Digging Myself A Hole...for a Passive House!

We have had relatively good weather.  This week has been above 0 deg C and the temperatures are climbing.  For the most part all the snow is gone and the ground has thawed.  The switch in weather has made it easy to get some work done.  This was a fairly busy week.  The excavation company, Wade's Excavating, dug down to solid ground. There was an amazing amount of topsoil on the lot.  In places, there was almost 2'.  It has all been pushed out of the way for now waiting to be moved around and used for final grades after backfilling.    After the hole was dug we hired Aubrey Burt (land surveyor) to come and roughly make out for the foundation pad.

Have a look at the pictures below.  The first picture is looking approximately west, as is the second.  With a minimal structural pad in place we will be above natural grade in the far north west corner of the lot (i.e. picture 2, the stake to the far right in the corner of the dug out.).  If you look at picture 3  below, you will see theres a bit of a pit.  We knew we would require fill there....and we need a lot!   That small round silver object over the stake, pushed into the dirt pile, is a spray can.  it represents where the bottom of the footing will be in the north west corner of the lot.  So it seems that using a 4' frost wall everywhere will not be an option anymore otherwise back filling to the top of the wall at the back of the lot will turn the lot into a ski hill.  Our plan is a stepped frost wall of varying depth from 8' to 6' to 4' from the low to high parts of the lot with backfilling 4' up the wall and follow the natural grade of the lot.  We may do siding part way down the insulated concrete or just leave it with the concrete parging over the foam.  Behind the garage will be nice and sunny and I have been thinking about using that area for my solar kiln for drying green wood for woodworking and firewood.  So leaving the foundation exposed there may not be a big deal.







Comments

  1. Thinking about your under slab as a heat source.....or If I were to build again, if I would consider it.
    As you know, a minisplit is a much lower cost option that ground source geothermal. Low ambient air temperature and frosting of the coils take away from minisplit performance for a part of the year, while for the shoulder seasons minisplits will give a better COP. I have pondered how to rise the input temperature to the minisplit during worst ambient conditions, beyond the exing attic set up.......and just a few degrees makes a big difference.
    So what if one used both........a small geothermal with plastic pipe under the slab for intermittent use. Full time use would chill the ground there too much, and the soil is a slow conductor, but would regain if not severely loaded. I think the temperature under your slab will be not less than 50F in winter, or certainly mid to high 40s, and better than if a dug well was used.
    Being a small ground source HP, and no well digging, the cost would not be great, and one could optimize air and ground with the dual system.
    I know you are a wood guy, and that is good, but as a contractor, getting customers, perhaps only one in a hundred would want wood over no fuss efficient electric. Then again, after DARKNL, no fuss electric may be a thing of the past, and your wood can sure be a great asset with the power lines down for 2 or 3 weeks.
    Yesterday , not a good solar day, nevertheless, it appears a 22 percent reduction from attic mount beyond what the HP normally provides. Calculations show 8.88kwh for heat reduced from solar gain to 6.89 for the full day.A good sunny day would do better.
    I can see attic thermal mass beneficial effect carry over to about 4 am, and considering adding one or two drums of water into the attic, to see if this mass extents it further, or gives insight to further thermal mass considerations. Incremental improvements add up.
    Kind of wish I was starting from scratch like you on a new house......maybe yet to come.
    Enjoy following your progress, and insights.
    To your question of attic mount HP for AC purpose. Not a problem for my installation, and used only about 2 weeks of the summer. Used in dehumidification mode, low speed , low energy. The attic cools some once in operation, taking air through. If in a warm area, a duct from the soffit to the HP intake may be used to bypass the excessive attic heat in summer. Most of Nfld in coastal areas would not need the duct, but inland may likely benefit from that, or exceed the unit operating temperature, or decrease its AC efficiency some.
    I wonder if your HP water heater will be an issue for too low RH in the house.in winter......my cottage when not occupied and the heat on, drops below 20 RH and takes a day to get it back up once occupied and a humidifier on.
    Looking at small ultrasonic humidifier, very low energy use, but not sure it is a good choice.

    Winston

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  2. the under slab as a heat source is something that is often used in Passive Houses. The Zehnder lines of HRVs and others (which don't use recirculation to defrost) often use a passive geothermal loop. Its basically a low wattage pump that circulates glycol through about 300 ft of pipe. the fluid passes through a heat exchanger. Air being pulled into the home through the HRV passes through the exchanger and is preheated in winter...in summer the same system can be used to pre-cool incoming air, or used for cooling after air passes through the HRV core. These systems are relatively expensive and the payback is long but in a passive house, it is about all the cooling you would ever need...especially in newfoundland. The big problem with these systems is that with large temperature swings in the shoulder seasons, they need good controls linked to exterior conditions in order to behave properly. In the warming season they are not that useful for systems that have recirculation mode and in fact can lower the efficiency of the HRV...in some cases. I would think that for newfoundland, using it for post cooling would work. Our cooling season is short so I am not sure about the benefit. You could run a pex line around the foundation footer (since you've already paid for the digging and will be backfilled anyways, hook it up to a small ecm pump activated by a simple toggle switch for hot days. The peak cooling demand of the house is only about 0.4 tons (I think). which is tiny and would require a loop of less than 300' according to my calculations. hooked up to a heat exchanger coupled to the hrv is all the cooling that would be required.

    Earth tubes are simple and passive also but they can introduce air quality problems.

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