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Dear Jamie

March 17th, 2009

Jamie Wolfe, skilled contractor, devoted NESEA member and one of the most nimble minds I know, was curious about my previous post on frost-protected slabs.  He wrote the following:

I’d like to know more about the building envelope. I did some presumptive math (1/4 cord = 5MBTU/768SF=4557BTU/SF/YR) – pretty impressive.

So these questions:
What did you model the heat loss at?
Did you test ACH?
What about ventilation?
What’s the rest of the energy profile?

OK – I guess I’m asking for that second post!

Jamie, our little building isn’t all that impressive.  Actually, as modeled and so far this first season, it’s looking more like 5500 Btu/ft2-year, which is still pretty good.  NESEA members are achieving similar results in projects far more complicated than this one.  I’ve had the added advantage of working for myself: I only had to answer to myself and to the data, and by using my own labor, I could afford a premium building envelope.  But perhaps a quick sketch of this project would offer a few tips, so here goes.



 

The office has a simple plan and a simple form, keeping costs down and detailing simpler.  It lies on an east-west axis, the south face within 15 degrees of true south.  It’s heavily glazed to the south (8.33% of total floor area), lightly glazed to the east (2.6%) and west (2.6%), and no glazing on the north.  The layout is simple, but compared to we were working in before, the space seems luxurious to us.

 

 

 

 

 

 



 

Of course buildings last longer than we’ll likely be in practice.  So the design is adaptable, to make a small guest cottage or rental unit someday.

 

 

 

 

 

 

For simple structures like this, we use ENERGY-10 for energy modeling.  (We’re in the process of adding REM Design to our toolkit.)  ENERGY-10 begins with a base-case model and compares the effectiveness of various upgrade strategies.  For our base-case model, we assumed the following (all in Imperial units):

• Concrete slab foundation with R 7.5 beneath and R10 at the slab edge;
• 2 by 6 walls with R-19 insulation;
• 2 by 8 roof with R30 insulation;
• better-than-average air tightness (Effective Leakage Area about 37 square inches);
• Marvin “Integrity” windows with double-pane low-e w/ argon glazing, u-value 0.33, Solar Heat Gain Coefficient 0.30.

We modeled and explored a series of upgrades before settling on our first set of strategies:

• Under-slab insulation increased to R15 and slab edge insulation increased to about R25;
• optimum-value-engineered framing to reduce wood use and thermal bridging;
• a wall system with a whole-assembly R-value of about 33;
• a roof system with a whole-assembly R-value of about 45;
• a robust, continuous air barrier and other air-sealing measures to lower Effective Leakage Area to 9 square inches;
• energy-recovery Ventilation with an assumed efficiency (60%) and a schedule equalling 0.2 “natural” air changes per hour.
• efficient lighting, “Energy Star” equipment and other measures to reduce plug loads.



 

 

The model showed significant differences between these cases: a 60% reduction in heating loads (from 25,500 Btu/ft2-year to 10,200) and a 59.5% decrease in overall energy use.

 

 

 

 



I felt I could easily pay for and justify all these upgrades, especially by doing the work myself.  The next upgrade needed more thought: a significant increase in the window budget for superb Canadian units: “Thermotech” windows from Ontario, with insulated, pultruded-fiberglass frames and triple, low-e, argon glazing achieving the remarkable combination of a u-value of 0.16 and a Solar Heat Gain Coefficient of 0.42.  Marvin “Integrity” windows are an excellent product, the Thermotech upgrade was expensive, the return on investment was low even if we burned propane, even lower since we burn wood… but heck, this is what we do and believe in, and if we charged ourselves the full cost for fossil energy and climate disruption, it wouldn’t even be close.  We bought the Thermotech windows.

 



 

 

With the great windows, our model showed our energy use at 5500 Btu/ft2-year for space heating and 17,000 Btu/ft2-year for overall energy use.

 

 

 

But as Henry Gifford, Chris Benedict, Marc Rosenbaum and others remind us, it’s one thing to model low energy use and another thing to achieve it.  Here’s how I got dirt under my fingernails.  I fully expect that many of you will suggest smarter ways to do what I was attempting – that’s really the point of the NESEA blog, for us to put stuff out there in hopes of stimulating discussion and thought.



 

I made a “load-bearing Larsen truss.”  This is a ladder truss fully capable of bearing the structural loads imposed by this little building.  I made a jig from 3/4-inch plywood, allowing me to place two 2-by-3’s in parallel, 7 1/4 inches from edge to edge.  I used 1/2-inch OSB cleats top, bottom and at the third points, nailed with galvanized 1 1/4” roofing nails.  I made similar assemblies, with full OSB covering, for top and bottom plates.  I framed these “studs” 24 inches on center.  I made rough openings 1 inch larger than called for, then closed them in with 1/2” OSB.  There’s a 2 1/4” gap in these assemblies into which insulation can be installed, vastly lowering thermal bridging.  The framing members are straight and consistent, they use small sticks, and they took surprisingly little time to fabricate.



I wanted additional R-value, additional diminishment of thermal bridging, and a robust air barrier.  I decided that a relatively-thin structural insulated panel (SIP) would do the trick nicely.  It’s possible to buy SIPS with 1 to 2 inches of foam in them, plenty for my needs.  But I had a few concerns.  First, it was much less expensive for me just to buy sheathing and foam, and install my own “sandwich.”  Also, I was willing to forgo a little foam in areas where solid nailing is critical, such as bottom of wall, corners, and around window and door openings.  And we live in a very windy spot near the Maine coast, so I wanted adequate lateral bracing.  So I used a first layer of 7/16” OSB, nailed with 8d galvanized ring-shank nails.  I placed 2 by 3’s or 2 by 4’s on the flat at those critical areas.  Elsewhere, I placed extruded polystyrene foam, 1 1/2 inches thick, taking care to seal all seams and gaps.  Then I placed an outer layer of 1/2” “Advantech” sheathing, nailed with 16d galvanized nails.  This outer layer of sheathing wouldn’t be necessary if I were using a clapboard or plank type of siding, but I was using cedar shingles (FSC certified, grown in Maine and milled in Quebec), and I needed a nail-base for them.  Otherwise, vertical furring strips would be fine.

 

 



 

The roof is a short span, so 2 by 8’s, 2 feet on center, worked fine structurally.  I added a 1 1/2” by 1 1/2” strip of extruded foam to the underside of each rafter, then placed a 2×6 beneath that, held to the 2 by 8’s with OSB cleats.  I placed 1-inch “Thermax” foil-face, polyisocyanurate insulation in each rafter cavity, on 1-inch foam spacers glued to the underside of the roof sheathing.  This space provides a channel for roof venting that is probably unnecessary – unvented roofs work fine if they’re done right – but done anyway with an abundance of caution.  The air-gap and foil facing may also slightly reduce summertime heat gain from radiant heat transfer from the shingles.  I was very careful to seal the “Thermax” at the bottom of each roof cavity, to provide strong coupling to the foam on the walls, thus helping maintain continuity of the air barrier.  The rest of the rafter cavities was for dense-packed fiberglass, and there’s an R-7.5 insulator at each rafter assembly.

 

 



 

Like many others, I was trained to believe that dense-packed cellulose or fiberglass, installed properly in a well-built assembly, can be an air barrier.  I still think that’s possible, but harder to do well than we think.  I treated the air barrier and the thermal insulation as separate assemblies – except that my foam air barrier also helps reduce thermal bridging.  I also painstakingly sealed every hole, gap, penetration, etc., that I could find.  Fred Unger says that when it comes to buildings and water, we should “think like a drip.”  I’d add that when you think about air movement in a building, “think like smoke.”  Since the air barrier is separate from insulation, I tested its effectiveness before installing dense-packed fiberglass insulation.  The results were excellent.  The area of the building shell is 1762.67 square feet.  The volume within that barrier is 5256 cubic feet.  Our blower-door test, at 50 Pascals of pressure, averaged over 2 minutes, indicated leakage of 85 cubic feet per minute.  This translates to 0.97 air changes per hour at 50 Pascals, or 0.046 air changes per hour “natural.”  It’s an effective leakage area of 9 square inches – what we calculated in our model.  At 50 Pascals of pressure, the air leakage is 0.048 cubic feet per minute per square foot of building shell.  During the test, we located a few sources of leakage and fixed them.

 

(On a side note, I first reported these results to Marc Rosenbaum – to my horror, I had made an arithmetic mistake and grossly overestimated the building’s volume.  I’m usually careful with arithmetic, so a mistake like this is embarrassing, but when it happened with Marc, one of my heroes and a paragon of careful number-crunching, it was mortifying.  New frontiers in “geek shame.”)



 

So we had excellent air-tightness before most of the insulation, but we still needed the insulation.  The great folks from R.H. Price Insulation, of Montville, Maine showed up and did an excellent job filling all the wall and roof cavities with dense-packed fiberglass.  (Cellulose would be another great choice, but Bob Price’s crew does fiberglass, and in this kind of application we haven’t seen any difference in performance.  Loose-fill in an attic is another story, where we think cellulose is superior.)  Bob’s crew installed a vapor retarder called “MemBrain.”  This is a polyamide plastic that changes permeability in response to difference in vapor pressure between an interior space and an insulated cavity – in other words, it can block vapor transfer into the cavity, or allow drying to the interior by vapor transfer out of the cavity.

 

 



 

With a building this tight, it was obvious that we would need mechanical ventilation.  I bought and installed a “RenewAire” energy-recovery ventilator.  We like these units: they recover sensible heat and humidity, they don’t need a condensate drain or a defrost cycle, and they are relatively efficient in the use of electricity.  We control the ERV with a programmable-schedule electrical outlet and a “run-time-percentage” controller.  I took great care with the installation, keeping duct runs short and well-sealed, and installing the best backdraft dampers I could find.

 

 

 

 

The ERV is half of the “mechanical system.”  The other half is a very small, well-crafted wood stove by “Mørso” of Denmark.  This is their model 1440.  It’s a convection stove: cast-iron baffles at the sides allow air flow there.  This keeps the sides cool enough to touch when the stove is firing, and lowers the side clearance needed to just 8 inches.  Unfortunately, it’s not available with a direct outside-air feed – I don’t know of any small wood stove on the market that does offer one.  I compensated by located a fresh-air supply from the ERV in the ceiling directly over the stove.  Even with that, it takes some care and experience to manage the wood burning properly.

 

 

 

 

 

 

 

Such a little stove requires little sticks of wood.  Our ENERGY10 model had indicated 5500 Btu/ft2-year annually for space heating, or a total of 4,224,000 Btu.  If you assume that the wood stove efficiency is about 65% then you’ll need about 6.5 million Btu worth of wood.  Seasoned mixed hardwood contains about 24,000,000 Btu, so our space heating needs are just over a quarter of a cord of mixed hardwood.  (This assumes that we’re keeping the upper level at the same temperature as the main floor, which we don’t, so our actual fuel need may be less.)  We need so little firewood, and it has to be in such small chunks, that I wondered if I could provide our fuel without cutting down any trees.



 

I could.  There are techniques called “coppicing” and “pollarding.”  Coppicing is cutting shoots of woody plants that sprout back.  Pollarding is cutting selected branches from trees that re-grow those branches.  In my case, I did the coppicing on Black Alders and the pollarding on Dutch Timber Willows that I planted years ago.  I’m so happy with the results that I took cuttings from my willows, sprouted them, and planted some more.

 

 

 

Alder contains 17.2 million Btu per cord, and willow only 14.5 million.  Since I mixed them roughly fifty-fifty, my fuel only contains an average of 15.85 million Btu per cord, 34 percent less than mixed hardwood.  With this fuel, we need at least 0.4 cord.  Therefore, I played it safe and cut about 1/2 cord of willow and alder.  I also bought a small supply of compressed-sawdust bricks called “Biobricks” as an insurance policy.  I calculate that these provide about 15,000 Btu each.  We’ve had a colder-than-average winter, but as of March 15, we’ve used about 80 “Biobricks,” or about 1.2 million Btu, plus about 1/4 cord of our “junky” firewood, or about 4 million Btu, for a total of 5.2 million Btu.  We rarely need to make fires now, with the moderating temperatures and strengthening sunlight.  It looks like we’ll meet or beat our calculated heating energy.

We’re delighted with our new little building.  We’re proud of its performance, but know that with hindsight we could have done a little better, that it’s much, much harder to pull off this kind of performance in more-complex projects, and that astute readers of this post will offer all kinds of comments and suggestions that we haven’t thought of.  Now we just need to start saving funds for an eventual grid-tied photovoltaic system to offset the 3 kilowatt-hours a day we use for electricity.

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Food Forests

February 17th, 2009

 



“Wenn ich wusste, dass die Welt morgen untergeht, würde ich dennoch heute einen Apfelbaum pflanzen”
(Translation: “Even if I should learn that the world would end tomorrow, I would still plant this apple tree today.”)
– Martin Luther

If we’’re to move toward sustainability, we’ll need an elegant economy of effort. Our most daunting problems are linked and planetary, but many of the solutions will be crafted, piecemeal and patiently, in our households, neighborhoods, watersheds and bioregions.

Alex Steffen summed up our predicament well:

…We each get 1.9 hectares, and we’re already using 2.3. Where’s the extra half a hectare coming from? It’s coming from nature’s capital… (Yet) just as a fair and sustainable footprint is a receding goal (shrinking as we use up more and more nature), so too is the idea of prosperity…

We’’re in ecological overshoot, but to have any hope of solving that problem, we must ensure a sufficient, decent and secure life to everyone.

What a great design problem!

Here’’s one piece of the puzzle, something many folks can do: plant trees which provide food. For better or worse, we all have to learn tend the Earth like a garden now. “Food Forests ” are one important way to do that.

The idea is simple in theory, rich and complex in practice: mimic a successional forest, using trees, shrubs, ground covers, herbs, fungi and roots that reinforce one another, enhance ecological health, and yield food, fiber, fuel, medicine and habitat for people.

Crops now cover an area about the size of South America . We’’re becoming increasingly aware of the damage our agriculture causes, and the benefits of more enlightened practices. Although we have obtained food from trees for millennia, our main practice has been to farm surfaces – now we need to farm in three dimensions, stacking crops in layers, from canopy to root zone.

Forest gardens and forest farms can be made at many scales, from urban backyard to whole countries, and in many climates, from tropical to arid to temperate.

Forest gardens can be a vital part of Green Cities, renewing the health and vigor of their scattered patches of vacant land. Forest gardens can shorten the journey from farm to table, help cool urban heat islands (where the effects of climate change are often worst), and even help decontaminate polluted soils through “mycoremediation”.

Where people are desperate, tree crops bring hope. We’’re learning how to plant forests that are worth more to people standing than cut down. Examples are all around us: many are aware of the Green Belt Movement founded by Wangari Maathai, but similar efforts are everywhere, often below the radar screens of media. One of my favorites is the work done by my friend Carol Kinsey through her organization Seed Tree.

Forest farms and gardens can serve the planet, but are necessarily place-based. Sources for edible trees, shrubs, herbs and even mushrooms are fitted to particular biomes. Every region has its heritage of edible tree crops. Like people, every cultivar has its own personality and needs.

Cultivating trees teaches patience. It’’s a work spanning years, decades and centuries, whether developing management plans, learning coppice rotations, or breeding disease-resistant varieties, such as the American Chestnut. Trees teach humility too. Losing an annual crop is difficult; losing an orchard is heartbreaking. Knowing that trees absorb carbon is heartening; realizing that trees alone can’t halt climate change keeps us honest.

In short, Edible Forests are a “Great Hope and Many Little Hopes.” The quote is from J. Russell Smith, whose book, Tree Crops: A Permanent Agriculture, first published in 1929, is perhaps the classic text on this subject.

Instruction abounds. There are many books, including recent, comprehensive instruction manuals, and online resources. But the best way to learn is to do: happy planting!

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Using Insulating Concrete Forms To Make A Frost-Protected Slab Foundation

February 13th, 2009

I devoted spare time this past year to building a new small office for our architecture practice. I thought of a simple way to use insulating concrete forms (ICF’s) to make a frost-protected, shallow slab foundation. It worked out well, so I’m sharing the idea, with hopes that other folks will improve upon it.

Here in Maine, foundation walls typically extend 4 to 5 feet (122 to 152 cm.) below finish grade to be safe from freezing soil. That uses a lot of concrete, an energy-intensive, carbon-intensive building material. Don’t get me wrong, concrete is great: plastic and cast-able when wet, strong and durable when cured. Treated correctly, a concrete slab can be a finished floor, and that floor can provide significant thermal mass within a structure.

Frost protected slabs help us use concrete for real advantages and avoid using it just to deal with freezing soil. The technique has been used for decades in the cold climates of Alaska and Scandinavia. The National Association of Homebuilders has researched and endorsed the technique. The idea is simple in principle, although the details are tricky. First, use well-drained material below and around the slab to keep water away. (Think about it: railroad tracks don’t “frost-heave” because they rest on about 3 feet (1 meter) of stone ballast.) Second, place a nearly-horizontal layer of rigid insulation around the perimeter of the slab, extending outward at least as far as the frost depth in your region.  This helps prevent freezing at the slab perimeter.

When making a frost-protected slab, one of the trickiest parts is forming the slab “haunch,” or the perimeter edge requiring thickening and reinforcement for structural loads. Another challenge is placing and leveling formwork on uneven ground. And a further challenge is providing ample insulation on outside of the slab edge – energy modeling reveals that slab edges are a large source of building heat loss.

That’s where ICF’s come in. They make handy, pre-insulated formwork. They’re easy to handle and place, and they have hard plastic splines embedded in them that can take fasteners – making it really easy to attach additional rigid insulation, expanded metal lath for stucco, and so on.

Here’s what I did. First, I had my neighbor Jim place clean, bank-run gravel to make a well-drained building pad. I compacted the gravel in 15-inch high “lifts,” then allowed a winter for the gravel to settle.




My buddy Stephen and I poured a footing, taking great care to make it as level as possible.

 

 







 

I coated the top with a cementitious “capillary break.” Then I snapped chalk lines for the slab perimeter and nailed steel track to the footing while the concrete was still “green.”




 

 

 

The next step was perhaps the most innovative. ICF’s usually come as 2 “panels” that are assembled using plastic connectors that double as holders for steel reinforcing bars in the haunch. I cut one of each pair of “panels” on a table saw, lowering it by about 4 1/2 inches (11.4 cm.), or the depth of the slab (except the haunch). Then I connected the panels, with the shorter ones to the inside. (The photo shows the corners placed first.) I could level the ICF’s and fasten them to the footing by screwing into their plastic splines through the steel track. Because we were careful with the footing, we only needed to level the ICF’s by 1/8 inch (3 mm.) over the whole perimeter.




The idea is then to place under-slab compacted fill and rigid insulation to the top of the interior “panel.” Then you’ve formed the interior and haunch of your slab. Here’s an illustration:






 

You’ll notice that you only benefit from half of the ICF’s insulation at the outside perimeter. The ICF’s I used provided 2 1/2 inches (6.4 cm.) of expanded polystyrene on the outside edge, with an R-value of about 9.5 (RSI-1.7). I added 3 inches of extruded polystyrene insulation (R-15, RSI-2.6) to the perimeter, fastening it with screws driven into the plastic splines. The total slab edge R-value is 24.5 (RSI-4.3). I also placed 3 inches of extruded polystyrene below the slab.

 

 

 

 

With the ICF’s all in place, it was time for under-slab fill, under-slab plumbing, insulation, vapor retarder and steel-bar reinforcement.

 

 

 

 

I have no skill with a screed or trowel, so I hired my friends Keith and Howard to pour the slab – they did a great job.  Once they finished, I sawed control joints to let the slab crack in a controlled way – I placed the saw joints in a pattern that mimics large tiles.

 

 

 

 

 

Later, I added the extra insulation at the slab edge. I protected the insulation by attaching expanded metal lath, using long screws to reach the plastic splines in the ICF’s, and applying two coats of cement stucco to the lath.

 

When the slab had cured, I etched it with acid and applied a concrete stain to make the slab the finish floor of our office. (I “embellished” the stain color by adding mineral pigments of my own.) Several coats of sealer and buffed paste wax gave the floor a nice patina. After several months in the office, the floor is warm, durable and attractive.

 

 

 

 

If I were doing the job over, I’d calculate the loads coming through the bottom of the slab haunch onto the footing – if a dense foam insulation could support those loads without compressing, I would add it to the top of the footing, eliminating a significant path for heat conduction. I placed a 2-inch (5 cm.) layer of rigid polystyrene insulation horizontally 4 feet (122 cm.) around the slab perimeter. That’s an R-value of 10 (RSI-1.8). If I were doing it over, I’d use 4 inches of foam for an R-value of 20 (RSI-3.6).

It was fun to figure this out and do the work. Through the years, the inspiring work being done by folks in the NESEA community, and the great presentations I’ve attended at NESEA conferences, have given me the gumption to try new ideas. If someone with my meager skill levels can do this, then a lot of you skilled folks out there will be able to really improve upon the idea!

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