In this section:
Five-step plan to controlling ventilation
Providing strong ventilation for places where it's needed
Provide a source of pre-warmed fresh air to come into
Provide a route for air to move slowly through the house
Pumping hot air from the mezzanine to the bottom of the
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.
STEP ONE- ELIMINATING DRAUGHTS
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:
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,
|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.
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
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
|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
STEP TWO- PROVIDING STRONG VENTILATION FOR PLACES
WHERE IT IS REALLY NEEDED
In a well sealed house the effective removal of damp or stale air is extremely
important to prevent condensation and mildew. in particular:
cellars and voids under ground floors
There are three solutions to ventilating
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.
STEP THREE- PROVIDE A SOURCE OF PRE-WARMED FRESH
AIR TO COME INTO THE HOUSE
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.
that the rest of the house is well draught sealed first, or it a porch
vent may only add further unnecessary ventilation.
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.
the vent at the top of the porch so that only warm air passes into the
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.
STEP FOUR- PROVIDE A ROUTE FOR AIR TO MOVE SLOWLY
THROUGH THE HOUSE AND OUT
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
FOR INCOMING AIR
At the back of the kitchen cool
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.
FOR AIR FLOW BETWEEN
Between the top of the extension and the rear bedroom
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
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.
STEP FIVE- PUMPING HOT AIR FROM THE MEZZANINE TO
THE BOTTOM OF 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
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
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.