Sub Floor Ventilation

Sub floor ventilation

Though the use of suspended timber floors is rarely seen in Spain, there is still good reason for providing ventilation to sub floor voids, even when concrete beams are used.

While solid ground floors are common in Spain, the infrequent use of damp proof membranes and problem of both rising damp and flash flood risk make the suspended ground floor a much more practical choice, (normally using pre-cast reinforced concrete beams with a block infill and concrete screed over).

Unfortunately the design and construction processes of some buildings ignores the requirement for sub floor ventilation (in the form of airbricks) which help to reduce; the general humidity in the void, condensation on the underside of beams and the lesser risk of a build up of Radon gas. Long term increased humidity will commonly pass through the floor slab and into the living space as damp proof membranes are rarely included in the design. This will typically cause mould growth in the living space especially at cold bridging points, e.g. external ceiling/wall to north facing bedrooms. In the very long term, condensation on the underside of the beams may cause the steel reinforcement to corrode and eventually fail. A general high moisture content of the floor slab will also add to its overall weight and potentially reduce its bearing capacity.

In addition to water vapour, it is also beneficial to limit the risk of a build up of Radon gas. A study between 1990 and 1991 by Universidad De Valencia revealed an average indoor concentration of Radon at just 25% of the recommended acceptable limit, though other areas of Spain where granite substrate is common such as the North West suffer much higher levels. While the lack of ventilation to a house in the Valencia region is less likely to result in Radon issues, some isolated areas do show higher readings, such that void ventilation is always the preferred option.

How do I know if I have got a problem?

If you have an obviously raised ground floor, but no evident airbricks or access door, it is worth making an inspection hole (normally through an outer wall, so that the underside of some beams can be seen. Once you have access to the void, inspection with a torch will soon reveal if there are problems. Inspect the void ideally before or after summer, paying attention to the underside of beams towards external walls and directly under external terrace areas. High humidity will normally be evident through water droplets on the underside of beams (especially in the morning) and possibly signs of corrosion (rust staining or surface concrete blowing to beam undersides). Just inspecting close to a ventilated access door is not a sufficient assessment of the whole void. Most voids that have a problem will invariably feel humid or even sauna like and may also smell damp.




How many vents are needed?

The positioning and size of  vents is more important that shear number. Ensuring that there is adequate through ventilation is key to successful ventilation, as a sheltered corner or area with venting from just one side, may still suffer significant humidity and condensation problems. If you already have some airbricks in place, but still find areas within the floor void demonstrating excessive humidity. Add extra or larger  vents to the problem areas. If the void is sub ground (typical to the rear void area on sloping sites) it may be necessary to add a riser pipe and create a vent above ground level outside. These can be included in outer walls or even discharge within a garden border area via a simple pipe up stand with 90 degree protected outlet.

It should be noted that the drying out of a previously damp sub floor area can result in some minor settlement cracking and hence it is best to save redecoration internally or externally for at least 6 months following the addition of ventilation. For best results ventilation is combined with a damp proof membrane during the original build or re-tiling works so as to prevent all gases from passing to the living space. If you have suffered mould growth in the living space, be sure to re-decorate the affected walls and ceilings with ‘Anti Moho’ paint.

NB:- Information for advice purposes only. Proper safety precautions should be taken  and legal procedures followed when carrying out all works.

Information provided by Mark Paddon BSc Hons Building Surveying. MCIOB. Structural Surveys throughout the Valencia and Alicante regions .  T: 962807247 M: 653733066 . Free 17 page  property buyer’s guide and advice available via e-mail request to This email address is being protected from spambots. You need JavaScript enabled to view it.

ã Mark Paddon 2008

Additional information