Handbook of Radon.
61. Experience with whole house pressurisation.
Conceptually, whole house pressurisation systems offer one of the neatest solutions to radon problems, especially in houses with complex construction. The reason for this is that their effect may spread throughout the house volume, unimpeded by obstructions that may be present underground and that might serve to reduce the effectiveness of underfloor suction systems. Unfortunately, the systems are not universally applicable but in the right circumstances they can be very successful.
This Section summarizes experience of these systems in the UK and USA. The emphasis is on principles, diagnostics, and practical experience. Details of particular products can be obtained from manufacturers' literature.
Pressurisation systems operate by introducing a flow of air into a building (usually a house) using a small fan. If a house is totally airtight (which it never is) the fan will build up pressure over a few seconds and then simply maintain over-pressure. Almost inevitably, radon will no longer be able to enter from the ground. At the other extreme, a house may be so leaky owing both to adventitious openings and deliberate use of windows that the fan may be unable to build up any significant pressure. It will then act simply to increase the local or overall ventilation rate, but the reduction in radon concentration may still be noticeable.
Real houses lie often between the two extremes and without testing for airtightness (see Section 59) it may not be possible to predict system effectiveness. However, if the house has draught-proofed windows and doors, and no substantial areas of unsealed timber floor or open chimneys, and if it is usually operated with all windows closed, there is some chance of a system working. A gas fire with a restricted flue has been shown not to affect performance to any great degree.
Usually the systems are installed at first floor level, drawing air from the roof space through a filter. Most systems do not incorporate a heating element, so air is introduced into the house at about ambient or roof space temperature. In wintertime this can lead to complaints of cold draughts, especially where the stairs lead directly into living areas, rather than into a hallway. Several householders have complained about this. Other problems have arisen in houses having secondary glazing. Because the fan may build up a slight over-pressure in the house, more air is caused to flow through gaps into the inter-pane space of secondary glazing systems. This causes a marked increase in condensation on the inner surface of the outer pane during cold weather.
In the USA, concern has been expressed about possible interstitial condensation in buildings not designed to run at an over-pressure: the highest risks may be in well insulated structures in cold climates, and structural elements of timber frame buildings may be at risk from decay. Therefore, some care should be taken when specifying these systems either where the house has secondary glazing or where it has a well insulated timber frame structure.
In summertime, and especially in houses where windows are kept closed most of the time, householders have complained about the increase in temperature in upstairs areas during sunny weather: a consequence of the systems drawing air from the roof space rather than from outdoors. In modern houses, there have also been complaints about the smell of 'tar' from the fan. This results from outgassing of bituminous felts used in roof construction. It should not be a problem in older houses unless they have been re-roofed, or in any house where a high performance felt has been used. Some concern has been expressed about whether the filters supplied with these systems are adequate to remove very small fibres of glass from roof insulation, and that may be present in roof spaces.
Another problem is that in some houses the roof space may be rich in radon owing to air flow from walls. The fan may therefore introduce radon into the indoor air perhaps increasing average levels upstairs whilst decreasing them downstairs.
In summary, problems have included:
increased inter-pane condensation in secondary glazing systems,
cold draughts down stairways directly into living areas,
excessive summertime temperatures in first floor rooms and
introduction of contaminants, including radon, from roof spaces into indoor air.
One anecdote concerns a system that caused cold draughts in the first floor bedroom. As usual, the fan was installed in the landing area. Unusually, the house had a door at the bottom of the stairs and this was closed at night to keep the cat downstairs. It was found that much of the air flow under these conditions went directly into the bedroom and down through the bare floorboards - presumably to find its way either into downstairs rooms or into the walls (see Section 59 for a discussion of leakage paths.)
Another anecdote (only included here because it again concerned a cat and a door at the bottom of the stairs) is of a house where the ratio of radon levels downstairs to upstairs was a record 20:1 prior to mitigation.
On the positive side, systems have proven successful in houses where these were more than usually airtight and where windows were kept shut, especially upstairs. Tests have indicated that opening windows on the first floor may result in air from a pressurisation fan 'short-circuiting' out of the window and failing to influence radon entry from the ground floor.
Provision of natural ventilation at ground floor level appears to have less effect on system performance. Nevertheless, radon levels in houses fitted with these systems (whether installed for condensation or radon control) may be highly dependent both upon the use of the house and upon any changes to the building envelope that alter its air tightness characteristics.
In spite of these problems, pressurisation systems are likely to remain one of the cheapest, most easily installed and potentially most effective of all options for radon remediation. Sometimes householders will commend systems for making the house 'fresher'. Amelioration of asthma symptoms has also been reported. The underlying problem in these cases may be mould and/or dampness, and could be addressed directly.
It is advisable to test houses prior to installation and to warn householders of the possible effect of innocent changes in window opening behaviour.
Running costs of the systems will be a few tens of pounds per year for electricity and between a few tens and over a hundred pounds per year in extra heating costs. It should be remembered that radon sump systems can incur similar costs.
More accurate estimates require knowledge of the fan flow rate 'in situ', the type of heating, and the severity of local winters. A minimum flow rate may be 100 m3 per hour, representing perhaps 0.3 ach. During a wintertime with a temperature difference of 18 K the marginal cost using on-peak electricity would be about £1 per day, falling to around 35p per day with gas central heating. A typical heating season may be 1500 or 1800 degree-days, thus the seasonal cost could exceed £100, or be as low as £30. Some fans run at much higher flow rates. In all positive pressure systems, the net change in house ventilation may be less than the system flow rate because incoming draughts are excluded.