Posted By WannaBuildGreen on 26 Nov 2018 04:53 PM
We bought a new house a couple years ago. We love everything about it: location, layout, finish, etc. Only thing keeping it from being absolutely perfect is that it's code-min: traditional 2x6 stick framing, fiberglass bat insulation, forced-air HVAC (natural gas heat). It's generally fairly comfortable, but there are cold spots when the outside temperature gets to the low teens or colder (climate zone 5a, western Chicago suburb). One room in particular is pretty bad: it's above the (unconditioned) garage, and has no attic space above it (i.e. just roof).
I'm just wondering what, if any, "easy wins" I might look into in terms of improving energy-efficiency and comfort of this house. As the house is only a couple years old, a major retrofit is out of the question. But, for example, adding some ceiling insulation to the garage might help with the room mentioned above. Also, the upstairs HVAC ductwork runs through the (unconditioned) attic, which obviously reduces the efficiency of the upstairs furnace---maybe there is some way to improve this situation?
Or, how painful is it to remove siding and install some rigid foam to increase insulation and mitigate the thermal bridging of the studs? We have concrete fiberboard (like Hardie board) siding.
Thanks for any thoughts or suggestions!
Pulling the siding for a foam-over is a big project with lots of ways to screw it up. It's much easier to do it correctly in new construction. This falls under the "major retrofit" category.
The biggest bang/buck on energy use is usually blower-door & IR imaging directed air sealing, followed by insulation. Even though the IRC calls out maximum air leakage of 3 air changes per hour at 50 pascals, I suspect most homes in IL are never actually commissioned with a blower door test.
An IRC 2012 or later code-min floor over an unconditioned garage would be R30, or a full cavity fill, which ever is less. If the joist bays just have some crappy R19s flopped in there, air sealing & filling it with blown cellulose is a reasonable DIY. But it won't necessarily fix your comfort issues with that room. The heat loss characteristics of rooms over unconditioned garages are very different from those of house over an unheated basement, or a slab on grade. Sometimes it makes sense to micro-zone those rooms with a hydronic heating loop (baseboard convectors or a thin profile wall coil) operating off the water heater, but do what you can on the air sealing & insulation front first. This would be a substantial, but not "major" retrofit, but within the skill sets of many DIYers.
Duct leakage and duct imbalance are also major comfort & efficiency hits. All seams and joints in the duct system need to be well sealed (duct mastic is your friend), as well as caulking the register boots to the subfloor/wallboard. All doored-off rooms with supply registers need a dedicated return register or jump duct to guarantee air flow, and to keep the air handler from pressurizing the room relative to the outside, making "the great outdoors" part of the return path. To commission an Energy Star ducted HVAC system the room to room pressure differences have to measure less than 3 pascals (0.012" water column) under all operating conditions, doors open/closed, all air handler speeds. If there are a few suspect cold room with no return registers, it's fairly easy to turn a partition wall's stud bay into a jump duct return, installing a grille on the stud bay inside the room near the floor, and near the ceiling on the opposite side.
The ducts in the attic are always a mistake, but fixing that isn't always easy or cheap- could be a semi-major retrofit to get it right. Are they flex or hard-piped? Is the furnace up there too? By "...efficiency of the upstairs furnace..." are you implying there are more than one furnace in this house?
Even some of the smallest gas furnaces can be oversized for normal sized houses, and large oversize factors contribute to the comfort issues. ASHRAE recommends 1.4x oversizing (max) for the heat load at the 99th percentile temperature bin (the "99% outside design temperature"), which for the western 'burbs if Chi-town is about 0F (it's -1F at O'Hare and Aurora, 0F at Midway.) Yes, it does get colder than that, but only 1% of all hours over the past 25 years are colder than that. At 1.4x oversizing you're covered capacity-wise for Polar Vortex coolth, but during normal cold weather the furnace will run very long, comfortable cycles, at a duty-cycle more than 50%. An oversized furnace delivers the hot-flash, satisfying the thermostat in 10-15 minutes followed by the long chill, which is the opposite of "comfort". The rooms a the end of a long duct runs may not see a high enough duty cycle to ever fully warm up if the furnace is 3-4x oversized, whereas it would if the furnace is right-sized.
If you have a heating history on the place, using only winter period gas bills run a fuel-use based load calculation, which will give you some idea of your 99% load and oversizing factor. The methodology for running those numbers lives here:
http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
Compare your heat load numbers to the nameplate BTU output of your furnace(s). It's a sad state of the industry that most are ridiculously oversized for the actual heating loads, which leads to more noise, more room temperature cycling, lower comfort.