In this section:
Five-step plan to controlling ventilation
Eliminating draughts
Providing strong ventilation for places where it's needed
Provide a source of pre-warmed fresh air to come into the house
Provide a route for air to move slowly through the house and out
Pumping hot air from the mezzanine to the bottom of the house

Any house that does not have air passing through it is fundamentally unhealthy. Ventilation is vital to replace moist, stale air with fresh clean air. The problem is that ventilation can also account for up to a third of the heat loss of the house. Just as "weeds" are plants we don't want, "draughts" are ventilation we don't want. The trick is to control the passage of air such that it is just enough to meet the ventilation needs and has an efficient route through the house.

We followed five steps to controlled ventilation in the Yellow House:

Step one: Block up all sources of unwanted draughts
Step two: Provide strong ventilation for places where it is really needed
Step three: Provide a source of pre-warmed fresh air to come into the house
Step four: Provide a route for air to move slowly through the house and out.
Step five: Pump hot air from the mezzanine to the bottom of the house.

The only draughts that we are aware of are from incoming outside air. However, there is a basic law to draughts- all air coming in is balanced by air going out somewhere else. To eliminate draughts one has to deal with both kinds:

Incoming draughts
Every time there was a windy day we went around the house feeling for draughts. We found that the main sources, in decreasing order of importance, were:

Doors and windows Draughtstrips around the sides
Gaps in the front room floor Fill in gaps with dark wood coloured mastic
Around front room skirting boards Fill in gaps with white mastic
Gaps in the bedroom floor Fill in gaps with pale wood coloured mastic
Around bedroom skirting boards Fill in gaps with white mastic

Gaps between floorboards look small, because they are narrow, but they can add up to a large hole. We estimated that, if added up, the gaps in the front room floor added up a hole 20 cm square. Because the floor is suspended over a ventilated void, this is like taking a brick out of the wall! We plugged these gaps with thin slices of left over Celotex and/or mastic of a colour that would blend in. The smart builders way to mastic a gap is to put masking tape on either side of the gap, then lay a line over the gap with a mastic gun and push it deep into the gap with a damp finger. When the mastic has dried, carefully lift the masking tape.

Outgoing draughts
These are harder to find. Of course, depending on which way the air was blowing, the gaps for incoming air often also doubled as gaps for air to pass out. However, there are places which specialise in outgoing draughts that on would never normally notice. The place to take special care are holes in the ceilings. Warm air collecting under ceilings exerts a powerful stack effect and if it has anywhere it can escape it will pull cold air in behind it. Otherwise any gaps at the top floor of the house will have tendency to pull air out. For houses with unconverted lofts, the greatest source of outgoing draughts is often from gaps around the loft door. In the Yellow House, the main sources of outgoing draughts, in descending order, were:

The old fire places Block them with rockwool or cap them
Doors behind the mezzanine Draughtstrips around the sides
Holes around pipework passing through the top floor ceiling into the loft Fill with foam and mastic


In a well sealed house the effective removal of damp or stale air is extremely important to prevent condensation and mildew. in particular:

• bathrooms
• cellars and voids under ground floors
• kitchens
• workshops

There are three solutions to ventilating these spaces:

1. Seal them off from the rest of the house and provide them with a strong through draught
2. Provide extractor fans that can be turned on when needed and/or have timers
3. Use heat exchanging fans that use the stale air to warm incoming replacement air.

In the Yellow House we used all three systems. We sealed off the void under the front floor very carefully and actually increased the ventilation by adding an extra vent. We also added a new vent under the stairs for the cool "root store" for storing vegetables and wine.

In the kitchen we fitted a Baxi heat exchanger fan through the wall. It recovers 85% of the heat in the extracted waste air to heat the incoming air. In defiance of the technical advice we placed it over the cooker and cut a standard carbon filter to fit in it. So far it has worked fine. Heat exchanging fans would work very well in a bathroom, but need to be fitted through an external wall, and neither of the Yellow House toilets had external walls. We therefore fitted fan extractors with timers in the bathroom and utility room.

Every degree that air can be warmed before it enters the house is one degree less heating that it will need inside the house. Thus, prewarming can greatly reduce heating bills. The typical way to preheat air is for air to enter the house through a south facing porch or conservatory.

In the Yellow House incoming air passes through the sun porch at the rear of the house. Even though this is only South East facing on overcast winters days it still adds 3-4°C to incoming air. On sunny days it will add far more. In spring and autumn air passing through the porch can actually be warmer than air inside the house.

There are some rules for prewarming in a porch (or conservatory) to work.

1. Ensure that the rest of the house is well draught sealed first, or it a porch vent may only add further unnecessary ventilation.

2. Install an adjustable vent which can be closed. In a well sealed house, air will tend to be pulled through the porch. However, when there is a strong wind in the wrong direction, warm air may be pulled out of the house through the porch. In the Yellow House we adapted one of the clear plastic vents with revolving fins that are typically installed in windows. With these it is easy to see from the direction that the fins are revolving whether air is being pulled in, or out, of the house and so close the vent if necessary.

3. Install the vent at the top of the porch so that only warm air passes into the house.

4. Install an adjustable vent elsewhere in the porch to control the cold air entering the porch. In 2002 we plan to build a porch on the front side of the building with a solar powered fan to pull warm air into the house.

It is tempting not to bother with a ventilation path and simply block most of the draughts and leave a few to provide background ventilation. The problem with such uncontrolled ventilation is that it will be very variable (being too high under strong winds and too weak under still winds) and inefficient (not ventilating all rooms equally). Providing a path for ventilation allows the flow of air to be controlled through each room and at the points where it enters and leaves the house.

When we renovated the house we installed adjustable vents throughout the house to allow us to control the flow of air. The key word here is "adjustable". It is impossible to predict air flows, and the beauty of adjustable vents is that you can tweak the air flow until you get a system that works. We installed vents in the following places:

At the back of the kitchen cool cabinet:
The incoming air cools food and preserves.

Between the sun porch and the extension:
Incoming air is prewarmed in the sunporch before entering the house.

Between the top of the extension and the rear bedroom and office
The extension roof is slightly higher than the first floor, so there is a small area of the rear room walls within the extension space. The vents here work extremely efficiently to draw warm air from the top of the extension into the top of the house. They can be closed if they are also drawing up sound and cooking smells.

Between the utility room and the upstairs bathroom
One of the panels in the bath opens up as a laundry shoot to the utility room directly under it. When open it doubles as a vent, pulling warm air up through the damp bathroom. This is a particularly useful vent and helps keep the bathroom dry.

And, of course, there's always air flow up the stairs.

Trickle vents in the mezzanine skylights
The stack effect ensures that these small trickle vents at the very top of the house gently pull air out of the house. These vents are open most of the year and are only closed during the coldest part of the winter. During the summer evening, we ventilate the house by opening just these skylights and the windows at the back of the house. A powerful stack effect through the skylights pulls the cool garden air right through the house.

Hot air tends to collect wastefully at the top of a house. This problem is particularly acute in the Yellow House because the bedroom is a well insulated double height room. As a result the mezzanine can become uncomfortably hot, especially when there is late afternoon sun. The usual solution, opening the skylights, would not only throw away the heat at the top of the house, but would also exert a very powerful stack effect pulling cold air into the house.

We found a way of turning this problem into an asset by fitting a fan to pull air down from the mezzanine into the back of the extension where it is needed. The duct collects air at the roof ridge, pulls it through the loft space behind the mezzanine, down through the office, through the utility room, and out at the back of the extension. The ducting was simply and cheaply constructed from 100mm plastic drainpipe and standard drainpipe connectors. Unfortunately we could find no readily available alternative to PVC. The ducting behind the mezzanine is thoroughly insulated with plumbing insulation and lagging recycled from the old hot water tank. Once the duct enters the warm interior of the house it is unlagged.

To fit behind the insulation in the mezzanine ceiling we narrowed the duct to two lengths of 50mm pipe which run up to two small holes in the apex of the roof. A more expensive option would have been 50mm flat ducting which is available from most manufacturers. The duct enters the extension through a vent with gravity flaps which close to prevent back draughts (without which warm air could pass back up the duct from the extension).

We contacted several fan manufacturers and found their technical departments extremely helpful. In the end we went with Vent Axia which recommended a high performance ACM 100mm in-line duct fan (63 trade price +VAT). This could have been placed anywhere along the duct, but, because of noise, the optimal location was behind the ground floor toilet in the utility room. A Vent Axia thermostat at the apex of the roof is set to operate the fan when the temperature at the roof ridge is 23°C, 5 °C higher than the temperature chosen for the house as a whole.

According to Vent Axia, the ACM will move 144m3 of air per hour on low setting. As the volume of air at the mezzanine level is only 32m3, the fan only needs to operate for 15 minutes at a time. To ensure that the fan does not exceed this amount, and to ensure that it is not constantly clicking in whenever the thermostat is triggered, it is plugged into a standard 24 hour security timer (5 from B&Q) set to run the fan for 15 minutes every hour in mid afternoon to early evening. Because the fan is controlled by both the thermostat and timer it only works if the air is warm enough and when the warm air is needed.

In practice we find that the fan is not needed in winter, but is very useful on sunny days in spring and autumn when late afternoon sun has heated the bedroom. These times of year are on the edge of the heating season, and recycling the mezzanine heat helps to increase the period of the year when no additional heating is needed.

Building control were surprisingly tolerant of the ducting, requiring only that a ring be fitted around it at the top of the rear bedroom to prevent fire passing up the duct and into the roof space. I would add that thermostatic ducting should only be fitted in a house with good smoke detectors, as it could potentially spread smoke throughout the house.


A vent in the wall between the downstairs utility room and the extension. The glass blocks below are part of the insulation and lighting strategy.

Another vent! This one is in the external wall of the office upstairs.

Fresh, warm air, heated up in the back porch (above), is pulled into the main living area via an adjustable vent in the ceiling (below)

Loose gaps where pipes go into the roof space were very carefully plugged to prevent uncontrolled draughts.

The fan above the cooker on an external wall.

Wherever possible, draughtstrips were applied to door and window frames.

The vent in the cool cabinet was carefully positioned to store store food below room temperature.

The fan in the bathroom reduces the risk of damp.