How it is calculated
Finland: the outdoor airflow is the largest of three figures — 0.35 dm³/s per m² of floor area, 18 dm³/s per home, and the sum for the living rooms (12 dm³/s for the largest or only bedroom and for any other bedroom over 11 m², 8 dm³/s for the other living rooms, plus 6 dm³/s with a sauna). Extract flows come from the guideline values for the kitchen and wet rooms. Supply and extract are designed to be equal, so the design airflow is the larger of the two. England: the whole dwelling rate is the larger of 0.3 l/s per m² of internal floor area and the rate for the number of bedrooms; the extract rates for kitchens, bathrooms, utility rooms and WCs are listed separately. Sources: FINVAC guide Opas asuinrakennusten ilmanvaihdon mitoitukseen (2019), based on Ministry of the Environment decree 1009/2017; Approved Document F Volume 1, 2021 edition, paragraph 1.24 and tables 1.1–1.3.
Finland: outdoor air = max(0.35 × A; 18; 12 + 8 × (rooms − 1) + 4 × large bedrooms + 6 with a sauna) dm³/sFinland: extract = 8 kitchen + 10 per bathroom + 7 per WC + 8 per utility room + 6 per closet or storeroom + 6 saunaFinland: design airflow = max(outdoor air; extract)England: whole dwelling rate = max(0.3 × A; 19 + 6 × (bedrooms − 1)) l/s, 13 l/s with one habitable roomm³/h = l/s × 3.6; air changes per hour = m³/h ÷ (A × H)Example
Finland: an 80 m² flat with a living room, two bedrooms, a kitchen, a bathroom and a WC needs 0.35 × 80 = 28 dm³/s by floor area and 12 + 8 + 8 = 28 dm³/s by rooms; the extracts add up to 8 + 10 + 7 = 25 dm³/s. The design airflow is 28 dm³/s, about 101 m³/h, which is 0.5 air changes per hour with a 2.5 m ceiling. England: the same two-bedroom flat needs the larger of 0.3 × 80 = 24 and 25 l/s, so 25 l/s or 90 m³/h.
Tips from practice
- Supply and extract are usually designed to be equal; in Finland the totals may differ from the design values and from each other by no more than 10 %.
- Cooking needs a boost: the Finnish guide asks for at least 25 dm³/s through the cooker hood. The extra air can come as more supply air, by turning other extracts down or through a separate outdoor air valve.
- A fireplace, a central vacuum cleaner and a cooker hood that blows outside all take air out of the home: plan where their makeup air comes from.
- In the Finnish guide a living room over 22 m² needs 0.35 dm³/s per m²; if yours is large, add the difference to the room sum.
- The result is a reference for planning and comparing. A ventilation designer sizes and commissions the actual system, and building control checks the design.
Questions and answers
How much ventilation does a house need?
In Finland at least 0.35 dm³/s per m² of floor area (about 0.5 air changes per hour with a 2.5 m ceiling), at least 18 dm³/s per home, and enough for the rooms: 12 dm³/s for the main bedroom and 8 for the other living rooms. In England at least 0.3 l/s per m² and 19 l/s for a one-bedroom home, plus 6 l/s for each further bedroom.
What is makeup air?
Outdoor air that replaces what a cooker hood, a fireplace or a central vacuum cleaner takes out of the home. Without it the home goes into underpressure and impurities can be drawn in from the structures. Boosting the kitchen extract from 8 to 25 dm³/s needs 17 dm³/s more outdoor air unless other extracts are turned down.
How do I convert l/s to m³/h?
Multiply by 3.6: 28 l/s is 100.8 m³/h. One dm³/s is the same as one litre per second.
How much should ventilation be boosted?
In Finland the ventilation of a home must be able to run 30 % above its normal design airflow; the calculator shows that figure. Cooking is boosted through the cooker hood, to at least 25 dm³/s.
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