
PEAD-A15AA7
SEER: 18.3
EER: 12.6
HSPF Region IV: 9.9
MSZ-FS06NA-U1
SEER: 33.1
EER: 19
HSPF Region IV: 13.5
Plus, and this may just be a marketing strategy, but I thought the whole point of ductless minisplits is that duct work has like 30% losses and so ductless units avoid those losses? Are these duct losses already factored into the efficiency numbers for the ducted unit?
I will check out the BetterBuilt NW Manual J tool. Sounds interesting.
In terms of making our house tighter, I'm sure I could improve it somewhat, but I'm not sure what else I can do to dramatically tighten the house. We already have R-49 on the attic floor, R-15 along the rim joists, and R-15 on the crawl space walls; plus, that 37.9K btuh Manual J that I did already assumes that all leaky, single-pane windows have been replaced by Energy Star, double-pane, 0.3 U-factor or below windows. I've been warned that trying to add insulation to the walls would be way too expensive (how much, I'm not sure). And we have a fieldstone basement foundation, so aside from like a major sprayfoam carpet-bombing of all the basement wall surface area (something I've been trying to avoid), I'm not sure how to effectively insulate the basement walls without creating new moisture problems. Given all of this, do you really think I can get our first floor design temp (11F) heat load down from 24,000 btuh to 18,000 btuh? Should I still insulate the attic knee wall even though the attic floor is insulated?
Good tip on sealing the new basement windows to the foundation. As soon as we get our 8 new first floor windows in next week, I'm going to look into having the 5 basement windows replaced ASAP....
You install a single slim ducted unit for the main floor. Generally best to place it near a main hallway where you can do a very short return duct (90 from the unit up into an interior wall above, large filter grill on the wall) and easy reach the rest of the rooms with simple runs of flex duct. Looking at your floor plan, if the basement staircase is not in the way, the best spot is probably under the 6x4 entrance closet.
The unit I linked is a pretty strong blower as long as you keep the ducting reasonably straight you won't need to oversize. Roughly, you are looking at two 5" feed to the bedroom, 5" to bathroom, two 6" to the living room, two 5" to the kitchen, and a 6" to the dining. Simplest is to home run most of these ducts to the unit.
Since it is a retrofit, easiest is floor registers.
The reason for the efficiency difference between the wall mount and the ducted unit is the power consumption of the blower. You can't move air through ducts without using a bit of extra power. Overall, this efficiency difference is a small price to pay for comfort.
Adding insulation is not the same as air sealing. Stuffing walls or floors with fluffy batts does not stop any of the air from moving through. In your case, you are loosing more heat through air leaks than you do due to lack of insulation in the walls. I wouldn't worry too much about insulating anything (basement included) until you get your leak rate down to around 5ACH.
A good read on how to seal your knee walls:
https://www.greenbuildingadvisor.com/article/insulating-behind-kneewalls
P.S. If your rim joist was insulated only by stuffing batts into that area, you should remove them and air seal properly. This generally means either spraying the area with one of the smaller two part SPF kits or rigid foam cut to fit and sealed in place with canned foam.
Thanks for the continually high quality advice -- I did not know how important air sealing was compared to insulation, so I've been reading up on that the last few days.
It sounds like the first thing I need to do is replace all the leaky single pane windows (first floor and basement), and then hire a professional company to do some quality air sealing. I spoke to one such company that only uses AeroBarrier (and only in empty houses, so they won't work), but they advised that replacing my leaky windows alone might be able to get the ACH down to around 10. I also discovered yesterday that the damper in our fireplace that we've never once used doesn't close all the way and has a probably 1/4" gap. So if we seal this, and replace the windows, we may be on our way to a sub-10 ACH before doing any other air sealing?
"If your rim joist was insulated only by stuffing batts into that area..." Yes. That's exactly what we had done. Mineral wool batts in the rim joists. No air sealing. And in the attic too, they did blown in cellulose under the floorboards plus fiberglass batts on top of the floor, and they claimed to have done some air sealing, but they were done pretty quickly and I have no idea how good of a job they did. It's frustrating because we've had two different companies each add insulation, and each company has had to come back more than once to correct some previous mistake (they left gaps in the insulation; they messed up a blower door reading, etc.) but in all these visits no one seems to have done any thorough air sealing.
A few other questions, if you don't mind:
1. If I get the ACH down to 5, should I still do a ducted solution for the first floor, or would the house be sufficiently tight then to just do 2 ductless units like I had planned?
2. If the answer is yes, that I should still go the ducted route: I still don't totally understand the physical layout of the interior equipment for this solution. In addition to that slim cassette, is there also an air handler? Or is that tiny cassette itself the air handler? I would have thought that with ducts, there would have to be a furnace- sized or at least a junior-furnace-sized box that goes somewhere down in the basement? Because I have a hard time picturing how that tiny cassette could generate 18,000 btus, or how it would have enough surface area to even connect to all that duct work. But maybe the picture isn't to scale....
3. Whatever the heart of this system is (cassette, air handler, etc.), I know you recommended putting it under the closet, but I was actually thinking that a more central location would mean less overall ductwork - like under the dining room, or under the entry-way between the dining room and the living room. It seems like the ducts to the first floor rooms would be much shorter from that position, unless all the registers need to be along the perimeter walls, in which case the ductwork would be long regardless of where it starts from. What would you think of this?
4. I'm not sure it will even be possible to run ducts to the bedroom and bathroom. The reason is that these two rooms sit over a crawl space while the living room, dining room, and kitchen sit over the main fieldstone foundation basement. The wall that separates the crawl space from the basement runs parallel to the N-S wall that separates the dining room and living room from the bathroom and bedroom. The only access to this crawl space is a 2'x2' or maybe 3'x3' open-air "window" between the basement and the crawl space, right below the door to the bedroom. This "window" already has some flexible plastic plumbing running through it from the water heater to the bathroom. So the problem would be that if I also ran ductwork through there, it would become pretty hard for people to then get into the crawl space if we ever needed any work done in there (plumbing, structural, insulation, etc.). The only other way would be to make another hole in the fieldstone foundation, which probably isn't a good idea? So then it would seem that the bathroom and bedroom would have to be heated by ductless + baseboards, or perhaps just baseboards and then we leave the door open 24/7 (easy enough for us to do - right now we're mostly using it as a playroom for our kid) and let the heat from the rest of the first floor seep in there and hope that the baseboards wouldn't have to do too much of the heat load in those rooms?
1. The one or two wall mounts work well in a well sealed and insulated open concept space. You'll have a hard time getting much comfort in a chopped space with questionable insulation no matter how much you air seal. I would go ducted if you can get it installed for a reasonable price.
2. The slim cassette is the air handler. It doesn't take a very big unit to move some pretty decent BTU. See attached an 18k mid static unit. The 8" trunk feeds an open living/kitchen space bellow, the 5" ducts are for two bedrooms plus a den and the 4" duct is for a bathroom.
All the ducts connect to a manifold on on the front of the unit which is simply a length of 8x12 square duct with the ends capped. You can make this larger if you need more takeoffs.
3. Location is really determined by the layout of the house. Best layout is one that allows for a simple return up to the floor above and easy runs to all the rooms. Longer runs are less of an issue than convoluted bends.
For a unit doing heating, you generally want registers on the perimeter bellow or near windows in older houses. This is the simplest setup that just works. Rest require designing registers and getting good throw and mixing in each room.
4. That crawlspace should really be part of your conditioned envelope. Unless you have major radon issues, getting the crawlspace perimeter sealed up and stem walls insulated as well as getting a vapor barrier on the ground will make a big difference in energy use, indoor air quality and comfort in those space.
One the crawlspace is part of your conditioned space, as long as not load bearing, punching holes through the rubble foundation for extra ductwork should be not a problem.
You don't need to do this and if costly to install, heating the bedroom and bath with baseboards might still be a cheaper option.
P.S. Unless your single panes windows are missing glass, I doubt it will get you down to 10ACH. Replacing the single panes will make a world of difference in comfort and will reduce your air leakage a bit but not much.
Your place has some major holes somewhere, you need to find those and seal them up. Fireplace is a good start, knee walls are also a good spot.
If your place is balloon framing, most likely you are missing blocking at each floor, at the basement and in the attic. These are like a huge open chimney to the outdoors, you need to seal them up properly.
You want to hire a company that specializes in air sealing. This is not hard but fussy work and needs somebody that knows what they are doing.
OK, thanks. I will start getting estimates from air sealing companies this week and one thing I'll ask them about is if they can add blocking at each floor.
In terms of the crawl space being part of the conditioned space: the passage between the crawl space and the unfinished basement is open. It's like 3'x3' and 3' off the ground. We did a lot of crawl space work last summer, right when we moved in. Asbestos abatement (there was crumbling asbestos pipe insulation - some of it still on the pipes, some on the dirt floor); mold remediation (the basement and crawl space had a major mold issue); additional pressure treated wooden support posts and additional concrete footings (support posts were in direct contact with the soil); and a vapor barrier on the ground. The dry basement company which did the vapor barrier did a good job. The most recent work we've had done in there is insulation: company number two added Owens Corning Foamular 250 ("Greenguard Gold" XPS foam board). Of course, they didn't do any air sealing before putting up the foam board. And of course, they left the foam board exposed, despite the warning on the foam board itself which says "This product is combustible. A protective barrier or thermal barrier is required as specified in the appropriate building code."
Anyway, it seems like the order of operations here is:
1. Replace all single pane windows (first floor and basement)
2. Air sealing until we get the ACH down to around 5
3. Solicit quotes for an 18K BTUh ducted system on the first floor and either a 6K or 9K BTUh ductless spit on the second floor
Interestingly, when I change the ELA score in Cool Calc from 201 (what it is right now) to 50 (equal to the goal of an ACH score of 5), the heating load drops to around 28,000 BTUh.
"Anyway, it seems like the order of operations here is:
1. Replace all single pane windows (first floor and basement)
2. Air sealing until we get the ACH down to around 5
3. Solicit quotes for an 18K BTUh ducted system on the first floor and either a 6K or 9K BTUh ductless spit on the second floor"
That's the wrong priority order.
1: Highest priority is air sealing- hand's down THE MOST important thing for being able to control heat and moisture flows, indoor air quality & comfort levels.
2: Window REPLACEMENT is usually the lowest priority. If the existing single panes are in pretty good shape and can be tightened up, a tight Low-E storm window would perform about as well as a code-min replacement window (at a fraction of the installed cost), and improve both the ACH/50 numbers. For double hungs the northeastern US regional player Harvey's Tru-Channel products are the tightest storm window in the industry, and have a hard coat low-E glazing option, but even the Larsen low-E storms sold through box stores are pretty good. Unlike full replacement windows, low-E storms will typically "pay back" in energy savings in under 10 years, under 5 in high energy cost locations. Unless the existing windows are really junk or you're going for higher performance than code minimum, DO consider this option.
3: HVAC replacement quotes can happen at any point in the process, but need to be based on the "after upgrades" picture of the house.
>"The most recent work we've had done in there is insulation: company number two added Owens Corning Foamular 250 ("Greenguard Gold" XPS foam board). Of course, they didn't do any air sealing before putting up the foam board."
The foam can be detailed as part of the air barrier.
How thick is the foam?
Thanks, Dana. Yes, I'm definitely gathering that air sealing is the highest priority. The only reason I thought it might make sense to replace the windows before air sealing would be to avoid two hypothetical situations:
1. air sealing company doesn't improve the ACH by much but insists that this is because the windows need to be replaced
2. window company ruins some of the air sealing work that's already been done around the windows by the air sealing company and then we have to call the air sealing company out a second time
But I could also see neither of these scenarios coming to pass. In any case, we had the eight first-floor windows replaced today (this had been scheduled for months). The basement windows really are pretty bad. I wish we could just go the low-E storm replacement route you suggested, but a lot of the wood in the wood frames looks soft and some of it has some ancient termite damage that the home inspector found last year. So I'm going to try to get some quotes on putting in some really tight vinyl replacements which can be secured as close to the fieldstone foundation as possible. The dry basement company that did the vapor barrier in the crawl space did a really tight, clean job with it, and they were trying to sell me on vinyl replacement basement windows last summer (before I really appreciated the amount of air leakage we have in the basement), so I'm hoping they would be similarly competent with window replacement. I'm also going to try and find a good air sealing company in my area. I haven't had great luck with the insulation companies, but this may be because the state pays the insulation companies rather than the homeowners paying, and I'll bet most homeowners don't complain about mediocre insulation work since that work is free to them.
Also, our basement still has way too much humidity in the summer. The $300 GE dehumidifier down there (left by the previous owner) was using 10 kWh a day last summer if I left it on 24/7. Putting down the vapor barrier helped (and we also repaired and repainted the rusty Bilco door, and replaced the crumbling, wooden, Pirates of the Caribbean basement door with a new fiberglass one). And we also put in a new Rheem HPWH in January, so I'm hoping that this will do most if not all of the dehumidifying this summer and that I can leave the GE dehumidifier off. But maybe the HPWH can only do all of the dehumidifying if we also air seal and replace those windows.
Oh, and the foam board is 2", according to the invoice from the insulation company.
Kieran973--it's been a while, any updates? I am reading through this and other posts trying to get a sense of how to address my own situation.