Tag Archives: Chapter 3 – Alternative Building Strategies

What is Natural Building, Or “Do I Have to Wear Dreadlocks to Build with Straw Bales?”

This article by Clarke Snell originally appeared in the New Life Journal.

Column redux: two months ago I delved into straw; last month I riffed on dirt. Add locally harvested wood, some stone, maybe a little bamboo, an enthusiastic attitude, and we are squarely in the realm of “natural building”. What’s it all about?

Before I go any further, I need to make a full disclosure: I took a lot of philosophy in college. Red flag! That means that words like “natural” really trip me up. What is natural? Some might say that “natural” simply means that which is in tune with nature. But aren’t humans natural, so wouldn’t anything we do be natural, too? I’m sure the makers of vinyl wall coverings will be happy to hear it.

Another approach is to say that natural building uses natural materials and that natural materials are those that are locally harvested and minimally processed. Okay, then is gravel natural? It’s local, durable, and doesn’t require a huge amount of energy to harvest. However, gathering it completely destroys the specific, local ecosystem from whence it comes. (Have you ever visited a quarry?)

Yet another spiel would be that natural is synonymous with indigenous. What is the indigenous building system for our area? Some variant on European wood construction, the wigwam or other native American technique? How about the rabbit hole or bird’s nest? Even if we could agree upon the representative historical composite system, what meaning would it have for us? Let’s face it, we aren’t indigenous ourselves.

My point here is that “natural” only has meaning when given a context. It used to be that the context was distinctly local. A small group of people using locally harvested materials interacting with a specific culture and single annual climatic cycle. Today our context is global. There are lots of reasons: billions instead of millions of people, communication in seconds rather days, travel in hours rather than months. People now travel from the Philippines to Dubai to get a job behind the counter at Starbucks. The CO2 my woodstove in WNC or your car in Atlanta puts off today may change the climate in India tomorrow.

To my mind, that means that being natural these days is a lot more complicated. When it comes to buildings, it can’t simply mean using local materials anymore. It has to mean taking the full impact of a building and the lifestyle it serves into account. There simply are no easy answers. All of us heading back to the proverbial woods isn’t a feasible option. How would 3 billion small huts each with it’s own fire for cooking and heating impact the global climate? How long would the burnable cellulose hold out? On the other hand, it’s hard to even use the word “natural” in the context that most of the human population now resides: the city. What is the natural way to build for people in a huge city that contains no indigenous resources for building?

This line of thought has led me to try and keep an open mind as I’m looking at the resources available for building. I’ve accepted that human dwellings have an impact, so my goal is to create the best building with the smallest impact. If in the full life-cycle of the building that means using some mass-produced materials, high tech systems, and heavy machinery, then I’m all for it.

It’s interesting then that even in this context, my focus is still on what are traditionally called “natural materials”. Part of the reason is the typical rationale that they are locally available and therefore create less pollution in harvesting, manufacture, transport, and installation (have a lower embodied energy). Just as importantly, though, they often provide much greater design flexibility than their modern counterparts.

Let’s take straw and dirt for example. We can use these two simple materials combined in different ratios and configurations to deal with a variety of different specific situations. Configured as a bale, straw can provide incredible insulation. Mixed with a clay/sand soil, the result is cob, a simple material with amazing structural strength. A different mix of the same ingredients creates clay-slip straw with more insulation than cob and more water resistance than bales. What’s more, all of these incarnations can vary easily in width and height and therefore don’t need to fit into cavities of predetermined size. This flexibility is important in the real (i.e. natural) world because every site and even every wall on every site is in some ways unique. Materials that can be modified and tweaked to deal with different incarnations of sun, water, wind, and life forms (mold and insects, for example) are often easier to match to a given real world scenario. It’s difficult to design mass-produced materials to have that kind of flexibility.

In the end, then, I wonder if the whole “natural” moniker isn’t really just confusing the issue. If we are trying to create buildings that work more efficiently with the realities of a give site, then materials and techniques commonly called “natural” can stand alone as solid, practical choices toward that end. You may not like dreadlocks. I may not like baseball caps. That doesn’t mean that we both shouldn’t consider incorporating some straw and dirt into the next building we build or have built.

(By the way, if you want to get into some down and dirty details on this topic, I’m teaching a WNC Green Building Council seminar on July 18th called “Natural Building: What Is It? How Do You Do It? Why Would You Do It?” Call the WNCGBC or check out this link for more info: http://www.wncgbc.org.)

How Fiberglass Window Frames Are Made

The best windows in Europe are made with thermally broken wood frames, but the best windows in North America have fiberglass frames. The European wood windows have my vote for the best made windows in the world. I consider them art. However, I have to admit that fiberglass is a pretty ideal material for window frames.

As wood takes on water and dries it can warp. If it takes on enough water it can rot. Fiberglass, on the other hand is impervious.

Expansion and contraction are another problem. Aluminum frames expand about 3 times as much as glass when they are heated and vinyl expands 7 times as much as glass. Wood does not expand when heated. It expands with humidity. The varying expansion rates can result in stressed frames, broken seals, and inoperable hardware, but fiberglass frames have nearly the same coefficient of expansion that plate glass does.

Fiberglass frames are made by a process called pultrusion, a combination of the words ‘pull’ and ‘extrusion’. Vinyl and aluminum frames are extruded, meaning the material is melted and forced thru a die. With pultrusion, continuous glass fibers are pulled thru a resin impregnation bath and then a heated die where the resin sets into the desired shape. Here’s a pultrusion machine in action:

Thermal Mass

In our climate, even on the hottest days of summer, the outdoor nighttime temperature drops below the indoor temperature. Using massive materials inside the insulated envelope, we can take advantage of that diurnal temperature swing to reduce the amplitude of the indoor temperature swings. The mass absorbs heat during the day and radiates it back at night.  If we do a good job of keeping that mass shaded during the summer there’s no need for mechanical cooling and the dog has a nice cool floor to lie on.

Thermal Inertia
Talking about thermal mass in more detail gets a little more complicated. I’m afraid we’re going to need a few definitions:

  • Heat capacity is the ability of a material to absorb heat.
  • Diffusivity is a measure of the speed heat moves thru a material.
  • Effusivity describes the ability of a material to exchange heat with it’s surroundings. It is similar to emissivity (as in low-e or low emissivity windows).

Good materials for thermal storage have high thermal inertia.  They have a high heat capacity, but low diffusivity and effusivity.  Metals don’t work well for thermal storage.  They can take on a lot of heat because they have a high heat capacity, but they can’t store it very long because they also have high diffusivity and effusivity.  Metal heats up quickly, but it gives it right back. Clay is much better for storing heat in a timeframe that’s useful for conditioning houses.  It has a high heat capacity, but low diffusivity and effusivity. It’ll take all day to heat up a cold earthen floor sitting in a room with a warm air temperature, but it will take all night for it to radiate that heat back. That’s what we’re looking for.


Even when the two items are identical in temperature, the metal feels colder. Why? Wood is not a good conductor of heat, so it is slow to absorb the heat from your hand. Metal has higher thermal effusivity, so the heat from your hand flows into the metal quickly – creating the sensation of it feeling cold.

Mathis Instruments

In our climate massive construction is awesome in the summer. The downside is that dense materials like tile, concrete, and compressed earth block also feel cool to the touch during the winter. That’s why European stone castle walls are covered with tapestries.

Mean Radiant Temperature
To derive your Mean Radiant Temperature, look around you and take the temperature of every surface you see. You are exchanging heat with all of those surfaces. Surfaces warmer than you radiate heat to you and all the other colder surfaces. You’re just another room surface exchanging heat with all the others. To be comfortable all the surfaces around you need to be within a few degrees of each other (and you), and in a well insulated house with good windows they will be. However, believe it or not, our skin does not have good temperature sensors. Instead, we have excellent heat flux sensors. All of the surfaces in a room can be exactly the same temperature, and some will still feel colder than others when we touch them. The surfaces that feel colder are the ones with higher effusivity. The castle tapestries have low effusivity so they feel warmer than high effusivity stone.

Radiant Heating
In a typical (minimally insulated and drafty) house, radiant floor heating feels great because the mass is heated up to about 80 deg F. The floor radiates heat up to other surfaces, and brings the mean radiant temperature up so we’re nice and toasty. The problems are:

  1. radiating 80 deg F from the entire floor is a lot of heat.  A house that needs that much heat is wasting a lot of energy, and it should be insulated better.
  2. you lose a lot of ability for a slab to absorb free heat coming in the windows from the sun if the mass has already been heated by radiant tubing.

In an efficient well sealed house, a conventional concrete radiant floor heating slab won’t have to rise above about 73 deg F to meet the heating load (assuming the entire floor is heated). You will wonder if the heat is really on because it won’t feel warm. Even though the floor slab is adding heat to the house and the mean radiant temperature is high enough that we aren’t radiating much heat to the other surfaces, concrete has a relatively high effusivity. It exchanges heat with us pretty easily and feels cool even with a slight temperature difference. In a passive solar house, high effusivity materials located in areas that get direct solar gain will feel tactically warm on sunny days, but those same materials in northern rooms without solar exposure or in southern rooms on cloudy days won’t.

If you use radiant heat, insulate the house well enough that a small area of radiant will heat the entire house. Locate it in northern rooms (especially bathrooms) that can’t be heated by the sun.

Concentrate high heat capacity materials in the south rooms where they will do the most good. Use low-medium effusivity materials to store heat. Assuming no radiant heat, a north bath or kitchen would be better off with low effusivity wood or cork floors and wood countertops, but the same room located on the south would benefit from medium effusivity concrete countertops and tile floors.  Likewise, soft earthen plasters will feel warmer than hard venetian lime plasters, and soft lime and gypsum plasters will feel warmer than harder cement based plasters.

This spreadsheet shows the effusivity and interface temperature (how warm the surface feels when you touch it) for a few typical materials. I assumed an 85 degree hand surface temperature and all other surfaces at 70 degrees, but you can go to the spreadsheet and change those values as you see fit: Google Docs | Thermal Effusivity.

Furnace Free in Vermont

In 1998 Marc Rosenbaum was working on a 22 unit cohousing development in Harland, VT. Amory Lovins told the client that she should build a passive solar house without any backup heat. Marc didn’t buy it, and they went back and forth discussing how it could be done. Then Marc published their correspondence at BuildingGreen.com. It’s definitely worth reading.

:: The Furnace-free House in Vermont

Low Mass Sunspace

A low mass sunspace is meant to serve as a heater, not a greenhouse for plants or a comfortable place for humans.

William A. Shurcliff:

It is hard to think of any other system that supplies so much heat at such low cost…

One could shorten the warm-up time of the enclosure and increase
the amount of heat delivered to the rooms by making the enclosure
virtually massless–by greatly reducing its dynamic thermal capacity.

This can be done by spreading a 2-inch-thick layer of lightweight
insulation on the floor and north wall of the enclosure and then
installing a thin black sheet over the insulation. Then, practically
no heat is delivered to the massive components of floor or wall;
practically all of the heat is promptly transferred to the air.

And since the thermal capacity of the 100 or 200 lb. of air in
the room is equal to that of one fourth as great a mass of water
(about 25 to 50 lb. of water), the air will heat up very rapidly.
I estimate that its temperature will rise about 40 F. degrees in
about two minutes, after the sun comes out from behind a heavy cloud cover.

At the end of the day, little heat will be “left on base” in the
collector floor or north wall and, accordingly, the enclosure will
cool off very rapidly.

New Inventions in Low Cost Solar Heating–
100 Daring Schemes Tried and Untried
Brick House Publishing, 1979

This works well with airflow between the sunspace and living space
during the day and no airflow at night.

Passive House Criteria

Certification Requirements:

  • Max space heating and cooling energy < 1.4 kWh/sf/year (4777 btu/sf/year)
  • Max primary energy usage < 18.6 kWh/sf/year (63,500 btu/sf/year)
  • Air tightness < 0.6 air changes per hour @ 50 Pa

The energy calculation uses these values:

  • Indoor temperature: 68 deg F
  • Internal heat gains (from lights, people, cooking, etc.): 0.7 Btu/hr/sf
  • Occupancy: 377 sf/person (other values between 215 and 538 sf/person may be used with an explanation)
  • Domestic hot water use: 6.6 gal/person/day
  • Domestic hot water temperature: 140 deg F (120 is standard in the US)
  • Domesitic cold water temperature: 50 deg F
  • Average air flow rate: 12-18 CFM/person

Passivhaus certified windows must meet these standards:

  • U value < 0.14 (R-7)
  • SHGC > 0.5

This presentation is a good overview:

Greenhouse vs Solar Heating for a House


For many homeowners, building an attached solar greenhouse is very appealing. They believe that they can extend their garden’s growing season while reducing their home heating bills. Unfortunately, there is a contradiction between the use of a greenhouse to grow plants and the use of it as a solar collector for heating the house.

• To provide heat for a home, a solar collector needs to be able to collect heat in excess of what plants can tolerate.

• Much of the heat that enters into a greenhouse is used for evaporating water from the soil and from plant leaves, resulting in little storage of heat for home use.

• A home heat collector should be sealed to minimize the amount of heat loss. Greenhouses, however, require some ventilation to maintain adequate levels of carbon dioxide for plant respiration and to prevent moisture build-up that favors plant diseases.

Greenhouse management practices also can affect heat storage. For example, a full greenhouse stores heat better than an empty one. However, almost half of the solar energy is used to evaporate water from leaf and soil surfaces and cannot be stored for future use. Solar heat can be complemented with heat from compost as described in the ATTRA publication Compost Heated Greenhouses. Besides adding some heat to the greenhouse, increased carbon dioxide in the greenhouse atmosphere, coming from the decomposition activities of the microorganisms in the compost, can increase the efficiency of plant production.

Because of the concentrated air use by plants, greenhouses require approximately two air exchanges per minute.

Shading provided by mature trees is not recommended. Older books on solar greenhouse design argue that deciduous trees can provide shade in the summer but allow for plenty of sunlight to enter through the glazing in the winter after the leaves are gone. However, more recent literature notes that a mature, well-formed deciduous tree will screen more than 40% of the winter sunlight passing through its branches, even when it has no leaves.