Posted by: Scott McCullough | August 2, 2011

Energy Efficiency and Renewables

Although we’re enthusiastic about being energy efficient and environmentally conscious, we have a limit to how far we’re willing to go. The primary reason being up front cost. We are aware of the basic premise that if we don’t pay for our energy now we’ll pay for it later. How thick should we do our insulation? What type of heating system should we do?  At this point we have it all planned for, and I think we’re doing the right thing for us.

Since we are doing a timber frame, the rigid insulation shell has very few thermal breaks.  We realize that our shell’s R-Value should carry more weight than the same R-Value in a conventionally framed house.  Right now we are planning on 5 1/2″ thick walls for R-27.5, and a 9″ thick roof for R-45.  This may change depending on the type of rigid that we choose in the end, since these numbers currently reflect DOW’s Styrofoam product.

Our foundation wall  insulation is something that we’ve never seen used before.  It’s a system called Thermomass, where the rigid insulation is in the middle of the poured foundation wall.  It has been used in the past more with commercial buildings and is now gaining more popularity in the residential sector.  We like the idea that we get continuous insulation to the mud sill without needing to deal with covering the insulation on either the inside or outside of the concrete wall, which is what is typically done.

Our basic concept for the heating system is to rely on a wood stove in the Living Room for the coldest months, but have a backup system that would keep the house at around 55 degrees when we’re not home.  Since we have a fairly large parcel of land we wouldn’t have any problem keeping a steady supply of firewood, without being detrimental to the quality of the forest.  The backup system to the wood stove is a four panel solar thermal collector system on the roof that supplies domestic hot water and a portion of the radiant slab heating on the first floor.  In conjunction with the solar thermal there will also be a 3.5 +/- kw grid tied photovoltaic system on the roof.  This will offset the electrical load need for our backup boiler, and also supply power for our lighting and appliances.  We understand that it really doesn’t supply power directly to the house, but sends it to the grid where we get credit for it on our NH Electric Coop account.  We are planning on having an HRV in the basement that will provide the house with fresh air.  It will be interesting shopping for a wood stove, as I am learning that it should definitely have an independent  fresh air intake to prevent back-drafting.

Back in June I had Mitch Sidd from HB Energy Solutions come out to our site and do a quick solar analysis.  Mitch used a device called a solar pathfinder to help him determine what trees would need to be cut down in order to get a year round sun exposure on our solar panels.  Turns out that we only needed to take down eight or nine more trees, which we could do at a later date.

As you can see in the image the pathfinder is made up of a reflective globe and a sun diagram.  Everything above the horizon is reflected onto the globe, and with the diagram below we could tell exactly which trees would need to be cut in order to get full sun on the solar panels.  The pathfinder is of course sitting on the ground, Mitch used his judgement to determine how the exposure would change if it were on the roof.

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Responses

  1. A few things to keep in mind about the “tightness” of your home and your plans for a wood burning stove.

    Putting a wood burning stove in a super tight home just doesn’t work. Well, at least it doesn’t work efficiently, nor as you desire… Without trying to insult your intelligence, a fire needs two things, fuel (wood in this case) and air (or better stated oxygen). You will need to take into account the sizing of the opening of your “exhaust” chimney flue and determine the required “intake” (depends on your stove) needed to keep a flame going in CFM’s. Once you can determine that number you will be able to figure out ways to bring that needed CFM into the stove. It’s sort of a backwards way of thinking about heating, ie. pulling in cold winter air to keep the fire going, which in turn heats the space… Your choice of an HRV is a wise one, since it will “preheat” the incoming air. Be sure to confirm that it’s CFM capacity and exhaust locations meet the demands of your stove. Another cool device I’ve found is this: http://www.hilkoil.com/

    For our home, we had planned on installing a wood burning stove ourselves and using the Hilkoil water heating loop tied into our solar, water preheat system (if you want more info on DIY solar water preheat systems let me know) as an additional cost effective supplement for the radiant slab; however, due to personal, fire burning safety concerns (we lost 1/3 of the house I grew up in due to an accidental fire caused by un-disposed ashes from a wood burning stove) as well as the additional expense of a masonry chimney, we decided against it. As a side note, the manual labor and supply of wood was not a concern for us.

    Also, there is a danger of the fire back drafting into the house (which I’ve experienced first hand). It’s all about air pressure levels and wind currents/ventilation characteristics. Each design is unique, and research can only get you so far, so please, please, please, as a caring friend, hire someone well versed in tight homes with fire places to help in your design!

    Also of note is that over-insulating has a certain diminishing level of returns. There comes a point where your wasting your efforts/money putting in more insulation simply to raise R-values. With that said, I believe your design choice will work well. Make sure you use air sealing techniques EVERYWHERE. Seal ALL penetrations, tape ALL joints in the rigid foam, spray foam ALL gaps/holes you find and get a blower door test down to find areas you’ve missed!

    I don’t want this to sound like a design lesson, since you both and I went to the same design school and have all been practicing architecture for years now but please take this as, partially my opinion, partially my experience, partially based upon my own home design research and finally, partially to help me consolidate some facts for my own use!

    Let me give you a rather crude example:

    My father lives in a farm house built in 1888 that has hand built, single-pane windows and the walls are rough, hand sawn small timbers with plank exterior “sheathing” and hand sawn clap-board siding (notice no air/moisture barrier nor any insulation?). The inside is lathe and plaster. His foundation is hand placed large stone, rubble and hand packed dirt (for lack of a better word). Although you can stand up in the “basement” there is no slab and no insulation. There’s no concrete and no footings (the walls splay out). His main heating system, original to the home, is a small open flame (now with a retrofitted glass front and flue control system) fireplace centrally located within the structure and positioned between the living room and kitchen. He has said that during even the very coldest Maine winter days, when the sun is shining he can sit on the couch and not need to have the fireplace going due to the solar gain he’s getting thru those thin (uninsulated) walls and single-pane (no-E, hahaha) windows. It creates a “green-house effect”. Since his house is so “airy” and uncomfortably drafty, his fireplace works beautifully. Wood stoves get SUPER hot and controlling the output is marginally handled. I admire your dedication to this planet, have enjoyed every word you’ve typed up so far and look forward in discussions with you about design! Later!


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