Posted By MattH on 04 Sep 2013 12:51 PM
I am looking at building an ICF house and my main concern is the proper heating and cooling. I live in Central Indiana, the house will be 2 story, approx 5000 sq ft. I have heard some horror stories about humidity and wondered if someone could see if the company quoted the correct products. Our rep from Nudura said to figure a r35 when pricing. The furnace has a standard blower and not variable, it includes an air filter in the electronic air cleaner and a fresh air ventilation kit. It does not include a humidifier. I thought I needed one of those. Thanks for any help!
Almost all gas-fired furnaces would be oversized for a 5000' house
with true R35 wall performance (assuming you had U0.25 windows and R50+
in the attic, and 2-3" of EPS under the crawlspace slab.) In a central IN
climate you'd probably do better from a comfort point of view at a
comparable operating cost to install a right-sized fully variable speed
Bryant Evolution Extreme
air source heat pump (without the gas-burner option) for both heating
& air conditioning. The installed price is probably cheaper than
oversized AC + oversized gas furnace, and the comfort levels much
higher. (In smaller or more open floor plan houses ductless heat pump
solutions would be more efficient.)
All good HVAC solutions start
with a room-by-room and whole-building heating & cooling load
calculation, based on realistic indoor temps and 9
9% & 1% outside design temps.
ACCA Manual-J methods are fine, but be aggressive. Use the real (ASTM
C518 rated) R values on the ICF foam, not any loaded figure their
marketing idiots dreamt up based on R4.5/inch foam (valid at central AK
outdoor temps, but not central IN) or the mass effect benefits at
extreme diurnal swings more applicable to the high-desert regions of the
US southwest. If your local HVAC designers are using the "lessee,
that's 5000 feet at 25 BTU a foot gives ya 75,000BTU/hr" type
calculations, buy them a box of crayons and a coloring book to work with
and find somebody else.
Using the heating system as the ventilation system is a bad idea in a
standard house, and an even
worse idea in a tight higher-performance
house , since the heating & cooling loads and thus operating duty-cycles will be low or even non-existent for months during the shoulder seasons. Ventilation is needed whether there is a heating/cooling load or not. The right solution is a separate ventilation system running it's own
duty-cycle or other controls. A typical cost-adder for a heat recovery
or energy recovery ventilation (HRV or ERV) system in new construction
is on the order of $2.5-3.5K.
A tight house will only need a
humidifier if you're running high to VERY high ventilation rates. If
you're religious about running bathroom and kitchen fans during the
appropriate periods, don't smoke, burn candles or use toxic aerosol
spray products indoors, you don't need anything like ASHRAE 62.2
ventilation rates to maintain good indoor air quality, and many people
will run an HRV or ERV under DE-humidistat control in winter to bring
the indoor humidity DOWN to ~35% relative humidity, which keep the
moisture loads on the building materials down in winter, limiting the
mold spore count in the spring warmup. During the summer outdoor air in
IN is far more humid than 35%RH indoor air, and duty-cycling the
ventilation rates would be the right seasonal approach. To know when to
switch over, whenever the outdoor dew point as reported by local
weather stations exceeds 40F, or when the daily lows are 40F or higher,
you won't be able to hold the line at 35% @ 70F with ventilation- time
to flip over to duty-cycling.
Make sure they put R35 in writing, so you can file the complaint with the FTC.
Their website fudges it with terms like "Performance Values up to R-50" to avoid the legal violation of calling it an "R-value" of R50, which requires an ASTM C518 test on the representative assembly, which it would fail (unless there is 11.5-12" of total EPS.)
Unless it's a nighmarish mad-architect's frenzy with a gazillion bump outs & dormers with lots of corners and leak points, most IRC 2009
code min houses will have true heat load to conditioned space ratios of well under 15 BTU/foot, and well considered designs can come in under 10 BTU/ft. If the ICF has 2.5" of EPS on both sides, depending on siding type that comes in at a true R22-R23 steady state, but in most cases the thermal mass effect would knock ~8-12% off the peak heat load requirements, sometimes more. With an R22 wall the windows dominate the heat loads:
In Indiana (currently based on IRC 2006) code-max on windows is U0.35 if climate zone 5, U0.40 if in climate zone 4 (
find yourself on this map and report back), either of which would be a waste of a good ICF shell except for windows on the south side and calculated & adjusted carefully for optimal wintertime solar gain against unwanted summertime gains. U0.35 performance is less than R3, U0.40 is R2.5- a single 10 square foot U0.40 window loses more heat during the heating peak than is being drawn by a 10' x 10' section (100 square feet) of a 2.5" + 2.5" ICF wall. A more reasonable upper-bound for window U-factor for an ICF in that climate would be U0.25, U0.28 if you must, except where analyzed for optimal solar gain/rejection.
In any new construction some consideration should be given for roof pitches, site shading, and rooftop photovoltaic solar panels. In 2012 the median installed cost of rooftop PV in the US was ~$5.30/watt, but in many area it's bouncing around $4 watt, before any subsidy is applied. During the same period using the same panels, racks, and inverters Germans were paying less than $2.50/watt, a price point that should be arriving in the US in the next 5 years (it's not as if German labor is so cheap, eh?). At $2.50 watt the lifecycle cost of PV is below the residential retail price of electricty nearly everywhere, and where net-metering is allowed (most state it is) as well as third-party-ownership, you don't even have to buy them to reap the financial benefit. There are already more than a dozen 3rd party ownership model solar companies like Solar City, SunRun, Sun Edison etc competing for residential rooftop space in places like TX, CA, NJ, & MA, and as this market heats up and the prices undercut the residential retail price every where the lid's going to blow off, just as it has in high-cost electricity markets already, federal & state subsidies or not. But if you only have gables, not roof pitches with southern exposure, the ROI on PV would still be pretty low, even at $2/watt.