Ventilation in a detached house is the topic people usually think about last, and the one that decides first whether a new house is good to live in. An old house “breathed” through cracks, the stove and leaky windows; a new airtight house does not, and that is good, because the heat stays inside. But the air still has to change, and for that there is heat-recovery ventilation.

Below are six reasons why an airtight house needs it, how the system works and what it asks of you. All our houses have heat-recovery ventilation, and this is our own experience of it.

Why an old house breathed and a new one does not

In an old house the air changed by chance, through the walls, the windows and the chimney: in winter the air change was large and so was the heat loss, in calm summer weather it was almost nil. A new house is designed to be airtight, because every leak is heat loss and a path for moisture into the structure. The measured air leakage rate of our houses has stayed between 0.13 and 0.35.

An airtight house without ventilation is a stuffy, damp house. An airtight house with mechanical air change is one where the air is always fresh and the heat stays inside. Ventilation is therefore not the opposite of airtightness but its other half. What airtightness means for the energy class is explained in our article on the energy class A house.

1. Moisture and mould

A family produces several litres of moisture a day: breathing, showers, cooking, drying laundry, plants. In an airtight house without air change it builds up indoors, condenses on window frames and the corners of exterior walls, and mould grows there. A mechanical system takes the humid air out continuously, not only when someone opens a window.

For a timber house this is also a structural question: moisture that cannot leave the house looks for a way into the wall. Heat recovery is one of three protective layers together with the vapour control layer and the drainage, which we wrote about in our article on timber house lifespan.

2. Indoor air quality

People produce carbon dioxide, furniture and finishing materials give off volatile compounds, and radon may come from the ground. All of it builds up in an airtight house if the air is not changed, and the result is headaches, tiredness and poor sleep that people rarely connect with the air they are breathing.

Heat-recovery ventilation changes the air continuously and in calculated amounts: fresh air into the bedrooms and living room, used air out of the kitchen, bathroom and toilet. The air moves in one direction, from clean rooms towards the humid ones, so smells do not spread.

3. Heat recovery

The heart of the system is the heat exchanger, where the warm outgoing air and the cold incoming air pass each other without mixing and the heat transfers from one to the other. The incoming air reaches the room already pre-warmed, and the heating system only has to cover the remainder. The fans themselves use little electricity — many times less than the heating energy the exchanger gives back.

Together with an airtight envelope and low-temperature underfloor heating, this is what gives our houses energy class A or B. Which heat source sits beside the unit is a separate decision, compared in our article on choosing between a ground source heat pump and an air-to-water pump. The exchanger’s working principle is described on Wikipedia.

4. Filtered air

The incoming air passes through a filter that holds back dust, pollen and part of the outdoor pollutants. For allergy sufferers this changes life in spring: the air indoors is clean even when birch pollen flies outside. Near a main road it is a health question as much as a comfort one, because air through an open window is filtered by nothing.

The filter is also why the system needs looking after: a full one cuts the air flow and strains the unit.

5. Comfort and quiet

In a ventilated house you do not have to open a bedroom window for fresh air in the morning, or listen to the street noise that comes in with it. Today’s units are quiet, and the ducts are sized and routed so that the air movement is neither heard nor felt as a draught. Ducts do carry sound between rooms if they are laid carelessly, one of the details we cover in our article on sound insulation in a CLT house.

In summer some units switch to a bypass, where heat is not recovered and cool night air comes straight indoors: with the massive CLT walls this gives natural cooling. How glazing and room height work together with air change is explained in our article on large windows and high ceilings.

6. The occupancy permit and the energy label

The energy calculation of a new house assumes a ventilation solution, and heat recovery affects the result directly. The occupancy permit application includes the air-flow measurement report, which proves the system has been adjusted to the design; without a proper solution, today’s energy performance requirements are practically impossible to meet. We wrote about the paperwork in our article on the occupancy permit for a house.

How the system works

The system consists of a unit in the technical room with two fans, a heat exchanger and filters; supply ducts to the bedrooms and living room; extract ducts from the kitchen, bathroom and toilet; and outdoor-air and exhaust openings on the facade or roof. The air flows are calculated by room and volume, and adjusted by measurement in the finished house.

In our houses this is designed together with the building: duct penetrations through the CLT floors and walls are cut in the factory, and the unit sits beside the heat pump and the hot water tank. The technical solutions are on the construction info page.

What ventilation in a detached house asks of you

Little, but regularly. The filters are changed usually twice a year, in spring after the pollen season and in autumn, more often in a dusty place. The heat exchanger is cleaned according to the maker’s instructions, usually once a year, and the outdoor-air and exhaust openings are kept free of leaves and snow; ducts are cleaned every few years. The unit runs all the time, even when the family is away, because moisture and volatile compounds take no holidays.

At handover the client gets the maintenance instructions and the maker’s guarantee, and we provide after-sales service. The annual check belongs on the same list as the roof, described in our article on flat roof construction.

Summary

Without heat-recovery ventilation an airtight house is damp and stuffy; with it the same house is fresh, dry and energy-efficient, for a couple of hours of attention a year.

Come and see a finished home to hear how quiet it is, or send the plot documents through the contact page and our architect will show how the ventilation is solved in your layout.