Walls, 2" spray @ outside, 3.5" wet-spray

I'd go with EPS foam on the outside and the wall filled with cellulose. There are other ways to get good air sealing.

As far as a cooling tower, this would not be a good choice.The amount of water dumped and the energy associated with the evaporative effect would not be cost effective.Cooling towers are very high maintenance. Water treatment is a must. Whole house fans are not a wise choice either.Reason one is that they change the building pressure and can cause improper venting of all natural gas appliances. Two they can bring in moisture into the space which will require more energy to remove.Trying to maintain a clean enviroment is nearly impossible. The last reason is if a fire were to develop within the home, the quantity of air fueling the fire would make for a fireman's nightmare.

Water usage is higher than I expected but not a deal breaker. 8% of flow rate, flow rate 9GPM so about 43 gallons per hour. Assuming 6 hours of run-time = 260 gallons/day for 75 days a year at Boise city water prices is about $47 in water. The 1/6hp fan motor would use about $13 in electricity over the same period maybe the same to pump the water. Not exactly free. Would still need some power for dehumidification. And the 6 hour figure was a total swag.

My current house is moderate efficiency and heat gain and I probably spend about $250/yr cooling it. The ROI on the time I've spent thinking about this probably does not justify the cost.

Thanks for everyone's thoughts.

Posted By jonr on 02 Jan 2013 02:43 PM

Decent quality mini-splits do pretty well in your climate, running a seasonal average COP of about 3 or bit better

I believe they are only reporting the COP for the ~60% of the time/load that the heat pump worked. So say more like COP = 1.7 if you make up the shortfall with resistance heat.

The NEEA study monitored in-situ retrofits on pre-existing conventionally-insulated houses, not whole house solutions for high-R houses. They actually "worked" all the time, but lacked sufficient output for carrying the whole load. Most houses aren't specifically layed out for optimal point source heating too, so sizing the ductless for the whole house usually doesn't make sense in that sort of retrofit- sizing it for the zone is more appropriate.

At +5F (across any range of humidity) almost all inverter-drive mini-splits can deliver ~15,000BTU/hr per ton heating capacity per ton of rated cooling. Many have output ratings somewhat below the 15K at -4F (-20C) too.  The Mistsubish H2i series guarantees a capcity of 70%+ of the nominal heat rating at outdoor temps of -13F/-25C.  In climates as temperate as Boise you're (almost) never below the rated output temps, let alone below the actual operating temp limits.  

The average COP of mini-split mostly affected by the average wintertime outdoor temp, which determines how much of the time it's running at an easy highest efficiency part-load.  Capacity is also a function of outdoor temp, and to a lesser extent the outdoor humidity (time spent in defrost mode is time spent not heating.).  The mean binned hourly December/January temps in Boise runs ~30-32, a temp at which mini-splits run COP>3.5.   The 99% outside design temp is +9F.  If you size the ductless for the whole house with a bit of margin for the +9F load it'll be only running a COP of 1.5-2 at design condition, but will be running better than 2.5 on average even in January,  and higher still during the shoulder seasons.

If the NEEA retrofit units monitored averaged a collective seasonal COP of ~3 on somewhat a random selection of mini-splits that tested at 8.5-12HSPF, you can bet that newer units testing at HSPF= 12+ will do at least as well if sized reasonably for the load.

Posted By NRT.Rob on 02 Jan 2013 03:13 PM
Yeah, I will note all this stuff about slab comfort in superinsulated buildings is a bit... off the mark. I stand barefooted or in socks here in my superinsulated shop every day, and I can most definitely tell the difference between an actively heated floor and one that isn't. floor temp oscillates from about 67 to about 73. 73 is very nice. 67 is kind of chilly and not as comfortable.

so as much as it's a meme in green building to think that minisplits provide equal comfort in tight buildings... I disagree. it may provide comfort people deem acceptable or even pretty good. but it's not the same as a radiant floor.

Yes you can notice the difference in bare feet, no doubt, but a lot of comfort can be made up with higher room temps and house-slippers too.  I don't know anybody who really believes that the comfort of a mini-split is the same as radiant floor, but many who deem it VERY acceptable.

The "pretty good" estimation really is true, (especially compared to any other air-delivered heating), as and so is your statement that "it's not the same as a radiant floor".   On a very low-load house the question becomes "how good is good enough", and at what installed cost.  It's a cheap and reasonably comfortable solution once the loads are low enough- you can't install any type of hydronic system for the cost of a mini-split.

A 73F floor is indeed pretty nice, but wouldn't necessarily be a common condition in many higher-R houses, even those well short of PassiveHouse standards.

Wont an ERV be able to maintain that indoors?

Is it possible that your humidity and dryness issues were related to having a leaky home before? When you bring in a lot of cold air and warm it up, it acts like a sponge, drawing moisture out of everything.

WRT dryness during the heating season in my current home... I think this home is pretty tight. All the doors and windows are top shelf stuff and the guy that built it was kind of a construction nerd. The HVAC on the other hand is a disaster. My daughter has the problem, her room is by far the hottest in the winter months and she always keeps the door closed at night. I'm guessing the HVAC imbalance as someone else suggested. She doesn't have nearly the problem with bloody noses at her mom's house.

I would get a humidistat and measure it at both places. If that supports the theory then you can start fixing it with hard data to verify.

Dana, actually, 73 is pretty common. it's hard to cycle a system and not hit that in a 70 degree room. and if the room temp increases, hey, now you've got a 71 degree room. maybe 72 in the shoulder seasons. your radiant output drops quickly when you are that close to room temp if the room temp ranges up at all. that's why some old timers mumble about radiant being self-correcting, output jumps as room temp drops and drops as room temp rises.

73 is an arbitrary number I threw out anyway... anything 70-73 is a very noticeable improvement over 67 in socks or bare feet.

I'm not trying to say everyone should use radiant floors in superinsulated homes. but even best case there IS a difference, and I find the greenies are often completely ignoring bedroom/auxiliary room comfort in their analysis.