DensPlus (and other non-paper-faced gypsum panels) are much more resistant to moisture than the green paper panels. That's why they can be used as tile backers. However, Dens Plus is much harder to prepare for painting than green board; it needs a skim coat. Frankly, other than in a bathroom, I've never used anything other than standard drywall in basements. But I always have a dehumidifier running to keep rh below 50%. And keeping the drywall a few inches above the floor and using XPS under the baseplates protects the wall from minor flood damage. Sealing the interior of a block wall--to the extent that the sealers can even do that--just means moisture accumulates in the wall and possibly causes damage.

Be careful with the first hour ratings. On a 100 gal tank you will get about 70-75 gallons of hot on that first load. You then get the balance of the first hr rating over that first hr. For the kind of lay-out you suggest I think you should split the house up into two systems and try to centralize each tank to its load. That minimized pipe losses with the recirc, and yours will be substantial. The very best recirc system is the Metlund Design system. It complies best with all the new Title 24 regs. You will need 3/4" insulation on your hot piping. Again T24. Be very careful with the pex sizing. I re-piped with Uponor and the brass fitting here in the Foothills of N CA. There is a bushing affect on the fittings. The ID of 1/2" is 3/8, 3/4=9/16 and 1" is 13/16. It does make a difference. I have a couple faucets I was surprised how poor the pressure is on my current well 40-60 psi set-up. I had intended to go to a constant pressure well pump but now will definitely do so. Everything is good about the 50 psi range. Not so at 40# I am a fan of tankless. I represented a major manuf of tankless for 20 yrs. As far as your suggested products I will only suggest that you avoid Bosch. They, imho, do not have the right technology. Your first step regardless of what you do for equipment is a thorough water test. Manuf of tankless have to list annual cleanings for maintenance, but I had one unit for 12 years and never touched it. It was a pre-production model and I had to replace it prior to selling the house. I cut it in half and it was as clean as a whistle. Many of your plumbing questions and answers begin with the compete water test. Tankless (2) could handle your loads until you bring the tubs into play. They represent a massive load, even for your Vertex's. To handle your tub flows you should have two units on each side of the house. The thing about the tankless is they will each make your 7 or so gpm 24 hrs per day, but 135/7=about a 18-20 min fill. Will you wait that long for your bath. I have a bath fill controller on my tub. I set it at 104 and 35 gal (the vol of my tub) and it will deliver 35 gal of 104* water and shut off the hot and beep to tell me my bath is ready. Pretty cool actually. Your shower should be okay with one tankless, but your shower head(s) must fall within the flow capacity of the water heater. I can go on and on with this topic. Get your water tested and report back. That should be step one regardless of what your are doing.

Caroline I don't live in Rapid City. I live N of Pierre. Its even closer than you think. Someone mentioned a humidifier. I can tell you right now it makes a huge difference. My DH keeps telling me, he can't believe how much warmer it feels in the house when it is on. I think it is the moisture that makes it feel warmer. I can tell the difference right away. The minute I turn the heat on in the fall I start the humidifier. I used to have two of the gallon ones going at the same time. A few years ago I went to a yard sale and someone had one of the console ones. I think I paid about 5 dollars for it. for it. It runs all the time now. Every night I have to fill at least one of the 3 gallon tanks but it is worth it to me. WE have our thermostat on 57 at night and after I come home from work we turn it up but only about to 65

We all know the windows are the culprit. Got to do something about them. Windows that large are costly to replace with something that stops heat and cold. That is the best option of course, but also the most expensive. For winter, my recommendation is climateseal interior storm windows. I am not a paid spokesperson, but I have them in my house and they very much help in winter. I find myself recommending them because I would prefer they stay in business if I move I can get them in my next house. They are significantly less in terms of cost to the best performing windows you would need. For extra large windows you may want to have a partition in the middle so they are easier to remove and clean. Here are some pictures. These are not cold to the touch, and when I took the one off I could for sure feel the cold. They magnetically seal to the metal L channels you install. All you need is a tinsnips, small drill bit, pan head screws, and caulking to install the channels. I do not know if these work well in summer for heat. I have the thermal series and take them off in the summer, but they do have a preservation and pro series. I am thinking the pro series combines preservation and thermal benefits. Ballpark guesses for quotes would be around 3-4k for the storm windows vs 10k for new good windows plus 10k more for installation. http://www.climateseal.com/ Some other options for summer would be heat control window film, awnings, and pulling the curtains closed.