Archive for March, 2011

March Update

Progress was still choppy this month, but we did manage to accomplish a few items.

  • Framed and insulated half of the windows
  • Initiated contact with a few heating and ventilation contractors to start the bid process
  • Installed telephone line
  • Received all our approvals for the solar installation, but we’re still waiting on the building permit to come through. Once that is complete, we can schedule an install date for the solar system.
  • Dug the trench to the house and ran the power into the basement. We had to drill another hole in the foundation because we didn’t plan a big enough sleeve when we poured the foundation walls. But we were happy to get this in before the forecasted 6 inches of snow tonight.

The next big item is getting the siding up. It is currently taking up most of the first floor space. We need space to work before we start the plumbing, electrical and venting rough-ins.

Window framing and insulating detail

This is the story of how we ended up with our window box detail.

We decided early on to use advanced framing techniques and work on a 24″ module. In early drafts of the design I used window widths that fit between studs. The advantage to this approach is that you use less wood, smaller headers and fewer jack studs. Less wood means a bit less money spent on framing materials and fewer opportunities for thermal bridging and cold spots around openings in the envelope.

Sounds great in theory, but then I discovered how difficult it is to fit windows into a 24″ module in a small house. Aesthetically we wanted the windows to line up with doorways and the spaces inside and frame the view, and also make use of passive solar. Eventually I gave up on fitting narrow windows into the structural grid.

The tradeoff was that almost every window interrupted the path of the load, so the headers above the windows had to be beefed up and more jack studs were required. Adding more structure around the window opening means more bridging from the outside to the inside of the window box, short-circuiting the insulating value of the double-wall construction.

We also decided to use a rainscreen detail on the outside of the wall (see Final Energy Analysis and Recommendations), which pushes the 3 1/4″ thick window frames out a bit, leaving a deeper window box inside. This exposes 4 1/4″ of exterior structure (2×6′s, 1 king stud and 2 jack studs) around the window frame. Doing the math, 4.25″ x 1.25 R/inch = 5.3 R, which is not a lot compared to the 40 R of the double stud wall.

Even though there are 12 window openings in the 2 bearing walls (north and south facing) it is only a small amount of bridging. Doing the math again, we have roughly 863 sf of interior walled surface area on the north and south walls, and 29.5 sf of bridging area next to the windows. That is roughly 3.5% of the overall interior wall surface.

Our contractor, Warren, came up with several good ideas to minimize the cold spots created by the bridging. First, move the header up to the top plate to add some space and insulation between the header and the top of the window frame. Second, add a layer of 1″ rigid insulation around the inside of the window box, to which the drywall will be attached. This layer of insulation is small (R5) but it doubles the R-value at these locations and breaks the bridge between the outer structural wall and the interior window box.

Earlier this week Warren added the bevel framing and insulation for one window. It goes together quite well, but in hindsight I would have just insulated the problem areas, the 2 sides of the window next to the studs, rather than the entire window box.

This weekend we are working on the remaining windows. Luckily the sun is out this weekend which makes working in the house fairly easy even when the temps outside are still below freezing. The passive solar is already working!

Review by GBA

Green Building Advisor is one of our favorite sources of information on green building and today they published a nice review of our blog. Check it out, and all they have to offer.

Blog Review: Up Hill House

Solar Power Planning

To get to net zero energy use, we must produce enough energy to offset our energy usage over a year. We originally planned to live in the house a few years before installing photovoltaic (PV) panels on the roof, but after we ran the numbers we decided it made sense to do it now rather than wait.

First a little context. How much energy should we be trying to offset? Our energy consultants estimated our total site energy usage at 5,995 kW hours per year (kWh/y). Based on this, they recommended a 6 kW array which they estimated would produce roughly 6,000 kWh/y. At $0.14 kWh that’s roughly $840 per year total energy costs (not counting the Basic Service Charge of $16.21/month and misc. charges). That includes heating, cooling, hot water, well pump and plug load (it’s an all electric house).

It is important to note that we’re using site energy numbers rather than source. Source energy is the energy that the power company must generate in order to supply power to the site. Roughly 2/3 of this power is lost on it’s way to the site. Some net zero homes try to offset their usage based on source energy rather than site energy. Our estimated source energy usage is approximately 16,187 kWh/y. This would require a much larger array, more space and, of course, a much bigger wallet!

The next step was to determine if we have enough space on our roof to generate that much power. We already made sure that the roof was facing due south and pitched the the correct angle to maximize production over the year. The rule of thumb is to pitch the panels at the same angle as your latitude. Our latitude is 43 degrees and our roof pitch is 45 degrees. We also have no shading during the primary energy production hours (unless you count snow as shade.)

We have approximately 646 sf of south facing roof area.This is enough room for roughly 30 panels, 3 rows of 10. One of the most popular panels being used right now is a 230 watt panel. 30 panels x 230 watts = 6,900 watts or 6.9 kW. This is DC power. Factoring in some losses converting DC to AC, and the solar potential for our geographic location, this equals roughly 7,000 to 8,000 kWh/y AC.

This is more energy than we require, but it makes good use of our available area and gives us some extra capacity for a future barn and electric UTV. I must note that over-producing is not necessarily economically beneficial for a homeowner. Any excess we produce one month is subtracted from our next bill, dollar for dollar. This is good for offsetting usage, but if we produce more than we use over a period of a year, the power company is only obliged to offer us payment at avoided cost. Defining avoided cost is for lawyers, but it’s safe to assume it means pennies on the dollar. (If you know how to calculate the avoided cost and where to get the data, please let me know in the comments!)

I originally estimated the system would cost in the range of $20k just for the panels and inverter, not counting all the other wiring, roof clips and rails, inspections and labor. I figured design and installation costs could double that number. I knew there were state and federal tax credits but I was surprised to find out how much federal stimulus dollars are available. In New York, NYSERDA administers this money, and rebates roughly a third of the cost of a solar PV system (including labor). I pay taxes, so it’s nice to get a little something extra back.

Th rebate brought the total cost of ownership down to a level that made sense to go forward with the installation now rather than wait. I called a few solar installers and settled on GroSolar. (GroSolar recently sold the residential part of their business to SolarCity.) Once the rebate and tax credits are figured in, the entire 6.9 kW system is less than $15k. Assuming we just count the $840 calculated above as saving per year, then the system would pay for itself in roughly 18 years, assuming electricity stays at $0.14 kWh.

We are scheduled to get our system installed in April. I’ll post again when I have more details on all the components of the system.

Mechanicals (ASHP & ERV)

Click image for 3 page PDF (80k)

With little going on at the house due to the weather, I’ve spent some time considering the systems and ducting required for the house. I put together an updated set of plans and specifications to start talking to a few heating contractors.

Click image for 4 page PDF (1Mb)

Our energy consultants recommended a mini-split air source heat pump and two approaches to selecting a model. We could either go with a model that is less efficient at lower temperatures and supplement the heat with electric resistance when temperatures fall below 5 degrees. Or go with a larger unit that will meet all our heating needs at lower temperatures but is less efficient overall. I am inclined to go with the smaller more efficient unit that will meet our needs for the bulk of the heating season.

I only wish the makers of the mini-split units would hire a real designer. Their units are u-g-l-y. I’ve tried to minimize the visual impact of the indoor unit, but it’s still a big plastic box on the wall. Thankfully the mini-split units only require a small refrigerant line connecting the indoor and outdoor units and no ducting.

We also discussed the ERV versus HRV question with the energy guys. It was actually quite easy. From everything I’ve read, the most important factor is low energy use and efficiency, not which type of system. Right now that still seems to be the UltimateAir RecoupAerator ERV. In our cold climate it will nice to recover some humidity in the winter. The unit will be off and the windows open for most of the summer.

But ventilation requires ducting. This was the main reason we decided to use open-web trusses, to make it easier to install all the ducting inside the conditioned envelope. It has also been a challenge to keep all the ducts out of exterior walls.

I attempted to model the ducting as realistically as I could (see 2nd PDF above), taking into account the location of walls, floors, web trusses and plumbing. I have not found many good resources online for designing duct runs, nor have I found a good online catalog of various types and sizes of ducts for small residential ventilation systems.

I’m sure any mechanical contractor eye’s would glaze over looking at these models, but it was a good exercise for me. I’m now armed with lots of questions when interviewing heating contractors, and it will be interesting to see if I’m even close.

There are a few difficult areas: 1. ) Getting the unit in the basement vented to the outdoors without a lot of 90 degree bends. 2.) One of the bathroom walls is located above a truss making it difficult to get ducts into the wall. 3.) A return duct for one of the bedrooms must either cross over a supply duct or go up and over the stair in a chase that is still inside the air barrier. 4.) About the time I started wrapping this up into a post, I read that it will soon be required to place the in/out-take vents at least 4 feet above grade. (See The Energy Star Homes Program Raises the Bar with Version 3, by Martin Holladay at GreenBuildingAdvisor.com) It’s a good idea considering the amount of snow we’ve received this year.

I will revise the drawings after we choose a heating and ventilation contractor.


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