Energy Efficiency Upgrades to an Old House
-house from 1910’s (tudor more or less, 4000 sq ft)
-northern humid climate (southern CT)
-hot water heat (old school gas boiler)
-no insulation to speak of
-partial attic above the third floor (meaning that third floor roof is directly to the outside outside in some places)
-lots of knee wall attics
-Humid basement in summer
-original slate roof with heavy board backing
I think all of the above is fairly typical
The first thing we did energy-wise was install minisplit system for the southern half of the house (all three levels) so we can survive the summer while figuring out what else to do. It makes the whole house livable, with the help of a couple window units.
For our next step we brought in an energy consultant who came up with a comprehensive plan:
(1) insulate and seal the attic with (a) R49 loose fill cellulose for attic floor, (b) R30 dense pack cellulose for interior sloped ceilings, and (c) R49 open cell foam for slopes of knee attics.
(2) Whole house air sealing (including basement rim joists) with foam, caulking etc.
(3) Big dehumidifier in the basement.
(4) After all of that is done, install minisplit for rest of the house to take over both cooling and heating.
Estimate for payback: 15 years (not accounting for interest rate and energy fluctuations and this is just the insulation, not accounting for the additional minisplits).
I have so many questions I don’t know where to begin, so I will toss these for your consideration:
(i) Should I worry about all of the added weight from the insulation?
(ii) Is it ok to insulate slate roof with foam in the knee attics?
(iii) Is it reasonable to eventually remove the gas boiler in favor of minisplits?
(iv) What’s wrong with skipping the insulation and going straight to whole house minisplits?
(v) Will insulating the attic make it too hot up there in the summer (it’s an empty space, not used for anything). Should there be a fan to blow hot air out of the attic and bring in cooler summer air?
(vi) Who cares if the basement is humid in the summer?
Thank you for any and all insights!
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Replies
Is there a reason you aren't asking your energy auditor these questions? While we all have our own preferences, their recommendations and estimated payback seem like a good start.
(i) Should I worry about all of the added weight from the insulation?
>No, unless your framing is seriously undersized.
(ii) Is it ok to insulate slate roof with foam in the knee attics?
>Probably not without extra steps. Old slate roofs usually need to dry on the backside and probably don't have a waterproof underlayment.
(iii) Is it reasonable to eventually remove the gas boiler in favor of minisplits?
>Yes, once the building envelope is well insulated and air sealed.
(iv) What’s wrong with skipping the insulation and going straight to whole house minisplits?
>They will have to run too much, leading to frosted coils, high energy bills and reduced comfort due to lukewarm air blowing continuously.
(v) Will insulating the attic make it too hot up there in the summer (it’s an empty space, not used for anything). Should there be a fan to blow hot air out of the attic and bring in cooler summer air?
>No, insulation slows heat flow in both directions so overall your house should be cooler in summer.
(vi) Who cares if the basement is humid in the summer?
>Your basement is connected to the rest of the house, and damp air tends to rise, so as your house gets tighter you could see moisture problems, usually near the ridge and sometimes at windows and other condensation points.
If the windows are original how will you ever have a "tight" house? What are your HVAC your load calculations?
Any new heating system today needs to consider future time of use pricing for electricity. It wise to understand your watt per BTU efficiency at low temperatures. I find these numbers are hard to obtain from manufacturers.
Personally I would always have gas heating backup in zone five that could run without utility power. This could be a gas fireplace. Even today in Chicago with heat pump I would no be able to charge myEV cheaply on time of use pricing because the cost of electricity on very cold nights would be prohibitive. The future is residential customers paying the actual cost of electricity at time of use.
Thank you for your replies and viewpoints.
I don't know if there is an easy answer to the big question: should I do this or not? It seems to me that the cost of insulation vs. the price of energy at the moment makes it a nice to have luxury that can be kicked down the road rather than a must. And the thought that the sealing will make the basement MORE humid than it already is kind of scares me. I am used to air circulating more or less freely throughout the house.
What I wish I could ask my energy consultant is: what would it take to capture 80% of the gains? If the answer is 25% of the cost, I'm all in. In many things in life, the first 80% is by far the easiest.
As to the windows--the energy consultants I've consulted have universally said the windows aren't a big issue (they will seal them with the usual weather sealants). I suspect a window contractor would stress all of the benefits of new windows (which are real, just expensive and unsightly (to me)).
In the door fan test, they found that the CFM50 is 4.5 times "standard" for this size house, and they estimate they would get it down to 2.8 times standard if I follow their recommendations.
You probably want a final HVAC plan before starting insulation. Your energy consultant should be at least partially justifying the work based on payback. Some of the sealing suggested is also good to keep mice out.
If your boiler is less than 25 years old it is unlikely a heatpump will have any payback. Your yearly heating bill is probably not much over $1000.
Don't worry about the basement. Get a direct drain dehumidifier going. A $300 model from amazon is fine. Figure out if you can send condensate to a drain or if you need a pump.
No reason to worry about sealing the house when the goal is 2.8 times standard. Don't forget about winter humidification. In my heavily restored/remodeled 1920s tutor I need a steam humidifier to reach decent levels due to air leakage. (3000sf and a yearly heating bill of about $700)
“Your yearly heating bill is probably not much over $1000.”
What if it was much higher, say 4.5 times higher?
That's about what I'd guess for energy bills for a house that size in CZ5. I agree with Don that without making envelope improvements, you likely won't see much savings by changing to heat pumps, but you will if you improve the envelope.
I strongly disagree with Don about the basement; every weatherization contractor knows that after fixing a leaky roof, the next most important task is to get the basement dry and air-sealed, not just for comfort and energy but to reduce the risk of mold and rot taking hold. A "disposable" $300 dehumidifier is a good start, but putting a modicum of effort into reducing the source of water, moisture and air will likely have a relatively short payback period.
If your house is so drafty that you have to rely on humidifiers to keep your house comfortable in winter, that's an excellent sign that you need to tighten the house. "Built tight, ventilate right," as Dr. Joe Lstiburek likes to say. Why in the world would you intentionally leave your house drafty, wet and uncomfortable?!
A better way to think of your investment in your home than payback period is the return on investment. A 15-year payback is comparable to a 7% simple return on investment. That may not be as good as the stock market average, but not too far off either, and about as safe as investments get. Plus, if done right, your house will last longer, your health will be better, and your comfort levels will be higher.
“ Plus, if done right, your house will last longer, your health will be better, and your comfort levels will be higher.”
>I care a lot about these things. Probably more than I care about the heating bill. But there’s too much conflicting “science”.
> But there’s too much conflicting “science”.
It's not science. No one can make a pre WWII house perform like a new building without a gut renovation. That's why the target for your house is 2.8 "standard".
If you want to spend well over $500K to gut renovate you house in an historically sensitive manner to save a coupe hundred a month on HVAC that is a choice. But no one has ever taken a house like yours and for few tens of thousands of dollars turned it into a house that needs an ERV to balance humidity in winter.
You can't possibly get a 1910 house tight enough to not need a humidifier in winter without a complete gut remodel and window replacement. Few people want to encase their old house basement like a crawl space. They want to use the space. Most old house basements have little air infiltration and substantial vapor infiltration.
"You can't possibly get a 1910 house tight enough to not need a humidifier in winter without a complete gut remodel and window replacement."
I am not sure I follow...never felt the need for humidifier in the winter. House feels very nice in the winter as long as the boiler is cranking away. While lots of air is moving through the stack (I am sure the door blower test doesn't lie), it's actually not noticeable to me. A couple of specific areas had noticeable drafts, but that was easily fixed with localized weather stripping.
>> What if it was much higher, say 4.5 times higher?
Then you house is leaking a huge amount of air. Look for a local manufacturer of storm windows. Insulation in the attic will only make a modest improvement unless it also improves air infiltration.
How do you do insect screens without storm windows?
“ You probably want a final HVAC plan before starting insulation. ”
>makes sense. I wonder why they didn’t offer one.
Slateandall, I'm not sure what Don Miller is doing on a green building website or what his background is, but sealing and insulating basements is done all the time and rarely DOESN'T make sense. There is not conflicting science, there are well-established techniques to do it safely and reasonably economically. Insulation contractors I know routinely get old homes like yours (or mine, built in 1830) down to the 3.0 ACH50 range using basic air-sealing techniques, insulating walls with dense-packed cellulose, and spray foaming the basement walls after water problems are taken care of. I agree that it would not be financially wise to invest in a deep energy retrofit, but "shallow energy retrofits" are often good investments, for the reasons I noted above.
I'm just a homeowner, but I'll throw in my two cents.
First, make a list of all projects in the house, then go through and methodically think about solutions. Some of those solutions are going to require organizing projects in a specific manner. For example, you may find that you have no overhead lighting in your upstairs bedrooms. You may want to run wire and install ceiling fans/lights. It's a heck of a lot easier to do that without insulation in the way than once you've insulated. So your electrical project there should take priority over the insulation. And, obviously, you don't have a $1 mil to do whatever you want to the house, so you're going to have to budget and strategize on these things.
I would recommend that you prioritize as follows: (1) Things that must be done (leaky roof, rotten doorframe, etc.), (2) Things that will prevent a problem in the near future, (3) Thing you really want done, (4) Things with a pay-off period, (5) Everything else. Why? Well, if you look at your large list above and you know, for example, that your water heater is 25 years old, you should be aware that it could fail at any moment and flood wherever it is. That's potentially going to be a costly clean-up, not to mention an extremely aggravating one. So, I'd categorize that as a #2 and prioritize that over the insulation.
As far as the minisplits go, I'll go ahead and tell you that you won't get any good answers. Like you, I have asked the question, "if they're more efficient, then why wouldn't I put them in first?" It's pretty simple, right? If the mini split can produce the same amount of BTUs for less energy input, you should be able to compare the cost of that input versus current input and find out how much money you are saving. And why wouldn't you want to save money now versus later? The only answer you'll get is "you'll save more money when you do." But that's not a good answer. In fact, it's an incredibly bad answer because it completely ignores the fact that there is some set number of BTUs to heat or cool your house over a particular period of time for it to feel comfortable to you.
I think the real problem is that the math is hard. The math is hard because the amount of output for mini splits varies based on outside temperature and how much they have to raise/lower the temperature. So, you may have one month with the same average temperatures as the next month, but the energy usage is different because one month the temperature was pretty steady and the next month it swung around wildly. Meanwhile, your "dumb" furnace really has one setting - on or off. The output is the same until the thermostat tells it to stop. But that doesn't stop the fact that you could, theoretically, compare prior time periods and see what the difference would be for your current setup versus a specific mini split setup. And, assuming the numbers are the way that are claimed by manufacturers, installers, etc., there is some relationship between the cost per kWh and gallon of gas that tells you the lowest temperature at which you save money by using the mini split, and, if you were to switch you would save money (if temperatures are above that number). Now, maybe, somewhere there is some formula that solves this particular issue, but I haven't found it, yet, although I am still looking.
"Like you, I have asked the question, "if they're more efficient, then why wouldn't I put them in first?""
The answer is that you will need smaller equipment once you improve the insulation and air sealing, and it is very important that HVAC equipment be properly sized for your loads. If you reduce your heating and cooling loads by half (not at all impossible in a house like that), then the equipment capacity would also be reduced by half. If you installed equipment that meets your current loads, it would be dramatically oversized for the house once you make your energy improvements. Oversized equipment costs more, takes up more space, wastes energy and often provides poorer comfort performance.
"The answer is that you will need smaller equipment once you improve the insulation and air sealing, and it is very important that HVAC equipment be properly sized for your loads."
Ok but what if I decide life is too short to worry about perfection and am willing to tolerate oversized minisplits for the next thirty years? I will have accomplished my goals of (i) cooling the house in the summer, (ii) supporting the heater in the winter and possibly saving $ on warmer days (maybe 50F?), (iii) having a gazillion zones for people who want their rooms toasty, (iv) maybe somewhat cleaner air due to the minisplit filters, (v) lengthening the life of the gas boiler because it will run less, and (vi) when the gas boiler does go kapputz replacing it won't be an emergency and maybe then I will consider insulation.
Seems to me we could be leaving a lot on the table by dismissing HVAC upgrades. (Not surprisingly my HVAC installer agrees...)
"The answer is that you will need smaller equipment once you improve the insulation and air sealing, and it is very important that HVAC equipment be properly sized for your loads. "
I'm going to half-disagree with Peter on this point. The reason is that what he says is true for traditional heating equipment. Think about a baseboard heater - the longer the pipe run w/fins, the more heat - so a calculation based on assumptions that are incorrect after upgrades will result in oversized equipment. The same could be said of traditional forced air.
However, with heat pumps, you're not talking about something that the installer is adjusting on site. Instead, the installer is picking from a discrete set of options specified by the manufacturer to assemble an entire system. So, take Mitsubishi for example. Assume that you get a quote for a multi-head system. The installer is going to pick from between 6k, 9k, 12k, 15k, and 18k heads that are all basically the same size. The total combination of those heads is going to determine the outside compressor size. You also need a head in every room that you want to condition effectively.
So, if you look at my house, I have two small/average bedrooms, a master, and a continuous downstairs. I know, you probably know, and certainly all four of the installers that I just had come give me estimates knew that if I wanted those small/average bedrooms conditioned, they would be installing 2x 6k heads. The same is true of any small/average room anywhere. There's literally nothing any of them could do differently to size those...because there are no other options. So you already know that I need at least a 12k rated compressor.
But, I suppose that's what I've said in the last paragraph is not necessarily true of larger rooms. I could see a scenario where you could have a large room where an insulation upgrade makes it more suited to a 12k unit than a 15k unit. Depending on your house setup, you may or may not have rooms where that is possible. So depending on your setup, you could see that the insulation has a big effect or little effect. Based on your comment above about "a gazillion zones," however, I am guessing that your house is pretty typical NE in that it has a bunch of little rooms. So that's how I come to a half-disagreement. I'm willing to bet that you - like me - have at least a few rooms that you want conditioned where you will be putting in a specific head size without need for any sort of schedule J calculation.
Well, no. You could condition the entire upstairs with a ducted minisplit, sized to meet the entire load. If you need 3 tons to do the upstairs prior to improvements and only 1.5 tons after, you're going to save a bundle on the equipment, need less ductwork, get better energy efficiency and comfort with the smaller system. And, 6k heads are too big for even a modestly insulated, large bedroom already.
I you install equipment large enough for the pre air sealed insulated load after the improvements it will be so large the it will do a poor job in the summer removing enough humidity from the air because of the short run times. This forces you to turn down the thermostat to get longer run times and remove humidity. You end up in a cold and clammy house.
Walta
Does this apply to mini splits that can modulate output and also have a dehumidify setting? In the last humidity waive (high of 70F with humidity over 70%) I set them on dehumidify it worked fine. (Even the basement was dehumidified which makes me think all the humid air went upstairs.)
“ I you install equipment large enough for the pre air sealed insulated load after the improvements it will be so large the it will do a poor job in the summer removing enough humidity from the air because of the short run times. ”
"Does this apply to mini splits that can modulate output and also have a dehumidify setting?"
Not having owned a mini it seems to me minis even when properly sized minis struggle to remove humidity because when they lower the speed it changes the ratio of sensible vs. latent heat removed in the wrong direction removing less moisture the slower it goes.
The fact that they found it necessary to invent a dehumidify setting only prove that it is a real problem they are struggling to deal with.
Walta
Sure, oversized ACs don't dehumidify as well as properly sized units. But people have widely varying goals when it comes to humidity. Some people keep the house open unless it is hot outside.
Also, no AC can keep household humidity consistently below 50% during cool weather except in the desert
I love this way to break it down!
"I would recommend that you prioritize as follows: (1) Things that must be done (leaky roof, rotten doorframe, etc.), (2) Things that will prevent a problem in the near future, (3) Thing you really want done, (4) Things with a pay-off period, (5) Everything else."
It comes from experience with a foreclosure. I know that some people will disagree with my #3 but the truth is that when there's something that you really want to change, it's something that you're going to fixate on every time that you see it. While it might be 'smarter' to do #4 first, the fact that you're going to silently curse every time you see the pink bathroom has some value in itself.
This discussion has been really helpful, and I think I am figuring out what I really care about:
(1) I want the air to be nice and clean. My imagination conjures up dusty air coming down from the attic and CO air from the basement utilities. However, the energy people did a particulate test and it was within target range, and they tested the appliances for CO and found them to be really good. So is this even a concern?
(2) The house lasted 105+ years I don't want something major to go wrong on my watch.
(3) I want to be cool in the summer.
(4) If I have to sell I don't want there to be a specific impediment (assuming buyer is actually interested in old house and doesn't really want a new condo).
(5) Spending 35K to save $200 a month is not good use of funds unless it also supports the above. And this is where I am stumped.
You shouldn't have any issues with CO (carbon monoxide) if your combustion appliances are in decent shape and are vented correctly. Air sealing a home IS NOT the way to correct any CO issues!!! You MUST fix CO issues at the source, by repairing any leaking appliances or flue pipes. DO NOT rely on interior air sealing to protect you here.
Dusty air from the attic would be something that could be addressed with air sealing work.
If you are looking for the best bang for the buck in terms of air sealing, there are certain areas that are notoriously leaky. Rim joist areas are one of these, recessed lights are another -- especially if they are installed in the attic floor. You can get most of the benefits of a big air sealing project by addressing these areas (and I haven't mentioned all of them, there is plenty of info elsewhere on GBA, and the EPA has a brochure for this too), but without all the effort and cost of going after all the little leaks. Go for the low hanging fruit first if you want to maximize your return on investment here.
Bill
I'm in a remarkably similar boat as you and think this is a critical path to explore as there are so many 100-200 yo buildings just guzzling oil and gas trying to keep warm. I've gotten my audit and waiting on results... I'm curious about your answer to Michael's original question;
"Is there a reason you aren't asking your energy auditor these questions? While we all have our own preferences, their recommendations and estimated payback seem like a good start."
My energy auditor is also a salesman.
Have you found any contractors willing to do air sealing? I also live in southern CT, and I’ve also been advised to air seal before installing mini-splits. The problem, however, is that seemingly no one in my area will actually air seal. Traditional insulation contractors have flat out told me that they don’t do air sealing — all they’re willing to do is closed cell spray foam jobs in specific areas of the house (ie: the attic or the rim joists), but no comprehensive whole-house air sealing. And the passive house certified contractors that I’ve found only seem to be interested in massively expensive deep green energy retrofits, during which they will do air sealing as long as it’s parts of a bigger project in the tens of the thousands. But I simply can’t find any actually existing contractor who, in exchange for money, will do a blower door test, then go around the house with caulk and/or canned foam and seal the sources of air leakage.
I believe some will do sealing in conjunction with insulation. I didn’t ask if they would separate the two.
Of course. Just be prepared for the actual air sealing in practice to be poor or non-existent. I’ve gone through two insulation contractors already. The first one sent an “air sealing crew” that did about 20 minutes of work before the insulation crew got started and the blower door test even after the insulation and air sealing was still 3600 CFM. The second company, who I hired to air seal and insulate our rim joists with acrylic based low-VOC sealant and mineral wool, completely ignored the air sealing, stuffed the mineral wool in the joists, billed Eversource $1,000 for the work, and then when I complained, they told me that the only way they air seal is when customers choose to insulate with closed cell spray foam — otherwise, no air sealing. So now I just have to hire someone to take the mineral wool down and do the whole job over correctly. And in general, one down side to the free insulation jobs through Energize CT is that it seems to incentivize sloppy work and low accountability; companies do jobs that serve no real purpose other than to have an excuse to bill Eversource for a few thousand dollars of work.
All that being said, if you do find a quality air sealing/insulation contractor, please let me know and I would reach out to them in a heartbeat.
We have worked on a fair number of Tudors in lower Fairfield County and lower Westchester.
Concerning your Humid basement-
Are there any adjacent crawl spaces that have dirt floors, perhaps under a sun porch, pretty common to Tudors around here. If yes that can be a big source of humidity in a house like yours. What about the basement slab itself, does it fully cover the entire floor or are there lots of cracks and missing sections? Perhaps you have an old coal bin down there that still has a dirt floor? If yes that would be another source of humidity.
If you do have any dirt floors don’t just cover them with a sheet of poly, put down the poly and 2” of foam board and then pour a concrete slab on top of it.
Every now and then I see a full depth basement in an old house that still has open vents in place of windows, any chance you have those? If yes, close them up and insulate them with foam board cut to fit.
Gutters, what about them? Do you keep yours clean? I mean like clean them 3 to 4 times per year, so they don’t clog up and overflow adding excess moisture to the soil at the foundation and ultimately your basement.
Most houses I see have very poorly designed gutter system with far too few downspouts and leaders so the minute there is a clogged leader they overflow.
Even if you have a decent gutter system what happens to the water when it gets to the ground, is it piped away from the house, does the grade around the house slope away from the house or toward it?
Has your landscape contractor piled mulch around the foundation plantings year after year ultimately pitching the grade and surface water to the house?
Do you have an irrigation system that runs every night that might have drip lines to the foundation plantings? I see more and more houses in the area with irrigation systems that are set to run 4 to 7 nights a week, if yes that’s going to increase your humidity. Turn the irrigation off for two weeks and see how the hose responds, the lawn and plantings will be fine, don't worry.
What is your source for domestic Hot water? Whatever it is consider changing it out for a heat pump water heater that will be more efficient than what you are using now and will also work to dehumidify your basement at the same time. We have done this on more than one occasion eliminating the need for a standalone dehumidifier while lowering the overall utility bills.
Air leakage. There are several sources of huge air leaks I see around here in all types of houses:
1) Do you have a basement bulkhead stair down to the basement from the outside with nothing but a steel or wood door separating the basement from the outdoors? We typically build a 2-3” thick insulated and weather-stripped door at the bottom of the bulkhead stairs, this will stop cold air from rolling down the stair well in the winter. I see you have hot water radiators and there is probably the related piping up along the basement ceiling, this insulated door will stop all the heat that radiates off your radiator supply pipes from rising up and out the un-insulated bulkhead. Now all that heat is going to be trapped up against your first floor, most people notice an immediate difference in the winter. We have built these doors many times and if its in the winter, Cesar, the senior carpenter, likes to bring a thermometer down in the basement at the beginning of the work and watch it rise once the insulated door is completed. On a cold winter day, we have seen the basement temperature increase by 5-10 degrees just over the course of one day.
2) Attic access doors that are un-insulated and with no weather stripping will leak huge amounts of air and can be a cause of ice dams in the winter from all that lost heat melting the snow off the roof, there are all kinds of ways to deal with this from site-built solutions to insulated zippered tents you can buy. But in the end you may want to insulate the attic roof itself as opposed to blowing in cellulose.
3) Sounds like its too late for this, did you already blow in the cellulose on the attic floor? I hope not. If not look at the attic floor, are there recessed lights sticking up thru it? If yes they should be sealed up with a box made from fire proof foam or Tenamat hoods (you can buy the hood s at Home Depot.) What about bath fans in the 2nd floor ceiling? I see them sticking thru the attic floor all the time leaking air like crazy, here again they can be boxed over with fireproof foam board to stop air leakage.
What kind of windows do you have, most Tudors I see originally had steel casements? If you have wood double hungs there is hope, they can be weather stripped, and a good storm window installed, that can make a huge difference.
What’s your Energy Auditors background, most that I have come across don’t seem to really understand heat loss, cooling and the most basic air sealing concepts. I have seen extravagant reports and costly recommendations for homes like yours, but they completely miss the basic things that I talk about above so even after the work is done the owners notice little difference.
My suggestions are all low cost and low tech, all the material is available at local lumber yards and big boxes. Do all these things before you spend anymore money on mini splits or insulations and see how the house responds. Attached is an article from Building Energy Magazine that addresses some of these things. - Jud
Jud, Thank you for your amazing reply
- Concerning Humid basement, we don’t have any dirt floors or open vents. The basement slab has a few small cracks.
- The house is built on a slope, so the front of the basement stone foundation ( floor to ceiling) is mostly underground. We maintain the gutters, but the soil has moisture after a rain. I believe this is the main source of humidity. I’ve heard the only way to improve it is with costly French drains.
- The back of the basement is above ground with a garage. The basement and garage are separated by a metal door that lets some air through. It’s on my list of things to deal with but I haven’t prioritized it yet. No irrigation system. Trying to be sustainable and always looking for grass and clover mixes that tolerate dry spells.
- I would love a heat pump water heater once ours is ready to replace. I do appreciate a separate dehumidifier though because it also filters the air (I’ve heard).
- windows: we have a combination of wood double hungs and wood swing outs. I’m a little bit stumped by the wood swing out Windows but haven’t prioritized them yet. The double hung already have outside storms but could use professional sealing.
Thanks!
If I owned your house with the issues you have I would insulate the attic roof and then put a fully ducted heat pump or just AC system in the attic.
Why? Because if you do that you will have the opportunity to put in a really good filter system, something along the lines of a 4” thick pleated MERV 8 to 13 filter, I believe Merv 13 will catch Covid. Every mini-split that I have ever seen has a super thin filter that at best catches large dust particles.
With a fully ducted system you can use a “Circulating thermostat”, Honeywell makes sever models as I’m sure other companies do. A circulating thermostat will turn the blower in the air handler on for something like 20 minutes every hour, in an old house like yours this goes a long way towards evening out the hot and cold parts of the house because it's going to be constantly moving the air around the house. For instance, on the South side of your house, which I believe you said it's very hot in the summer, it will move some of that heat around to the cold side of house and vice versa in the winter. We've done this many times with good results. A circulating thermostat will also work really well if you go with the AC only ductwork option and keep your radiators, in the wintertime when the radiators are cranking and you leave the thermostat in circulation it'll move that heat around the second-floor evening out the cold and hot spots, again we've done this several times with good results.
As far as insulating your attic, in my last post I attached an article which explains how we often insulate attics, you can do that system if you want, that system allows the roof to breath and dry out and maintains maximum head room if that’s important. If you want to go with spray foam (I'm not a big fan of spray foam at all I try to avoid it as much as possible) first sheath the underside of the attic rafters with ¼” plywood or even something as thin as Masonite, then apply the spray foam to that. This system will create a huge airspace between the plywood and the underside of the roof deck & your slate roof will have plenty of air for breathing/drying, but make sure that you have a roofer put a Ridge vent on the roof. I wouldn't be so worried about having any kind of soffit vent, given that the slate roof is likely nailed to 1x4 skip sheathing there will be lots of vertical and horizontal air movement and the ridge vent should be enough to let the slates breath/dry out as needed.
Hi Jud,
I also live in Fairfield County and I have a similar age house with similar issues as the original poster. Would your company be available to give me an estimate for some of the insulating and air sealing methods you talked about above? Thanks.