Moisture and energy efficiency in ICF Cape Cod design
Hello,
My wife and I are designing and building a Retirement home in King George, VA (almost in view of the Potomac River). I’m concerned about managing moisture in what I hope to be a very energy efficient home for the long run.
The home is ICF in the Basement and on first floor (1700 ft-sq on 1st floor) and then has a room truss above for a second floor with two rooms and a bath room. Front of house has a direct southern exposure. The first floor is also planned to be an ICF “Lite Deck” concrete floor that I plan to run radiant heating tubes, while cooling is “planned” to be via Mini-Split HVAC system. My room truss Cape Cod style 2nd floor spaces are planned to be insulated with a combination of some type of foam board (EPS or XPS or ???) between studs and some blown-in insulation past the room-truss rooms (R-50ish); I plan to have a Ridge Vent and soffits with the FHB recommended 60/40 Air Intake to Air Exhaust to create a slight Positive Attic Pressure.
Since I intend the home to be super tight and with a good effort to eliminate air leak sealing in the challenging Room Truss spaces above the first floor; lots of spray foam and acoustical sealant…
I am starting to second guess my approach and am also worrying about a super tight home and moisture build up … I am trying to study as many FHB and GBA articles as possible but I really don’t have a handle on what to do in my design or design requirements to drive out moisture (especially in the hidden attic roof areas). Oh, I also plan to install some kind of Air to Air Heat exchanger that I was thinking of providing outside air via an earth tube to pre-heat or pre-cool the air (the tube would run around the foundation footing area but above the drain tile tubes).
Please help me with my design nervousness as a basic concept (ICF and such; good/bad or things to consider) and also what I could do to ensure I have risk areas like moisture build-up well mitigated by design. I guess in the moisture concern I am worried about condensing air. Since I am second guessing myself … any other things I should consider for an energy efficient home for the long haul…
Thanks so much,
Eric & Christine (worry warts) 🙂
PS – I know … that’s a lot of worries for one “question”
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Replies
Eric and Christine,
First, if you are going to install ductless minisplits for cooling, why not use them for heating as well? Then you can save all of the money you were going to spend on a hydronic heating system.
You wrote that you want to be sure that you can "drive out the moisture," and that you are worried about "moisture build-up" and "condensing air." These worries are vague, but I will try to address them.
A well-built home has a well-defined thermal barrier surrounding the conditioned indoor space. Adjacent to (or contiguous with) this thermal barrier is your air barrier. If the thermal barrier and air barrier are well-designed and well-installed, they effectively separate the indoor environment from the outdoor environment.
You want your indoor environment to be comfortable. You do that by specifying effective HVAC equipment, by specifying enough insulation, and by planning your windows and glazing options carefully. So indoors, you will be warm in winter, and cool and dry in summer.
If it's humid outdoors in the summer, you really don't care. You don't have to manage the humidity that is on the other side of your thermal barrier. All you need to do is make sure that the air inside your home is comfortable.
Problems occur in homes with leaky air barriers. But if your home has a tight air barrier, I wouldn't worry about the humidity levels outdoors if I were you.
Earth tube pre-conditioning of the ventilation air is an expensive proposition in drier climates, and an actively bad idea at VA's subsoil temps and summertime outdoor dew points. An Energy Recovery Ventilator (ERV) ventilation system that does a both a temperature and humidity exchange with tehe incoming & outgoing energy streams makes sense, but don't over ventilate- dial back the ventilation rates during the sticky summers to allow the mini-splits to keep up with the latent load (humidity.) With a higher R higher-mass/lower-gain house it's likely that our latent cooling loads will dominate the air conditioning load more than you're used to.
If your plan is to use multiple mini-splits rather than a multi-zone single compressor ductless system, making one of the mini-splits with a large zone a Daikin Quaternity gives you the means of dehumidifying to a relative-humidity setpoint, and they dehumidify in either cooling or heating mode, with or without imparting sensible heating/cooling.
ICFs start out with an unusually high moisture content stored away in the concrete, which does eventually dry. The structure itself tolerates the moisture well, but the interior finish-walls can have issue if you use highly vapor retardent materials like foil or vinyl wallpaper. Standard latex is recommended, applied months, not just a few weeks after the concrete is poured.
Over the intermediate and longer term, the quality of bulk water management (particularly the window bucks) trumps all. If more moisture is being dumped into the concrete at every rain squall the concrete never dries, and the interior stays damp. But as long as bulk water is always re-directed to the exterior, you'll be fine.
Eric,
You didn't mention exactly what the second floor is to be for, but if you don't really need it for something specific, then why not keep the design to one floor? Climbing steps is not a favorite retirement activity.
If the house can face due south as you say, then use passive solar design techniques to the fullest. Read up. I'm talking about things like keeping the footprint fairly simple, rectangular, with the long axis facing south. Virginia winters have been milder lately than they used to be, but it is still a heating-dominated climate. Put the most lived-in rooms mostly on the south side, with utility rooms, closets, even bathrooms on the north side. Use plenty of high SHGC, double pane windows, properly shaded for the summer, on the south side. I (in Maryland) have south facing windows with area that is almost 12% of my conditioned floor space, and the overheating you sometimes hear about does not happen for me, and I don't have much thermal mass. I have R40 walls and R60 attic with 4ach at 50 pascals--again, read up if you need to. Use even a larger area of windows than I have if you can, but consider having some well-insulated thermal mass if you go higher. Use triple pane windows on the north side. I have triple pane, low-e windows on the north side, but I would advise NOT to have the low-e because the visible transmittance has been very disappointing after having lived a long time with clear windows, and also comparing my north side transmittance with my south side (double pane, SHGC=.0.6, about as high as you can get these days.) In my area and in your area, the sun supplies an incredible amount of free btus for winter heating--certainly not as good as it could be, but very usable. I have used Energy Plus, HEED, BEopt, energy design software, and they all tell me I am getting about 60% of my heating season heat from the sun, for free, using passive solar. Yes, it does make the cooling load somewhat higher, but the heating benefit far outweighs the cooling penalty.
You may notice that you don't hear most of these GBA guys talking much about passive solar. Trust me, don't make the mistake and think it doesn't work. It does work. Most of these guys are from New England--there's not a whole lot of winter sun in New England. I have been following this GBA site for 5 years, and whether they know it or not, admit it or not, there has been a clear, definite avoidance of solar energy, in general. I guess it's because they just don't get much sun in winter. Maybe it's something like me not talking much about making Maple syrup. I love it and tried to tap maples around here, but results are pretty poor--so I pretty much don't talk about it anymore.
Forget the radiant floor, unless you really have to have that warmth under feet. Minisplits, heating and cooling, are really a good, much simpler solution, particularly if you have no natural gas (like me). Just keep the floor plan open if you intend to use ductless as more or less heating/cooling point sources.
Forget the earth tubes. I once considered them, but with superinsulation, air tight design, efficiency of minisplits, the expense, complication, need for fans, possible mold issues just really aren't worth it.
Finally, it really sounds like you need a designer, or read up a whole lot more. Even if you aren't a designer, energy programs such as HEED and BEopt are free and easy to use, and easy to compare options in energy design. Knowledge, your knowledge, is the best remedy for your warts.
Eric,
I agree with Sonny Chatum on his advice concerning passive solar design principles. That's why, in my article called Green Building for Beginners, I listed the following four principles as the most important principles to follow in my "advice" section:
"Aim for a compact shape with few ells, bays, and bump-outs.
If possible, orient the long axis of the house in an east-west direction.
In most U.S. locations, it makes sense to include more windows on the south side than the north side. (This advice doesn't apply in hot climates or the Southern hemisphere, however).
Spend some time researching window specifications; choose glazing that is appropriate for your climate."
I disagree with Sonny, however, when he writes, "You may notice that you don't hear most of these GBA guys talking much about passive solar." In addition to the fact that my own advice prominently prioritizes passive solar principles, I can cite other GBA articles on passive solar, including these:
Cost-Effective Passive Solar Design
Resilient Design: Passive Solar Heat