ISBN: 9781890132644 Year Added to Catalog: 2000 Book Format: Paperback Book Art: b&w photographs and illustrations, appendices, bibliography, index Number of Pages: 8 x 10, 384 pages Book Publisher: Chelsea Green Publishing Old ISBN: 1890132640 Release Date: September 1, 2000 Web Product ID: 45
Design Challenges: Heat Loss, Air Quality, and Moisture1
Moisture, air quality, and thermal control and relevant strawbale design issues in all climates, but they are most crucial in cold and/or wet climates. Bales are a good choice for cold-climate walls, because they insulate well. They can only perform to their highest potential, however, when incorporated into an overall strategy for superinsulation. This chapter will define, for owner-builders and others not already familiar with them, the basic concepts and terms relevant to heat loss, air quality, and moisture control-the three technical criteria that form the foundation of cold climate design. Don't be dismayed about encountering this technical discussion before we dive into the practical aspects of building with bales. Dealing effectively with these design challenges is without a doubt the key to building a successful, durable straw bale house. If we look only at conserving heat, the ideal house would have thick, well-insulated walls, floors, and ceilings. Because we want no cold air to enter or warm air to leave, there would be no fans or chimney stacks. Neither would there be any doors or windows, except maybe for some insulating glass on the south side, for solar gain. On the other hand, a house designed only with air quality in mind might hardly be a house at all. A cave (with no campfire allowed) might serve well, or a roof and a couple of flimsy walls, for protection from wind and rain. If it were an actual house, people might not be allowed inside, on account of their CO2 emissions. Clearly, cold-climate design strategies must involve a compromise between heating efficiency and ventilation. We can see this need for compromise in real houses that people are struggling to live in. Some of the early attempts at airtight, superinsulated construction have poor air quality because they were designed only to conserve heat. On the other hand, many old houses allow an excessive amount of outdoor air to leak in. If you've ever lived in one, you know all about closing off rooms, keeping close to the stove to stay warm, and waking up with a sore throat on account of excessively dry indoor air. The key to comfort lies in striking a balance between heat loss and fresh air. To balance them wisely the designer must understand the relationship between them. A building's ability to hold heat depends on the quality of its doors and windows, the insulating properties of its shell components, and the rate at which air passes in and out. To some degree it also depends on the mass inside the building, though this has more to do with the quantity of heat energy that can be stored than the rate at which it escapes. (See sidebar, below, for more on this.) Door and window makers publish product guides for designers that provide the insulation and infiltration (air leakage) characteristics of their products. Compare them and buy (or build) the best ones you can afford. Insulation is largely what this book is about-bales of straw as wall insulation. Bales provide an R-value of 1.45 per inch when laid flat (heat flow along the length of the straws, "with the grain") and somewhere upwards of 2.06 per inch on their edge (heat flow perpendicular to the straws, "across the grain"). Regardless of how you use them, bales provide a superinsulated wall to retain heat inside a building. Infiltration, which is responsible for 28 percent of the heat loss in a typical, new, code-built house in the U.S., is a more complicated issue. Anyone who has spent a winter in an unplastered straw bale house can tell you that bales alone do not prevent massive quantities of cold air from leaking into a building. They need the inside and outside coats of plaster to provide a cavity of still air. This is true of most insulation materials. Even if insulation materials could block the flow of air, however, they would not stop the majority of infiltration into the typical house. The plaster and drywall planes of walls and ceilings are usually relatively airtight; most leakage occurs at seams between these surfaces, and around windows, doors, lights, electrical outlets, vents, and other openings; at framing intersections; and through holes drilled between conditioned and unconditioned spaces for pipes, wires, and heating ducts. An efficient building shell controls leakage at all of these points. Many people say that a building needs to "breathe," so that its occupants might do likewise. This is obviously the case; the key to balancing heat loss with ventilation is to regulate the breathing. In the same way that a person breathes through a portal known as the mouth, by the action of a pump known as the lungs, and takes in more or less air depending on the body's need at the moment, an efficiently designed house breathes through windows and fans, and takes in more or less air, according to the activities going on inside. (This "breathing" is not, unfortunately, a very useful term, as it is also sometimes meant to indicate a wall's ability to slowly diffuse water vapor and other gases. Openness to vapor diffusion is a good idea in bale walls, but it has nothing to do with inhaling or exhaling air. This ambiguity has caused endless confusion among bale enthusiasts. If we had our way, we would purge this term from the construction lexicon. See "Vapor Permeability," and "Paul's Diatribe Against 'Breathable,' below.) Often, in mainstream construction, little effort is made to close the many small gaps in a typical building's shell, the idea being that these will provide fresh air to the occupants. This is only crudely true. The problem with this approach is that the amount of fresh air bears no relationship to the needs of the people inside. It might or might not be distributed where they need it, and its volume is determined by the weather. Thanks to the stack effect and to wind pressure (see sidebar, "The Driving Forces Behind Air Movement"), such a house will exchange more air with the outdoors as the temperature drops (as the difference between indoor and outdoor temperatures increases), or as the wind blows harder. It is highly unlikely that these would consistently be the times when a house requires extra ventilation. This fact does, however, create an extra incentive for owners of old houses to host parties on the coldest nights of the year. You are already paying for a copious supply of fresh air, so why not help your neighbors through the loneliness of winter? When the temperature difference between indoors and outdoors is not all that great, such as in spring and fall, a house with many small gaps but no real controls may not provide enough ventilation for its occupants. Though it might seem obvious that people would simply open their windows at such times, this does not always happen. In Western societies, most people are now away at work during most days, trying to keep up with their next mortgage payment. In the spring and fall, it is typically only in the middle of the day when temperatures are warm enough to make people want to open a window. By the time they get home, and begin their evening rituals of breathing, cooking, and showering, the temperature will usually have dropped enough that they are less inclined to open the windows. It may not have dropped enough, however, for the stack effect to provide ample fresh air. Let's look at a building whose air intake is controlled by the needs of the occupants. To begin with, such a building would be detailed to minimize air leakage. Openings between heated and unheated spaces would be avoided wherever possible, and the unavoidable seams in the building's framing would be sealed. Wind and weather, therefore, would have a much reduced effect on the environment inside the house. All combustion appliances would be of the "sealed" variety, drawing none of their combustion air from within the building. In these appliances, exhaust and intake are ducted directly to and from the outdoors, and all combustion takes place within a sealed chamber. Equipment of this sort has no effect on airflow patterns, and cannot pollute interior air. In this building with minimal air leakage, fresh air is provided by quiet, efficient, timer-controlled fans, often the same fans that handle heavy moisture loads from showering and cooking. Two ventilation methods are suitable for cold climates: heat recovery and exhaust only. They both deserve careful consideration.
1. Much of the information in this chapter is derived from The Moisture Control Handbook by Joe Lstifurek and John Carmody, published by Building Science Corporation, Westford, Massachusetts. This is the best all-around book on moisture theory. Heat Loss and Indoor Air Quality
more from chapter 2: Moisture Water is the main enemy of straw. Straw that gets wet and stays wet will surely decompose; kept dry, it seems to last forever. In the straw bale construction world, as well as the wider building world, many people misunderstand how moisture gets into and out of walls, and such misunderstanding can lead to ineffective house design. Water enters straw bale walls from four major sources:
1. From the interior, in the form of condensation from leaking air or diffused vapor. 2. From the ground, in the form of groundwater or condensation, transported primarily by capillary action in the foundations, and also by vapor diffusion. 3. During construction, as liquid water from almost anywhere. 4. From the exterior, as rain or, to some degree, melting snow. This, obviously, is the most dangerous long-term source.
Interior Moisture Sources We begin with interior sources of moisture, not because they are the most important, but because the definitions of terms that are required here will be useful all through the rest of this chapter.
Vapor and Relative Humidity Most readers of this book have heard about water vapor, that mysterious stuff in the air that, when it gets into the fabric of a building, can cause moisture problems. What is this stuff? Air is a mixture of many gases, including water. In this gaseous state water is known as water vapor. How does it get into the air? Out of doors, it is largely the product of evaporation or transpiration-from oceans, lakes, rivers, the ground, and plants. Indoors, it comes from obvious sources such as showers and cooking, and also from less obvious sources, including people breathing, transpiration from house plants, evaporation from washed dishes and clothing, and even from drying firewood. In addition, just like outside, evaporation from the ground into the living space is potentially a huge source. Unvented cooking and heating appliances that burn any fuel also create water vapor as a by-product of combustion. The ability of a given volume of air to hold moisture varies with temperature. The warmer air gets, the more water vapor it can hold; the colder it gets, the less it can hold. This is why outdoor winter air is dry in cold climates-it is simply not capable of holding much moisture. "Relative humidity," then, is a term for expressing how "full" air is-how the amount of water in the air compares with the maximum amount of water that the air could hold. In more mathematical terms, relative humidity is defined as the percentage of possible water vapor that is present in a given volume of air, at a particular temperature. For the sake of illustration, let's say that a volume of air is capable of holding 100 molecules of water vapor at 75 degrees. If those 100 molecules are present, the air is said to be at 100 percent relative humidity. The air is saturated; it is holding all the moisture that it possibly can at that temperature. Now let's remove 50 of those molecules. The air is still at 75 degrees, and can still hold 100 molecules, but now we only have 50 molecules of water vapor left; the air is now at 50 percent relative humidity. If we further reduced the number of vapor molecules to 25, the air would be at 25 percent relative humidity, etc. So, it is possible to change the relative humidity of air by changing the amount of moisture in it (see fig. 2-2). It is also possible to change relative humidity by changing the temperature of the air. That same 75-degree air, at 50 percent relative humidity, is holding 50 molecules of water vapor while able to hold 100 molecules. If we cool the air, we reduce it's maximum capacity. If we cool it enough that its capacity is dropped to 50 molecules, it will now be at 100 percent relative humidity; the 50 molecules present are 100 percent of the new maximum. Likewise, if we warm the air we decrease the relative humidity because warm air is capable of holding more vapor.