Green Building and Solar Panel Installation at Santa Fe Habitat for Humanity
Santa Fe Habitat pretty firmly believes in environmental responsibility, ranging from proper material disposal, watershed protection, and recycling of just about everything (even bent nails!) to building the most energy-efficient house possible. Under Construction Director Rob’s guidance the affiliate has been consistently moving towards building very tight houses in order to significantly decrease the amount of energy required to heat and cool the structure. In non-construction lingo this means that we seal as many joints as possible to prevent air leaks. Blower door tests (which are essentially pressure tests designed to measure air exchange) on the most recently constructed houses have reported scores of less than 1.0 for ACH50 ratings, which approaches the threshold for passive houses and is in general, excellent.
There seems to be a fairly lively debate in the home construction world as to whether or not or not tight houses are a good thing since limiting air exchange results in the build-up of poor-quality, even toxic, air indoors – not to mention the trapping of humidity which will lead to the growth of mold in most climates. Proponents of airtight houses point to the (much) lower costs for heating and cooling that reduces reliance on fossil fuels and electricity (which is frequently generated through the burning of national gas).
Santa Fe Habitat, like many builders constructing tight houses, uses an Energy Recovery Ventilation (ERV) to mitigate the affects of poor air quality; this system exchanges stale air, vented from bathrooms, (though pressure pulls the air through the house and out those specific vents) and replaces it with air pumped from the outside into other living spaces. During the exchange process, the two vents will actually condition the temperature of each other resulting in warmer air from indoors increasing the temperature of the cooler incoming air in winter and vice versa in summer. Differences in moisture between the air streams are addressed through the same conditioning system which keeps a fairly stable humidity in the house. Even with the energy required to run the fan/exchange system, these systems require only a fraction of electricity compared to a traditional HVAC unit.
My personal feelings on the airtight debate are evolving – meaning, I’m still learning – but I’m of the opinion that you have to go one way or the other: I think that incorporating too many elements of an airtight house without a ventilation system is half-assed – and downright dangerous in high-humidity areas – but that both a mechanically-ventilated tight house and a “regular” house that breathes through un-sealed joints (which are what most houses would be categorized as) are fine in almost all scenarios. I do think that extreme climates are better suited for tight houses however since greater benefits in energy savings are realized. Santa Fe, which at 7,000 ft elevation in an arid climate experiences daily 30-degree temperature swings and fluctuates between an average low of 17 degrees F and 86 degrees F throughout the year, can certainly benefit from tighter house construction, but I personally don’t think it’s strictly necessary for homes here. That being said, overall this method of construction has proven itself to be extremely beneficial to Habitat homeowners in Santa Fe, reducing their monthly energy bills and making these houses financially-sustainable, which I think is a wonderful thing – and an element of affordable house construction that is often overlooked. I am thus in agreement with the philosophy of making these houses as energy efficient as possible within the constraints of cost, both for the good of the environment and the benefit of the partner families.
Measures taken to reduce air leaks and increase the tightness of the house include the sealing of joint between the sheets of OSB sheathing on the exterior, the caulking of top and bottom plates of walls, and the foaming of potential gaps where the OSB is nailed over the studs. Basically, anywhere where 2 pieces of wood are joined or wood is joined to the concrete slab, we seal it with caulk or spray foam.
In conjunction with airtightness, Santa Fe Habitat also puts a lot of effort into insulating their houses in order to achieve the maximum energy-efficiency. The exterior walls of the houses are covered in 2-inch thick rigid foam insulation, as is the roof. The same blueboard is even placed under the slab before the concrete is poured to provide further insulation from the ground. In accordance with advanced framing techniques, the affiliate builds with more space between the studs in the wall as well (24” o.c with California corners and no trimmers vs. standard 16” o.c., for you construction geeks), which basically means less lumber and more room for blown insulation. At least 20” of insulation is blown above the ceiling into the space of the trusses as well. Both the airtightness and insulation have resulted in an overall excellent energy-efficency rating (HERS score of 36 on their most recent test) that has proven savings for the homeowners on their energy bills.
I don’t have a photo of caulking and foaming on the interior but here are some exterior shots of air sealing and rigid insulation installation:
Santa Fe Habitat has has its eye on installing solar panels on their homes since before I volunteered here as a Care-a-vanner last year but it wasn’t until just this September that the first permit for solar panels was issued by the power company. Purchasing them at cost and doing their own installation has allowed the affiliate to make the expense worth it for the homeowners overall by rolling the initial cost into the mortgage for the equivalent of a few dollars per month; the panels are expected to pay themselves off within a few years. With 8 panels per house generating a maximum of 285 watts, these panels should cover 100% of daylight hour electricity usage for the average Habitat homeowner and will send excess power to the grid, crediting their account and offsetting the cost of nighttime power usage. The solar initiative is thus another example of making these homes financially-sustainable for the partner families.
Though the affiliate unfortunately had to delay the incorporation of panels into construction, this did mean that Alexander, Callahan, and I to be involved in the first-ever installation which was pretty exciting (luckily the board has approved a plan to retrofit existing Habitat houses at cost and renegotiate mortgage payments for those homeowners who are interested). The physical installation of the panels – which is what the 3 of us have done – is simple enough though I understand the electrical end can sometimes be a bit more tricky (I haven’t been involved with the tie-in to the box).
Some basics of how our system works: Each panel has it’s own microinverter for converting DC to AC power which prevents the array from going down should one panel be shaded or have a malfunction. Each microinverter is harnessed together as a last step and a single line leads directly to the box. Though individual microinverters can be more costly, central or string inverters that convert power for multiple panels will cease to function should a single panel have an issue, much akin to the way a string of old-style Christmas lights would cease to light up if a single bulb blew out. This could result in potentially costly repairs for the homeowner. Generated power from each panel travels through the harness wire directly to box for immediate use and/or feeds into the grid – we do not use batteries to store energy both because it adds cost to the system and because it requires a different type of permit.
The installation itself is pretty straightforward: We first plan out the array and placement of the brackets/chairs (1) in reference to the orientation of the roof, (2) at the correct distance from the parapet wall to avoid shading, and (3) around the plumbing vents. We then attach rubber pads to each metal chair to help stabilize it and place the 2-4 bolts that will fasten it to the panel. This can actually be done on the ground at the same time as someone is making measurements for placement on the roof. Once we transfer everything to the roof the microinverters can be attached to each panel with careful attention paid to the distance of the inverter from the panel edge so as to not interfere with the chair. Then the panel can be placed on the chair and secured. Finally, we run a wire (harness) between the panels and place appropriate ballast on the chairs according to the truss load specifications. Installation took less than an hour the second time we did it.
Photos from our first installation:
Process on the installation on the second house:
Finally, Callahan set up her go pro to capture a time-lapse of our second installation! Check it out: