era | Wind
When do you feel comfortable?
A breeze can make or break your comfort in a space.
It can cool you down on a hot day, or make your body freeze when it’s chilly.
It is 3 am, my daughter woke me when she had a scary dream and my face is sore.
I have a cold and it is hard to sleep.
It is a cold night and like I said, my face hurts.
But it had me thinking that when I was sitting outside yesterday I felt amazing.
I was still sick then too – don’t get me wrong – but I was sitting on big cushions in the sunlight. The air was cold, but I was sitting in a place out of the wind, so I was warm from the sun and not affected by the breeze.
This is my favourite spot when I’m not well, in fact it’s my favourite spot when I AM well! – fresh air, no cold wind and warm sunlight.
Oh, and a coffee on the side.
Wind.
Someone once told me that to her, architecture is about keeping the wind and dirt out of a childs eyes. I love this sentiment, and I don’t think that it applies only to children.
We live in a place which is particularly windy and predominantly from one direction.
How do I know this? I feel it when I walk outside – get blown away some days in fact!
I can also see it in the wind rose for this area.
What is a wind rose, I hear you ask?
‘A wind rose is a graphic tool used by meteorologists to give a succinct view of how wind speed and direction are typically distributed at a particular location.’ – Wikipedia
It is also used by architects to design buildings and their spaces.
Wind, particularly cold wind, can severely affect your thermal comfort in winter – especially if you have a cold. Alternatively, a cool breeze in summer can keep you nice and cool on a hot day. And again a warm breeze blowing through can warm you up.
Depending on where you live will depend on which direction and for how long the wind will blow, however the outcome is the same. The movement of air over your skin instigates positive or negative response in your body affecting your thermal comfort.
Here’s a bit of info about that – I love this stuff, but skip past if you don’t want to get into the detail.
‘Your skin is usually warmer than the air around you. Imagine that the air was very still. Then all the little bits of air next to your skin would warm up as heat conducts from your skin to the air. As the air gets warmer and warmer less and less heat would transfer from your skin. If they were at the same temperature, there would be no direct heat transfer. Now suppose you take that hot air and move it away and replace it by some other air which is still cool. That air can now get heated up by your skin, which is the same as cooling down your skin. The heat moves from the hot place to the cold place. This is one thing that the wind does. It is cool air that is moving that can push away warmer air that is near your skin. By constantly moving away the warm and replacing it by cooler air, it keeps cooling your skin. This process of getting better heat transfer by forcing air to move past a warmer surface is called forced convection.
The other important way that wind cools your skin is by evaporation of sweat. Even if the air is the same temperature as your skin or even hotter, wind can still cool your skin. Our skin contains sweat glands that leak water (in the form of sweat) out of pores. Imagine that the air is very still again. The water molecules in the sweat on our skin will diffuse out into the air. But that will make the air right next to our skin more humid and eventually, the water molecules will stop leaving your skin. This is like the heat accumulating in the thin layer of air next to your skin. The wind again pushes that moist air away and replaces it by drier air in your surroundings. OK, but how does that cool you. Well, the water on your skin is liquid. When it vaporizes and turns into a gas of water molecules in the air, that takes heat. That heat comes from your skin, which means your skin cools down. This is water "boiling" at a much lower temperature than we think of as boiling water. But it takes the same amount of heat for each droplet of water. It just happens at a lower temperature. This is how wet clothes and wet dish air dry. They dry faster when the wind is blowing and when the air is drier. Same for our skin. If the air is dry, it can cool us more. If the air is humid, then it can't take away as much moisture and therefore less of our sweat vaporizes, so humid air is less effective at cooling us.’ – Kim Aaron, Spacecraft Mechanical Engineer – www.quora.com
If we know where the wind is coming from, when it comes and how strong the wind is at a given time of year we can plan a building and its spaces
to either:
- Protect from a cold wind in winter
- Encourage a cool wind in summer
H o w d o y o u p r o t e c t f r o m t h e w i n d ?
Work out where the wind that you do not want to feel and at what time of year you do not want to feel it is coming from.
Then design a wall between the direction the wind is coming from and where you will be sitting, creating a place out of the wind otherwise known as a wind shadow.
Ho w d o y o u e n c o u r a g e t h e w i n d ?
Work out where the wind you do want to feel and at what time you want to feel it is coming from.
Then open the building to that direction.
The wind may be coming from the same direction for both scenarios.
As such, use an operable unit such as a door, window or screen which can be opened or closed in order to protect in one instance and encourage in another.
Thermal Comfort.
Attaining thermal comfort is associated with low rate of energy expenditure and the absence of sweating and shivering. There are four environmental factors which affect thermal sensation.
In other words, your comfort - how you feel.
air temperature
humidity
air movement
mean radiant temperature of surroundings
And a human being has three ways of controlling their thermal comfort.
Metabolic Rate
Clothing
Shelter
Shelter.
‘A shelter is the main instrument for fulfilling the requirements of comfort.
It modifies the natural environment to approach optimum conditions of livability. It should filter, absorb, or repel environmental elements according to their beneficial or adverse contributions to man’s comfort. The major elements of climatic environment, which affect human comfort, can be categorised as: air temperature, radiation, air movement and humidity.’ [Olgyay, 1992]
‘One of the principal functions of any building is to modify the physical characteristics in it and around it so as to make them more acceptable for the occupants in the performance of their various tasks.’ [Greenland and Szokolay, 1985]
‘Air movement affects body cooling. It does not decrease the temperature but causes a cooling sensation due to heat loss by convection and due to evaporation from the body. As velocity of air movement increases, the upper comfort level is raised. However, this rise slows as higher temperatures are reached.’ [Olgyay, 1992]
‘For a simple vertical air space such as a wall .. about 60% of the heat transfer is by radiation, 30% by convection and 10% by conduction.’ [Greenland, 1991]
There are a few ways in which your body can be cooled when you are in a space.
‘The live human body constantly generates heat. If this heat is not dissipated by conduction (direct contact-touch), convection (air movement induced by temperature differential) and radiation (loss of heat energy to nearby surfaces of lower temperature), the body will feel uncomfortable and in extreme cases go into stress and ultimately cease to function (die).’ NEVILLE QUARRY
Convective: Stemming from transfer of heat across the boundary film and in the body of the air by convection
Radiative: Where heat is transferred by radiation between the building surface and the other surfaces to which it is exposed. This component is also a function of the long wave emittance of the surface, which can vary from 0.05 for polished aluminium to 0.90 for most ordinary metallic materials irrespective of colour. [Greenland, 1991]
‘American scientists have tried to establish a physiological measurement, combining the effects of temperature, humidity and air movement, called the effective temperature scale (ET). They place the comfort zone between 30 and 70% humidity
.. [however,] considering the range of observations and opinions there is no precise criterion by which comfort can be evaluated.’ [Olgyay, 1992]
Basically, you can cool your body by the movement of air - a breeze - or by the surface temperature of a material surface - the building.
Ventilation.
The three ways of achieving controlled ventilation are:
exploitation of existing winds
stack effect
low energy forced ventilation – this is strictly speaking an active system but as most fans possess power ratings of less than 100 watt, this kind of ventilation is frequently incorporated with passive control
Wind.
For wind calculations several factors have to be considered. First is the decrease in measured wind speeds at levels close to the ground, second is the modification of the operative wind pattern by local topography and the immediate surroundings, third is the comfort evaluation – the desired breezes versus the unwanted winds.
The wind effects of the free atmosphere are modified and slowed down at low levels, and at ground surface the air is almost at rest. [Olgyay, 1992]
‘.. the need for cooling and relief from vapour pressure in periods of high absolute humidity should be considered in planning design conditions. To set a maximum it is customary to use outdoor temperature data .. which were equalled or exceeded not more than 5% of the time in the summer months.’ [Olgyay, 1992]
A hill has modification effects on both wind and precipitation distribution .. A wind flow is diverted by a hill in both its horizontal and vertical stream patterns, causing higher speeds near the hilltop on the windward side and less turbulent wind conditions on the lee slope. The resultant wind distribution on a hill creates high velocity areas below and at the sides on the crest; the lowest speeds are near the bottom of the hill in the wind “shadow”.
Precipitation on the windward side is carried over a hill by the wind which strikes the slope, and falls on the lee side, where irregular weak air movements prevail. However, high mountains cause exactly the reversed precipitation distributions. When air is forced to ascend on the windward side, this produces adiabatic processes of condensation and precipitation.
‘In hot-humid areas air movement constitutes the main comfort-restoring element. Sites off-set from the prevailing wind direction, but exposed to high air stream areas near the crest of a hill, or high elevations on the windward side near a ridge are preferable.’ [Olgyay, 1992]
Exploitation Of Existing Winds
A wind blowing around a simple building produces a positive pressure region on the upwind side and a negative pressure region on the downwind side. It is the combination of the positive and negative pressure areas which generates the ventilation through the building.
As it is air speed which is the important cooling component of ventilation, attention should be drawn to its control, especially when prevailing winds are light. The maximum air speed is produced in a room when the upwind openings are smaller than those downwind. The successful utilisation of this effect depends on management of windows and behavioural response by the occupants. They have to know how to adjust the windows and where to sit and so benefit from the cooling effect of the moving air.
Windbreak.
A windbreak, according to C.G.Bates’ description, diverts the air currents upward, and while they soon turn back and again sweep the ground, an area of relative calm is created near the ground .. The type of windbreak used has a definite effect on the resultant airflow pattern and on the area of protection. Solid wind barriers, or walls, cause eddies over the top which reduce their effectiveness. In general, three belts with greater density and thickness will produce a larger effect in wind protection. [Olgyay, 1992]
‘.. in structures where an outlet is not provided no airflow will occur inside the building. Similarly, it is evident that large openings placed opposite each other, and positioned at the high and low pressure areas respectively, will provide the maximum air changes within the structure. However, for summer cooling comfort sufficient speed is of more importance than the amount of air change. By using a smaller sized inlet opening, “Venturi effect” occurs, securing maximum air speeds within the structure .. the reversed arrangement, with large inlet and small outlet, is inefficient because the high speed occurs behind and outside the building.’ [Olgyay, 1992]
That is what happens in my favourite space - the one where I am warm in the sun and sheltered from the cool breeze.
.. take a breath and
Let’s get nerdy ..
Air change by gravitation is one of the motives for the use of high ceilings in warm environments.
The approximate inside air speed can be expressed as:
Vi = Cm
P
As the number of air changes can be calculated from the rate of air flow [Q], this expression provides an estimation of the resultant air speeds and furnishes a measure for the aperture sizes needed under specific climatic conditions.
Where
Vi = mean inside speed, ft/min
C = number of air changes, cu ft/min
P = airflow pattern, cu ft
m = mean distance between inlet and outlet, ft
The Venturi Effect as interpreted by Hassan Fathy
This effect, which is quite easy to understand in general terms, has recently been given more precise expression in the following formula:
The rate of air flow through a building in cubic feet per hour
=
3,150 (area of inlets in square feet)(wind speed in miles per hour)
This formula holds if the wind in the immediate vicinity of the inlet is at right angles to the plane of the wall.
If the areas are the same size, then:
Area of outlet value
Area of inlet = 1 3,150
If the outlet is larger than the inlet, then:
Area of outlet value
Area of inlet = 2 4,000
= 3 4,250
= 4 4,350
= 5 4,400
If the outlet is smaller than the inlet, then:
Area of outlet value
Area of inlet = ¾ 2,700
= ½ 2,000
= ¼ 1,100
‘Thus we see clearly that the greater the ratio of outlet area to inlet area, the greater the airflow through the building.
A shaded area with a through draught will always be relatively cool …’[Fathy, 1973]
The Inertia Effect as interpreted by Victor Olgvay
An inlet and outlet placed symmetrically will result in a straight inside flow pattern since the external pressures are equal. With asymmetrically arranged openings, in accordance with the difference in component pressure forces, the air will enter the building at an oblique angle. The inside flow will tend to follow its original direction by inertia until, overcome by differences in pressure, it will turn toward the outlet.
Divisions inside the house
Straight flow secures the speediest air movement, and any change in direction slows the effect . Any abrupt course change caused by furniture, equipment, or partitions will cut air speeds markedly. Therefore the placing of internal divisions should be arranged in consideration with the flow pattern.
Location of outlet and inlet openings
A relatively large ratio of outlet to inlet size secures the speediest, and hence most cooling air flow within the building. The location of the outlet is irrelevant to the pattern of the incoming flow, and speeds will be retarded only if energy is consumed by directional changes.
Directional effect of inlet attachments
features outside the building near the inlet openings can influence the flow pattern markedly. An overhang at ceiling height intercepting and diverting air masses toward the inlet improves the ventilation effect. Similar solid overhangs, when placed directly above the window opening, cause the air to flow toward the ceiling because they eliminate outside pressure effects from above.
Ventilation by temperature differential
Temperature differences existing between the air inside and that outside the building, due to the weight disparity, cause the warmer air column to rise by displacement gravitation. The higher the temperature difference, the larger the height between the inlet and outlet and the greater their size; the more rigorous will be the “stack effect”. The approximate rate of such air change when the area of inlets is equal to the area of outlets can be expressed as:
Q = 540A√H(ti – to)
Where
Q = rate of air flow, cu ft/hr
A = area of inlets, sq ft
H = height between inlets and outlets, ft
ti = average temperature of indoor air at height H, °F
to= temperature of outdoor air, °F
There is SO much to talk about when it comes to finding a home that fits you, that suits your lifestyle and that is
SPECIFIC TO YOU.
My hope is that you find your comfort spaces!
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Til next time!
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