Wednesday, June 4, 2008

Wind Protection and Natural Ventilation - June 1

Hello Everyone,

This e-mail starts where I left off with the Psychrometric e-mail. In that e-mail I closed with, “All you have to do is design an envelope that provides sun shading and direct gains, as well as natural ventilation and wind protection.” I will now elaborate on how to resolve these seemingly contradictory requirements, starting with natural ventilation and wind protection.

I have designed housing for the high plains and mountains of Wyoming; and, I have designed housing for the Sonoran Desert of Arizona. Both of these places have extreme climate conditions. In the mountains the summers are mild and the winters are extremely cold. In the desert the winters are mild and the summers are extremely hot. Designing housing for each of these climates requires a single-mindedness. You are either designing to heat and conserve heat; or, you are designing to cool and defend against heat. (In practice this is a bit of an oversimplification, but conceptually it is accurate.)

Charlotte is located in a temperate climate. By definition a temperate climate is a climate that neither the heating season nor the cooling season dominates. According to the National Oceanic and Atmospheric Administration (NOAA), the mean normal heating degree days for Charlotte is 3,162 per year, and the mean normal cooling degree days is 1,681 per year. Single-minded think will not work.

The psychrometric chart from Climate Consultant 3 shows natural ventilation for cooling as being appropriate for 10.7% of the year, and wind protection as being appropriate for 5.9% of the year. So, how do we do both without one being at odds with the other?

Natural Ventilation

There are two basic types of natural ventilation, wind induced and stack effect.

When wind moves over and around a building it creates positive and negative pressure zones. Wind blowing around a building with a rectangular foot print will create positive pressure on the windward side of the building and negative pressure on the remaining three sides of the building. Wind blowing over a build with a sloped roof having a pitch of about 2 in 12 or more will create positive pressure on the windward slope and negative pressure on the leeward slope. Wind blowing over a building with a roof having a slope of 2 in 12 or less will create negative pressure over most, if not the entire roof. All you need to make wind induced ventilation work for you is to have wind, AND two openings in the building envelope, one in a zone of positive pressure and the other in a zone of negative pressure. Of course you are not guaranteed the wind will always blow from the same direction, so you will need more than two openings that can be control independently to make use of wind coming from any direction. There are also other issues concerning the cooling of objects and surfaces, verses people; but, I will save that talk for later.

The second type of natural ventilation is stack effect. This type of ventilation uses warm air to facilitate ventilation. To optimize this type of ventilation, a vertical “thermal stack” or “thermal chimney” is incorporated into the design. The way it works is warm air rises out of the top of the stack or chimney creating negative pressure within the house. Fresh air is brought into the house though open windows, doors and/or vents. The advantage of stack effect ventilation over wind induced ventilation is it works when there is no wind. Design guidelines for the cross-sectional area and height of the stack or chimney are available, but as a general rule, the taller the stack or chimney the better it works. Another rule of thumb for Charlotte’s climate is the cross-sectional area should be approximately ten percent of the floor area being serviced. That means for a house of 1,200 to 1,400 square feet, the thermal stack or chimney should have a cross-sectional area of 120 to 140 square feet. Obviously, this is much larger than the chimney on a residential fireplace. It is the size of a bedroom. Consequently, in energy efficient houses, spaces like, atriums, solariums, stair wells, and multi-story rooms are often designed to function as thermal chimneys. Again, there are other issues, which I will talk about later.

Wind Protection

There are two ways wind protection reduces unwanted heat losses and gains. The first way is it reduced conducted heat transfer through the building envelope by allowing a still air film to develop on the outside surfaces. Air is a very good insulator, so if a still air film is allowed to develop it reduces the rate of heat flow.

The second way wind protection reduces unwanted heat losses and gains is by reducing air infiltration. Air infiltration is the uncontrolled movement of air through the envelope. (Ventilation is the controlled movement of air through the envelope.) The rate of infiltration is expressed in air exchanges per hour. A house of average construction will have approximately one air exchange per hour. That means, the equivalent of all the air in the house is being replaced every hour. For most houses, air infiltration is the largest single source of unwanted heat losses and gains. It exceeds the conducted heat losses and gains associated with the windows and doors.

For these reasons, wind protection is listed as an appropriate design strategy. The problem is, how do you provide wind protection and natural ventilation, especially wind induced ventilation?

Designing for Wind Protection and Natural Ventilation

The first thing to do in designing for both wind protection and natural ventilation is to minimize infiltration. If there was no infiltration there would be little need for wind protection. Having said that, there are two reasons why this is unfeasible. First, with the exception of building an underground house, the cost of building a near airtight house would exceed our budget. Second, to maintain indoor air quality, a minimum rate of air exchanges must exist. A half an air exchange per hour is commonly considered to be the safe minimum. The balance point between these two issues is to build a reasonably tight house and maintain air quality through ventilation. If air quality is maintained with ventilation, rather than infiltration, there is the opportunity to use a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV) to save energy. (We will focus on the design of the envelope, HVRs and ERVs in the design development phase.)

Let us move on to the air film. The importance of preserving an air film has become somewhat exaggerated with the development of better insulated buildings. In the not too distant past, it was common to build minimally insulated, if not un-insulated, walls and roofs. For this type of construction, the insulating value of the air film represents a significant portion of the total insulating value of the envelope. Now a day, codes require insulation in the walls and roofs. Consequently, preserving an air film is less important; especially, since its insulating value can be regained with a modest amount of additional building insulation. (As with infiltration, we will focus on this when we detail the envelope in the design development phase.)

Now some of you may be saying to yourself something like, “He didn’t resolve the contradictory requirements. He just marginalized one requirement to where it was longer a requirement.” Fair enough! So, for those of you that can not accept my solution, please read on.

If we were designing this house for the high plains of Wyoming we would want to provide some form of wind protection and would not worry too much about compromising wind induced ventilation. (As I said earlier, designing for this type of extreme climate requires a single-mindedness.) Two means of providing wind protection are with trees and berms.

Conifers, such as pines, firs and spruces, can be configured to form semi-permeable winter windbreaks. Semi-permeable windbreaks let some wind pass though but cast a deeper wind shadow than impermeable windbreaks, such as garden walls.

Berms (small hills) can be used to deflect the wind upward. The challenge in using berms is to prevent turbulent airflow from developing between the berms and the building envelope. Turbulent air will scourer the air film off the envelope.

Having these two means of providing wind protection at our disposal the only thing that needs to be known is where to place the trees and/or berms. For most of the high plains this is an easy call with the prevailing winter winds coming out of the northwest. And, since we are designing for an area with few features to alter wind direction it is a straightforward solution.

For our site in Charlotte, we can generate wind wheels, with Climate Consultant 3, that shows which directions the wind blows for each month of the year. (I have posted the wind wheels on the college server if you want to see them.) Looking at wind wheels it is clear there is not a dominate direction the wind blows. Furthermore, there is not a significant seasonal difference in direction or speed. Couple this information with the fact we are designing one house for an urban context, where countless features can alter wind direction, and the other house for a site with steep topography, and it becomes virtually impossible to make a meaningful decision about where to locate windbreaks.

The fact of the matter is, while wind protection is an appropriate design strategy in Charlotte it is not well suited for this project. If we were designing an expensive house we might think about designing a double skin house – what use to be called a double envelope house. This type of house uses an outer envelope to protect an inner envelope. Ventilation is achieved by aligning windows in both envelopes. When you want fresh air you open the window in the inner envelope and reach through to open the window in the outer envelope. (I designed and built three double envelope houses for the high plains and I can tell you they work.)

In the next e-mail I will talk about sun shading and direct gains.


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