Cellar & Subterranean Ventilation Guide

With the increased need for living space within existing properties, there has been a surge in the conversion of subterranean areas. With this surge, there has been a higher need for the correct ventilation to both dry and control the humidity issues that are prevalent within these areas.

The needs of subterranean ventilation are more complex than the needs of any overground structures, however, with this is something that we are going to explain within this article, shedding light on what is needed and how we can accomplish this with Vapourflow products.

The difference with subterranean areas compared to overground is mainly due to the pressure differential and the average ambient temperature of the areas. The issues we see with current systems people are advised on is the overuse of JUST extract (negative pressure) units; the use of these units are generally causing more issues than they solve- The units create a negative pressure, which means the area is more attractive to any water/water vapour within the surrounding area. Water and air in any state will always look to equalise out, which means they look for one of two aspects to migrate to: Lower pressure or cold spots for water to condense. The negative pressure that would be in the subterranean areas can lead to humidity from the property to be pulled down (which will generally be at a higher temperature than the ambient subterranean temperature leading to air expansion causing even more issues) or pulling water through the fabric of the property (plaster/masonry/wood), even to the extent of pulling water through the masonry from any earth resting against the external walls.

The most effective solution to deal with subterranean areas is the use of Positive Pressure Units and Negative pressure units, with a higher pressure incoming than leaving the area. Using this system, we are purging air into the area at a much higher rate than is being extracted, which creates a positive pressure barrier within the space. This positive pressure barrier is the most effective solution to dry the area, the fabric of the building and control the humidity. This barrier will also inhibit the ingress of any water through the fabric, and from air from the main property entering the area.

Another aspect to consider is the temperature of the incoming air, which is tempered when it drops below 14°C by a 200W heater; whilst the unit turning off if the external air temperature rises above 25°C to inhibit warm air expanding within the naturally cooler climate within the subterranean area.

It is best practice to have the external incoming air vent a minimum of 30cm above the ground level to stop the any sitting water from evaporating into the air being drawn into the incoming air stream.

To create this system, we recommend the following units:

Warm Air Dehumidifier Inline (WAD Inline)

Dryvent Inline

The WAD Inline will draw in external air, tempering it when it drops below 14°C and shutting down when the incoming air temperature rises above 25°C. The Dryvent Inline will continually extract at low speed and switch to high speed during times of high humidity. With the WAD Inline set to between 50-75% speed (done through the Vapourflow App), the incoming air rate will always be higher than the extract rate.

The benefit of using the inline models, is the ability duct the units to the most effective places for the area. It is best practice to place the vents at the opposite ends of the area to create the largest area of airflow and reduce the chances of areas becoming stagnant, which can create cold spots.

Other specialised cellar situations, such as Wine Cellars have other parameters, which we will cover in our next blog.

2 thoughts on “Cellar & Subterranean Ventilation Guide

  1. Jaqui Chapman says:


    We have a very deep and damp cellar with passive ventilation via external pipes to each side of the space. We may need one or two units. Do you offer a site survey to look at the exact requirements and to specify what would be needed? We are in NG33

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