Be Positive on Ventilation

From http://www.housebuildersupdate.co.uk/ the illustration shows air being drawn into the house from the eaves and then pumped around the 'whole' house and the warm moist air being pushed out of the building envelope 

Positive Input Ventilation (PIV) systems are now starting to be much better known. This may well be due to the profits that the companies are making from the installation of these types of systems across the UK by the social housing sector.

Many problems have been encountered by this sector and the private sector by the installation of energy saving measures. Sealing up houses with draught proofing, insulation, double glazing, new doors, removal of chimneys etc has meant that the warm moist air that we generate in our homes gets stuck inside. This high humidity means that the excess water has to go somewhere and this tends to be onto cold spots in the fabric of the building. The levels of condensation have radically increased and this leads to mould issues as well as potential rot etc.

So, the industry have come up with a technological fix in the form of PIV.

Basically the simplest form of PIV is where fresh air is drawn into the building using a fan and then pushed out (generally from a single point) into the house. The fans are designed to run constantly and so are low energy consumers and quiet (but I would always check their wattage and db levels before making your final choice) The lower humidity (and cooler air) then effectively 'waters down' the higher humidity air constantly by forcing it out through any cracks / trickle vents etc that are in the building.

Having a lower humidity in the house means that walls can dry out slowly and condensation problems can be eradicated. Lower humidity area is also easier to heat and so perversely the house can become warmer by having fresh air pumped into it. However there are some potential issues. The cool air has to come in somewhere and this immediate area will be cooler due to this. So it is important to locate the input vent well. The higher pressure created by the fan will force air out from the easiest locations (out through extraction fans, trickle vents, ill fitting windows etc.) and so the benefits of installation might not be felt throughout the house. This is especially important when one thinks that most of these are installed through the first floor ceiling and many houses have most of the humidity in the ground floor kitchens, bathrooms etc. Having doors open creates better air flow but might not be practical where noise, smells, privacy etc require doors to be closed. So care is required when assessing the suitability of the product to your particular situation.

PIVs can be fitted as a DIY project, but there are a few horror stories where people have found that it is better to bite the bullet and get one of the manufacturers to install it for you. At least then you get piece of mind, a guarantee / warranty etc.

Whilst I see PIV as a potentially viable sticking plaster for homes, I would always aim to tackle the fundamental causes of damp / mould by using appropriate materials and ventilation strategies. However, in an imperfect world, the cost of installing a PIV system (around £700 - £900 fully installed price or £300 just for the machinery) may well be sticking plaster that you need.

There are, of course, variants within the PIV world, so you can have multiple output vents, integral heaters, automatic controllers, etc, etc. Of course, the more you pay then the better the controllability of the system, etc. So again you will need to assess which system suits you best. The specifiers / engineers from the various companies should be able to assist with this choice.

The best well known PIV systems come from Nuaire and Envirovent, but all the main ventilation companies produce their own versions.

Good luck.