空気の古さ（空気の年齢）とは
外から入ってきた空気が、家の中で何分たったかを「空気の年齢（空気齢）」といいます。換気の研究や設計で、空気がよどむ場所を探すのに使う本物の指標です。
年齢が若い所は新しい空気が届いている所、年をとっている所は空気が入れかわらずによどんでいる所。人の吐く息やにおい、ほこりも、こういう所にたまりやすくなります。
24時間換気と換気回数
2003年から、新しく建てる住まいには24時間換気が義務になりました（建築基準法。シックハウス対策）。1時間に家の空気の半分以上（0.5回/時）を入れかえる換気扇を、ずっと回しておくしくみです。
換気回数 0.5回/時 は、家じゅうの空気が約2時間で1回入れかわる量。ただし空気はまんべんなく入れかわるわけではありません。給気口を閉めたり、ドアの下のすき間をふさいだりすると、行き届かない部屋ができます。
窓は2か所あけると風が通る
風が家に当たると、風上の壁は押されて気圧が上がり、風下や横の壁は吸われて気圧が下がります。窓を風上と風下の2か所あけると、この気圧の差で空気が通りぬけます。
窓が1か所だけだと出口がないので、風の乱れで窓の近くが入れかわるくらい。向かい合う窓がないときは、換気扇を回して反対側の窓をあけると、通り道が作れます。
換気扇と給気口
換気扇は、家の中の空気を外へ押し出すぶん、どこかから空気が入らないと回りません。入口（給気口・窓）が少ないと家の中の気圧が下がり、すき間から無理に空気を引きこみます。換気扇の力は気圧の差が大きいほど弱まるので、思ったほど換気できなくなります。
キッチンの換気扇を強にすると玄関ドアが重くなるのはこのため。給気口を開けるか、窓を少しあけると楽に回ります。
サーキュレーターの使い方
換気は「家に入る空気の量」を決めますが、部屋の奥まで届くかは家の中の流れしだい。サーキュレーターで空気をかきまぜると、よどんだ所の空気も、入ってきた新しい空気と混ざります。
窓をあけて換気するときは、窓の近くで外に向けて回すと、部屋の空気を押し出す力になります。
このシミュレーションのしくみ
入れかわる量は、換気の設計で使う「換気回路網」の計算です。窓・給気口・すき間は「オリフィスの式」、換気扇は「強く引くほど量が落ちる」特性、風は壁にかかる圧力（風圧係数。低い建物の Swami と Chandra の式）で表し、入る量と出る量がつり合う家の中の気圧を求めます。窓1か所の風の乱れによる入れかわりは Warren の式（0.025×面積×風速）です。
家の中の流れは、床から約1mの高さの水平な面での2次元の流れの計算（1マス10cm、空気は縮まないとする）です。乱れは、ねばりけと混ざりやすさを大きめにして表しています。サーキュレーターとエアコンの風は、その場所で風を吹き出させて表しています。
空気の古さは、流れに運ばれながら1秒ごとに1秒ずつ年をとる量として計算しています。出口（換気扇や風下の窓）から出ていく空気の平均の年齢は、どんな流れでも「家の容積 ÷ 換気量」に等しくなる、という性質で計算を確かめています。
簡単にしたところ: 高さ方向の流れ、温度の差（暖かい空気が上へ行く・すき間風の温度差換気）、家具・人の動き、ドア下のすき間の抵抗は入れていません。すき間の量は鉄筋コンクリートのマンションの目安（相当すき間面積 2 cm²/m²）、換気扇の風量は一般的な製品の目安です。実際の家の換気量を測るものではありません。健康の判断には使えません。
おもな出どころ
- 建築基準法施行令 第20条の8（シックハウス対策の24時間換気、住宅は換気回数 0.5回/時。2003年7月施行）
- Sandberg（1981）「What is ventilation efficiency?」Building and Environment 16 巻: 空気齢と換気効率の考え方
- Etheridge と Sandberg（1996）『Building Ventilation: Theory and Measurement』Wiley: 換気回路網、空気齢、出口の空気齢が名目換気時間に等しいこと
- Swami と Chandra（1988）ASHRAE Transactions 94 巻: 低い建物の壁にかかる風圧係数の式
- Warren と Parkins（1985）「Single-sided ventilation through open windows」: 窓1か所の換気の式
- すき間の量（相当すき間面積 2 cm²/m²）と換気扇の風量は、一般的な建物・製品の値の目安です
使っているもの: 文字に Dela Gothic One・Zen Kaku Gothic New・IBM Plex Mono（SIL Open Font License）。
What is the “age of air”?
The “age of air” is how many minutes have passed since a bit of air came in from outside. It is a real measure used in ventilation research and design to find places where air gets stale.
Where the age is young, fresh air is arriving. Where it is old, the air is not being replaced and is stagnant. Breath, smells and dust tend to build up in places like this.
24-hour ventilation and air changes per hour
Since 2003, new homes in Japan must have 24-hour ventilation (Building Standards Act, to prevent “sick house” problems from indoor chemicals). A fan runs all the time and replaces at least half of the home’s air every hour (0.5 air changes per hour).
0.5 air changes per hour means enough air to replace all the air in the home about once every 2 hours. But the air is not replaced evenly. If you close the vents or block the gaps under the doors, some rooms do not get fresh air.
Open two windows to let the wind through
When wind hits a house, the wall facing the wind is pushed and its pressure goes up, while the walls on the far side and the sides are sucked and their pressure goes down. If you open windows in two places, on the windward and leeward sides, this pressure difference pushes air through the home.
With only one window open there is no way out, so only the air near the window is replaced by gusts. If there is no window on the opposite side, run an exhaust fan and open a window on the other side to make a path for the air.
Exhaust fans and vents
An exhaust fan pushes air out of the home, so it only works if the same amount of air can come in somewhere. If there are few inlets (vents and windows), the pressure inside the home drops and air is forced in through gaps. A fan gets weaker as the pressure difference grows, so it ventilates less than you might expect.
This is why the front door feels heavy when the kitchen fan is on high. Opening a vent or a window a little lets the fan work easily.
How to use a circulator fan
Ventilation decides how much air comes into the home, but whether it reaches the back of a room depends on the airflow inside. Stirring the air with a circulator fan mixes the stale air with the fresh air that has come in.
When you open a window to ventilate, running the fan near the window pointing outward helps push the room’s air out.
How this simulation works
How much air is exchanged is calculated with a “ventilation network”, the method used in ventilation design. Windows, vents and gaps follow the “orifice equation”, fans follow the rule that “the harder they pull, the less air they move”, and wind is shown as pressure on the walls (wind pressure coefficients, from the formula by Swami and Chandra for low buildings). From these, it finds the pressure inside the home at which the air coming in equals the air going out. The exchange through a single window caused by gusts uses Warren’s formula (0.025 × area × wind speed).
The airflow inside the home is a 2D flow calculation on a horizontal plane about 1 m above the floor (cells of 10 cm, with air treated as incompressible). Turbulence is represented by making the viscosity and mixing larger. The air from circulator fans and air conditioners is made by blowing air out at that spot.
The age of air is calculated as an amount that is carried by the flow and grows older by one second every second. The calculation is checked with a known property: for any flow, the average age of the air leaving through the outlets (fans and leeward windows) equals “volume of the home ÷ ventilation rate”.
What is simplified: vertical airflow, temperature differences (warm air rising, drafts driven by temperature), furniture, people moving and the resistance of the gaps under doors are left out. The amount of leakage is a rough guide for a reinforced-concrete apartment (equivalent leakage area 2 cm²/m²), and the fan airflows are rough guides for typical products. This does not measure the ventilation of a real home, and it cannot be used for health decisions.
Main sources
- Order for Enforcement of the Building Standards Act (Japan), Article 20-8 (24-hour ventilation against “sick house” problems; 0.5 air changes per hour for homes; in force since July 2003) (in Japanese)
- Sandberg (1981), “What is ventilation efficiency?”, Building and Environment 16: the ideas of age of air and ventilation efficiency
- Etheridge and Sandberg (1996), Building Ventilation: Theory and Measurement, Wiley: ventilation networks, age of air, and the fact that the age at the outlet equals the nominal time constant
- Swami and Chandra (1988), ASHRAE Transactions 94: the formula for wind pressure coefficients on the walls of low buildings
- Warren and Parkins (1985), “Single-sided ventilation through open windows”: the formula for ventilation through one window
- The amount of leakage (equivalent leakage area 2 cm²/m²) and the fan airflows are rough guides based on typical buildings and products
Fonts: Dela Gothic One, Zen Kaku Gothic New and IBM Plex Mono (SIL Open Font License).