The job of the exterior of our house is to keep water out and to maintain a fairly constant temperature indoors.  When the exterior alone doesn't keep the temperature in our comfort range, we use energy to provide heating and cooling.  In order to reduce how much energy we need to maintain comfort, we seek to make the exterior be a very good barrier to heat movement.

The Physics of Heat Transmission
Although it is not necessary to understand the Physics of heat movement, it is useful to understand it in general terms.

• Heat moves from hot to cold in an attempt to make all objects the same temperature (in fact were it not for the nuclear and chemical reactions that produce heat, the universe would all be the same temperature).
  
• Heat moves in three general ways:
1. Via Conduction this occurs when two objects are in direct contact, for example the air against a window, the soil against a foundation.  In houses, this is typically the most significant method of heat transfer.  Conduction moves in all directions at the same time.
2. Via Convection this occurs within a fluid medium (e.g. air or water) and is the result of the warmer part of the fluid rising while the colder part sinks.  Convection results in the entire fluid rapidly reaching the same temperature.  The old saying that "heat rises" is really a misstatement that should say "warm air rises".  Heat has no sense of direction, but warm air being lighter rises due to being displaced by colder air which has a greater pull of gravity.
3. Via Radiation this occurs between a warm object and a colder object when they are separated only by a medium which is transparent to infrared radiation.  This is easiest to understand by just standing in the sun: while the sun is very far away, it is also very big and very hot while space and the atmosphere block very little of that incoming radiation.  With smaller, and much cooler objects, radiation is a much less significant source of heat transfer, although its affects can still easily be noticed.  In a home, windows are transparent to some heat radiation (more about this in solar power),  but the rest of the building is relatively opaque.
   

(picture: heat loss mechanisms of person in middle of a room, body @98, walls @65, ceiling @70, air @70, floor @60)

• Properties of Materials
1. Transmission/Resistance: Commonly known as the "R" value of a material, this represents its ability to impede the flow of heat through the material.  Its inverse, called the "U" value, is less well known.  It can be calculated by 1/R, and is the value more commonly used by architects and energy specialists.  A larger "R" value represents are greater resistance to heat flow, while a smaller "U" value has the same effect.  For example a "R" value of 10 is equivalent to a "U" value of 1/10 or .1.  These values are determined experimentally and include the effects of both conduction are radiation, but only under the conditions tested.  Although a materials "R" value can change in different conditions, that change is usually small and so is typically ignored.
2. Specific heat: This value represent the quantity of heat a material can hold.  Although most materials don't hold much heat, some, like stone and water hold quite a bit.  Later we'll see how we can take advantage of this property to help keep our house at an even temperature.
3. Absorption/Radiation: These values give the percent of radiation a material will absorb or radiate relative to a perfect "black body", which theoretically both absorbs and radiates 100% of the energy difference between it and surrounding bodies.  These values are not often used, except in building solar collectors, where we want to absorb 100% of the suns energy, but not radiate any of that back to the colder surrounding air.

(picture: window, wood wall, stone wall, with heat properties shown)

Building Envelope - Heat Loss Calculations
Because heat moves in all directions, when calculating the heat loss of a building, we much consider all surfaces that divide the inside, heated space from the outside.  We refer to that dividing line as the Building Envelope.  Any weak spot in that envelope is a potential unnecessary source of heat loss.

The amount of heat lost or gained is determined by the size of the surface area, the "U" value of the material and the difference in temperature between the inside and outside.  This method of calculating heat loss is only accurate when the material has an insignificant ability to store heat.  Since the only common materials that store a significant amount of heat are stone, concrete and clay, we will examine this case later, but even there we will show that only a minor modification is necessary to make our calculation right.  (Note that in all cases, these heat loss calculation are not accurate in the physicist's sense, but are close enough that we can predict our energy bill fairly accurately.  Standards are set by ASHRE, the American Society of Heating & Refrigeration Engineers).

(picture of house with various temps and heat loss amounts shows by arrow size)

There are two main methods of heat loss in a building: through the envelope by a combination of conduction, radiation and convection and that of air infiltration of outside air through leaks in the envelope.  We need to prevent both, but in the case of air infiltration we must be careful, because we need some fresh air entering the house at all times (more on this later).

While heat loss calculation aren't rocket science, they can be challenging for people who don't like math.  Later, we provide a worksheet with some sample calculations for those who like to see examples. The following are the principles you will want to know.

To calculate the "R" value of anything that is composed of multiple different materials, just add up the "R" values of each of the components.  For example:
R(wall) = R(sheetrock)+R(insulation)+R(siding)

(picture of summary of heat loss of part, and resulting temperature gradient)

To calculate the heat loss through any given structure, multiply the surface area of the structure times the "U" values of the material and then multiply that by the difference (commonly represented by the Greek letter Delta)  in temperature between the inside and outside.
Q=A*U*(T_inside - T_outside)   or   Q=A*U*DT

When a structure is made of different materials, for example a wall that contains windows and door, just calculate the heat loss through each of the components separately, then add their heat losses together to get the total amount.

Q(wall)=Q(framed area)+Q(windows)+Q(door)

Calculating heat loss through a basement or slab on grade is more difficult for a number of reasons: because soil can hold a large quantity of heat, because the temperature in the ground is not the same as outside temperature (in fact it varies little by season) and finally due to this, building loose more heat through their perimeter (i.e. the part of the foundation or slab nearest to the surface of the ground outside).  Because of this, standard practice is to only insulate basement walls and 2-4 feet under the slab near those walls. The ASHRE method is to calculate heat loss for this situation is to look up a perimeter heat loss factor (called "F") in a table based on the "R" value of perimeter insulation used.

For any house which receives a significant amount of passive solar heat (or has radiant heat in a the slab), the assumptions behind this factor leave much to be desired, and in fact it is common practice to insulate under the entire slab in these cases.  An alternative method is to calculate heat loss through the main area of the slab using year round underground temperature as the difference in temperature between the two space, then calculate the heat loss through the perimeter area using the outside temperature to get the difference in temperature, while ignoring the effect of the heat storage ability of the earth.  This method gives an upper bound on heat loss, and actual heat loss should always be lower.

Calculating heat loss through a crawl space or unheated basement is done in the identical way that walls and ceilings are done.

To calculate the heat loss through all the entire envelope, just add all the heat losses for the individual structures together.

Finally to all this, we must add the heat loss due to warm air leaking out (or being exhausted out by a fan) of the envelope. This heat loss is due to the quantity of heat stored in the escaped warm air (which is the same quantity that must be added to the incoming cold air to heat it).  To calculate this, you need to know the volume of the house (ie sq ft of floor times ceiling height) and how much air typically leaks out (see next sections), which is often stated as how many times per hour the entire air in the house is lost to outside and referred to as Air Changes per Hour or ACH.  For a modern tight house, a standard value is about 1/3 of an air change per hour: so it takes three hours to replace the entire air in the house with fresh air.  In older homes it is not uncommon to see 3 air changes per hour (i.e. the entire air in the house is replaced with outside air 3 times per hour!).  This gives the quantity of air lost, which is then multiplied by the amount of heat air holds, which is quite small, .0017 BTU per cubic foot and the difference in temperature between outside and inside.

Q(infiltration) = V(air)*ACH*.0017*(T_inside-T_ouside)

Build tight/Ventilate right
We know from history that by tightening up our houses we reduced heat loss, but also created air quality problems.  As a rule of thumb a typical family home should get about 1/3 air changes per hour to provide fresh air for its occupants (obviously, the fewer the occupants the less fresh air needed; throw a party and you need more--for more info on fresh air requirements, see ventilation in the healthy house section).  People often ask if we tightened our homes up too much, but when we look at how air leaks into an unsealed home in various weather conditions, we find that such natural ventilation is not that reliable, sometimes giving us too little fresh air, but more often giving us far too much.  

Air leaks out of a house due to two main driving forces: wind and a temperature difference, each of which creates a pressure difference between inside and outside forcing air through the cracks in the house.  What this means is that on cold windy days we loose a lot of heat, and on calm days where the inside temperature is near the outside temperate, even if we open the windows wide, we get very little air movement!  Clearly, we'd like to even these extremes out as much as possible to provide for a steady supply of fresh air.

Of course there is a compromise, and most people don't want to live in a hermetically sealed building.  There is a point where we've tightened our house up enough so that we're not paying a big energy penalty, but not so much that mechanical ventilation is the only source of fresh air. Since air leakage varies with weather, even a very tight house will leak 2-3 times in winter what it does in spring & fall, and could be five times as much (or more) and a particularly cold, windy day.  As with many areas of Green Building, there are no fixed answers and each person must find their own compromise.  In most buildings it is difficult to get it super tight anyhow, so this question won't come up unless you're taking extra air tightening measures.

Air Sealing Techniques
As discussed above, we need to prevent too much air from leaking in and out of our house.  In general, the majority of air leaks will be found around doors & windows, followed by any place where two parts of the building meet, such as the walls against the floor or ceiling. Air, like water will find a way through any place that can possibly be gotten through.

The tightness of a house is measured in the number of times per hour all the air in the house is lost, and is often abbreviated by ACH (air changes per hour).  To determine how tight a house is, a test called the blower door test is done.  To do this, all openings in the house are closed except an exterior door and that opening is filled with a device the size of a door that contains a powerful fan, a flow meter and a pressure gage that measures the difference between inside and outside (see figure).  The fan removes air from the house until there is a pressure difference of 50 Pascals (a metric pressure measurement), at which point the fan speed is adjusted so as to maintain that constant pressure difference. The amount of airflow is now equal to the air leakage of the home (at a pressure of 50 Pascals). 

Remember that the amount of air leakage on any given day is determined by the wind velocity and the temperature outside, so the result of the blower door test is not how much air your house will actually leak (which varies greatly), but a relative measure.  By using a formula, the air leakage under mild weather conditions can be estimated.

The blower door can also be used to find air leaks by walking around with a device that gives off smoke and looking for places where moving air moves the smoke.  This is typically done once the structural work is completed, but before any finish work is done to allow for easy fixes for any leaks found.

The most important and least cost technique is to make sure you have adequate caulking and weather stripping around all windows and doors, especially in older homes.  During construction, your contractor, or an air tightening specialist should walk around the house sealing all the potential leaks, typically with either caulk or expanding spray foam. 

Some structural building methods such as strawbale, adobe, SIPs and ICFs create a relatively impervious air barrier and so need no additional sealing.  Wood and metal frame construction covered with a sheet material (such as plywood) create a relatively porous structure and often additional sealing, of which the most common form is housewrap (such as tar paper or Tyvek), which is a sheet of a fabric like material that is wrapped tightly around the house to act as a barrier to air movement,  often called an air flow retarder.  It is important not to confuse air flow retarders with vapor barriers, even though sometimes one material will serve both functions.  For more info on vapor barrier retarders see moisture control  in the healthy house section.

See also:

epb1.lbl.gov/blowerdoor/BlowerDoor.html  for a technical discussion of blower door tests.

searching Google for "blower door" returns many good quality results

Insulation materials and methods
Insulation is the material added to a building structure when the building materials themselves don't provide the desirable amount of resistance to heat transfer. The amount of insulation that can be added is limited to the available space between the framing material, and is typically the  most significant factor in determining how well a wall insulates.  Since the framing material itself is at best a mediocre insulator, framing act as a thermal bridge leaking heat, and reducing the overall "R" value of the wall. Advanced framing is a method of making six inch thick walls (instead of the traditional four inch), without increasing the amount of wood used.  Alternatively, foam board sheets can be attached to the exterior of a standard 2x4 wall also yielding a better insulating wall.  The use of light-gauge steel framing to replace wood creates a problem because it conducts heat so well, and so must always be used with an exterior layer of foam board insulation to stop the thermal bridging of the steel.  In all cases light-gauge steel framed walls have a lower overall "R" value to a wood framed wall.  (For a further discussion see the Materials section) 

In other building methods, such as strawbale or adobe, the building structure also acts as the insulation.   Newer, alternative construction methods such as Structural Insulated Panels (SIPs) or Insulating Concrete Forms (ICFs) are manufactured wall building systems that also provide insulation (for more information on these see the Materials section).

Insulation material comes in three primary categories, each of which has its own particular application:

• Fill: Material that you use to fill a cavity between structural materials (e.g. between wood or steel framing).  Examples of these are loose fill fiberglass, cellulose, and spray in foam.
• Batts: Material that holds itself together in a blanket like fashion, and is attached to structural framing.  Batts often contain a backing or other form of vapor barrier in addition to the insulation.  See the Health section for a complete discussion on vapor barrier.
• Boards: A rigid material (typically polystyrene) that comes in sheets like plywood. 

There are many kinds of insulating materials, each of which has its own set of advantages and disadvantages, and none of which are the perfect solution.   Not all insulation materials are appropriate for all situations.  There is much information out there provided by manufactures of the various products, each listing their own benefits and the disadvantages of their competitors.  As could be expected they are mostly very biased.  A sampling of some web sites is included.

A quick note on "R" values of materials: Everywhere you look, you'll find somewhat different "R" values listed for the various materials.  A materials "R" value will differ based on how it is manufactured and how it is installed and possibly other conditions as well.  The numbers used here are typical, and should be used for relative comparison purposes only.

Fiberglass: In its familiar form, glass fibers are spun together and formed into batts with a glue and then typically also attached to a vapor barrier backing.  This glue is a skin irritant, making this form of insulation unpleasant to handle.  Fiberglass in this form is a good insulator (between R3 and R3.5 for every inch installed), but does loose some insulation ability as the outside temperature gets very cold due to having large air spaces which will transfer heat by convection.  Fiberglass is also available without the glues in a blown in systems that creates a higher density, and hence somewhat higher "R" value, and presumably less susceptibility to convection heat transfer.  There is some concern that fiberglass fibers break down with age and create tiny sharp filaments causing a disease called silicosis.  Virtually all cases of silicosis are job related.  Although there is some real risk, that risk is very small because insulation is contained in the wall.  Much information is available on the web,  for example by entering "silicosis fiberglass" into the Google search engine.

Manufacturers are beginning to include some recycled glass content in their fiberglass products, but currently old fiberglass insulation is difficult to recycle due to containing many contaminants. The raw material (sand) is readily available, and although glass manufacture requires a relatively large amount of energy, a long product life and recycling could mitigate that impact. 

Mineral Wool: A fibrous product made out of various mineral byproducts, and all having similar properties to fiberglass.  Mineral wool was common used on the 1940's and 50's but is no longer popular.

Cellulose: Typically made of shredded recycled newspapers soaked in a solution borates (related to borax cleaning powder) to increase its fire resistance and insect resistance.  Cellulose is either blown into a cavity dry, or sprayed on wet (when using wet spray, make sure the insulation dries completely!), and has a similar "R"  value to fiberglass.   Because cellulose is installed quite densely, it is not subject to convection current losses.  Any liquid water (condensation or whatever) in the wall mayl leach out the borates over time leaving the insulation vulnerable.  Although borates are considered safe, some people are sensitive, so anyone who has chemical sensitivities needs to find out is they are susceptible before using cellulose.  Due to it lower amount of processing, Cellulose is popular among the natural building crowd.

Cellulose is a 100% recycled product, but it not recyclable due to contaminants and the breakdown of fibers.  It is however completely biodegrading, and the original product is a renewable resource.  Its manufacture is a relatively low energy process, although until manufactures eliminate chlorine bleaching, there are dioxins produced.

Cotton: Made of waste cotton (typically industrial waste, not post-consumer), cotton insulation is a product that has limited availability and currently a significantly higher cost. Because it is a plant material containing cellulose, its properties and problems are like those for Cellulose.

Polystyrene: Polystyrene is a plastic (known mostly by the brand name "Styrofoam")  is made from petroleum that is "blown" with some kind of gas (i.e. filled with lots of bubbles) and formed into boards.  Historically the gas used was either a CFC of HCFC, materials that are greenhouse gases and contribute to ozone depletion.  More recently, manufactures have switched over to using Pentane or Carbon Dioxide gas instead, which are more benign.   It's "R" value is somewhat better than Fiberglass, about R4 per inch, although there is some contention that most of the foam board products loose "R" value over time, particularly the ones blown with CFCs of HCFCs as those gases are responsible for the higher "R" value and leak out over time. Although Polystyrene isn't as flammable as cellulose, it will burn, and produces toxic smoke when it does.  Polystyrene is the main component of SIP and ICF building systems (see Materials), and due to being available in a form with high compressive strength, is the insulation of choice for around foundations and under concrete slabs.  It is also sometimes used on roofs and in other places where its board form is more convenient.

Environmentally, Polystyrene leaves much to be desired.  Its made from a non-renewable resource.  Although technically possible to recycle virtually no one will even take clean Styrofoam packing material, so dirty old insulation is even less likely.  Because it is made from the polymerization of a simple gas, its technically possible to make it out of an alternative renewable material, but as long as there is abundant cheap oil there will be little motivation to do so.

Polyurethane/PolyIsocyanurate: Like Polystyrene, these are plastics blown with a gas, but promising a higher "R" value: up to R7.5 per inch for PolyIsocyanurate.  Since each product has a somewhat different manufacturing process, one must examine the specific product to determine is actual properties and environmental impacts.

Polyurethane can be "foamed in place" in an existing wall cavity. Icycene brand polyurethane is marketing itself as a more environmentally friendly foam, and has been used on a number of "Healthy House" demonstration projects sponsored by the American Lung Association.

Air Crete: This unique product is cement with a lot of air in it, installed as a foam.  Its "R" value is similar to fiberglass, but it has all the environmental properties of cement (and therefore does not have problems with silicosis).  Its raw materials are abundant, but it takes a lot of energy to make it.  Its is recyclable, but there is currently not a strong market for cement products.

See Also:

www.eeba.com - the energy efficient building association, publishers of the "Builders Guild" series of construction manuals.

www.oikos.com - web based bookstore specializing in green building, has a complete line of books on energy efficiency.

www.owencorning.com - Makers of "pink" fiberglass as well as Miralfex, a glue free fiberglass

www.certainteed.com - Makers of Insulsafe 4, a glue free fiberglass loose fill.

www.dow.com - Makers of Styrofoam brand Polystyrene insulation

www.icycene.com - Makers of Icycene brand Polyurethane foam insulation

www.pima.org - the PolyIsocyanurate manufactures of America

www.polyurethane.org - the Polyurethane industry

www.airkrete.com - Air Krete's website

www.bondedlogic.com - a maker of cotton insulation

www.buildinggreen.com - web site of Environmental Building News, an authoritative, independent reviewer of Green Building.  Alas, much of the information is only available if you subscribe to the on-line version of GreenSpec.

Windows
Windows provide light, ventilation and in many cases passive solar heating, but are otherwise a source of great heat loss.  A decently insulated wall easily achieves R21, while most windows do no better than R3: meaning that a window loses seven times more heat per square foot than a wall does!  So clearly we have to pay attention to how we use windows.

Window Construction
While in the past, windows where just sheets of glass installed by a carpenter on-site, today's windows are hi-tech assemblies consisting of an insulating glass unit mounted in a frame, with optional hardware to allow it to open, and sometime integral shading devices.

Window frames come in a variety of materials:

Wood- wood is beautiful, healthy and renewable, but requires significant maintenance in most climates.  Compared to insulating materials, wood is a mediocre insulator, and so wood frames result in increase heat flow through the window.

Clad wood - to make wood more weatherproof, manufacturers cover the exterior of the window in a durable materials (typically aluminum), which dramatically increases the lifetime of the window, but still results in a poorly insulating frame.

Vinyl - due to its low cost and low maintenance, vinyl frame windows are very popular.  The manufacture of vinyl is highly toxic, and some environmental groups have called for banning vinyl completely.  Like most plastics, vinyl expands and contracts much more than other building materials, creating stress on the frame and insulating glass unit which can cause the seals on the glass to fail.  When the hollow frame is filled with foam, the result is a much better insulator than wood, but without the foam vinyl is no better.

Fiberglass - low maintenance and durable, fiberglass frames are becoming more popular, although they are more expensive than Vinyl.  Fiberglass frames have a similar rate of expansion with temperature as the glass itself, and so avoid those problems.  Like vinyl, the frames can be filled with foam insulation to create a highly insulating unit.  Although there are no specific environmental concerns about fiberglass, the resin used is a petrochemical derivative and so isn't a sustainable resource and may have unknown environmental problems.

The spacer used to hold the panes of glass apart are made out of a variety of materials, some more better insulators than others.  Older units used simple aluminum spacers, while newer ones use metal in combination with other materials to make a better insulator, often called "warm edge" windows.  In addition to improving the "U" value of the window, warm edge windows increase the surface temperature of the inside of the glass, making the room feel warmer and reducing condensation.

Low-E coatings are a metal-oxide that is applied very thinly on one of the surfaces on the inside of an insulating glass unit.  These coating come in two main varieties, one for cold climates where solar gain is undesirable and one for all other climates.  In the hot climate version, the Low-E coating blocks virtually all of the non-visible light reaching the window.  In the other version, only the longwave infrared is blocked, because the major of solar gain is in the shortwave part of the spectrum, while the heat radiating from a building is mostly in the longwave part of the spectrum.  The Low-E coating can have a significant effect on the overall "U" value as well as the Solar Heat Gain Coefficient (see below for understanding these numbers, also see the solar section for further discussion on the effects of windows on passive solar gain). 

The air space between the panes can be filled with air, or an inert gas (argon or krypton) that has a better "U" value than air and improves performance by anywhere from 5 -20%.  While the insert gas does leak out over time, it leaks very slowly and is non-toxic.

"Superwindows" combine an insulating frame (often foam filled fiberglass or vinyl), double or triple glazing (sheets of glass) separated by an insulating spacer, one or more "low-emissivity" (called Low-E) coatings to reflect heat back into the building, and an inert gas fill which insulates better than air (see figure).

Window Ratings
Windows are rated according to a standard set by NFRC (the National Fenestration Rating Council) and consist of four values that tell about the performance of the window:

"U" value - the value is the heat transmission of the window, and tells how much heat the window will loose.  In the past manufacturers measured the "U" value at the center of the glass, because it is often higher than for the whole unit.  While this practice has been abandoned, buyers should verify that the "U" value given is for the entire window unit.  Typically values are "U" for a double pane, Low-E, argon gas fill window is around .33 (i.e, R3), while a triple pane superwindow achieves a "U" of about .15 (i.e. R6.6).  By comparison, an old style double pane windows has a "U" of about .5 (i.e. R2).  Because the frame of the window often lets more heat out than the glass, larger window units have better overall "U" values.  Likewise, using true divided lights (consisting of multiple glass panes instead of one),  reduces the "R" value significantly.

"VT" - visible transmission, the percentage of the available visible light that is allowed to pass through the window.  Even clear glass isn't perfectly transparent, and multiple glazing and Low-E coatings reduce this value.

"SHGC" - the solar heat gain coefficient, is the percentage of the available solar gain that is allowed to pass through the window.  As with visible transmission, this value is lower when multiple glazings and Low-E coatings are present.  A double pane window with a Low-E coating that stops solar gain allow only 30-40% of the solar gain through, while a Low-E designed to allow solar gain lets in only slightly less than plain clear glass, about 75%.  By comparison, a superwindow which has a U of .15 has a SHGC of only 50%. Note that these numbers are for glass only, and must be reduced to account to the space taken up by the frame and any pane dividers that shrink the overall glass area.

"AL" - air leakage, the amount of air leakage through the window.  Note that this does not included the air leakage around the window unit where it is attached to the wall, which should be sealed tightly.  The air leakage amount for a window that opens is higher than one that doesn't (due to greater difficulty in sealing) and that for double hung or sliding windows is greater than that for awning or casement.

Resources

Residential Windows, by Carmody, Selkowitz & Heschong contains everything you would ever want to know about windows.

Builders Guide to Cold climates (etc), Joe Lstiburek, 2000