Pitched RoofingCondensation & Airflow within your loft space

10 March 2021by purpleadmin

Condensation & Airflow

In traditional design and continuing to new methods, it is imperative to keep good airflow within your loft area. The idea of allowing cold air to flow freely through what is essentially the interior of your building may sound counterproductive but it is one of the most important parts of keeping your loft condensation free. While you may think this cold air may present you a problem when heating your house, this is where the importance of insulating is equally important. Poor insulation quality will also add to your condensation woes. Let’s look at some common questions regarding condensation in a loft space and some tips on how to reduce it.


What is condensation?
Condensation is formed from the moisture held in the warm air as it cools. When this warm air comes into contact with a cold surface, the moisture within it will no longer be retained and forms water. This is what causes a condensation droplet. Modern living with central heating systems means that our houses produce much more of this warm air than your building may have been built with intention of. While modern living methods changed with the times, so does the way we construct our buildings to accommodate these.


Why does condensation happen in a loft?
Warm, humid air will enter the loft space through any gaps including your loft hatches and light fittings. This air will also pass through an under-insulated ceiling. Leakage of this air can be prevented by using vapour barriers and forming a sealed ceiling. BS 9250 code of practice dictates the requirements for a design of the airtightness of ceilings in pitched roofs.

The next consideration is if the property has been newly built or renovated. Building materials in these instances will need a period of ‘drying out’ time. Where the moisture held within the materials themselves will enter the air, this will add to the day to day moisture created with living in the household. In this case, it is even more important to have further ventilation than usually expected, such as opening windows, even when heating the house, and dehumidifiers. While the house is heated it will increase the amount of ‘drying out’, also increasing the water vapor in the air.


Figure 1. Condensation collecting on a plastic Monarfol underlay


How can the moisture a household produces be lowered?
While just purely breathing in your house adds moisture to the air, there are ways that you can reduce the amount of this warm air moisture within your home and from entering the loft area by taking steps to not create it at all through some of the ways that we live. These include:

  • Closing bathroom doors and using extractor fans/opening the window while and after showering or bathing
  • Cooking with lids on pans and allowing ventilation through opening a window or using an extractor system
  • Using trickle vents on windows to improve continuous ventilation
  • Not drying clothes on radiators or within the house except for dryers the collect condensation or extract it outside the house


Problems with condensation in the colder weather
While condensation can be created at any time where warm moist air presents itself to a cold surface, these problems are more prominent during these colder conditions. While the air is colder outside the heating we use inside will be turned up, inviting the perfect conditions for condensation. The presence of condensation isn’t always a cause for concern and can most likely be a one-off event during this time of year. It is important in these circumstances to allow this warm air out as soon as possible by using the techniques described above.


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Preventing condensation if the loft space
Using the above steps can reduce the amount of water vapour that is initially in the air. But even with these steps, you are inevitably going to have some sort of vapour within your air. If your roof space consistently suffers from condensation or there is an excessive amount, here are some retrofitted techniques we can use such as increasing the ventilation capacity of your roof covering.

  • Tile vents
  • Mechanically fixed ventilated ridge system
  • Underlay spacers

A newly installed roof should be installed within the BS5250 code of practice. This is the standard for controlling condensation in a building. Within a roof covering it should allow ventilation from the eave of your roof up to ridge level. This is done by adding fascia or soffit vents and the use of mechanically fixed ventilation systems. These mechanically fixed ridge systems also cover the BS5534 code of practice for slating and tiling fixings.

Newly installed roof coverings should also allow for the insulation to be inserted up to the wall plate in the eaves, a method that could not be used on older installations due to the covering of the cavity airflow. New products such as rafter rolls allow for the insulation to be installed in this area without affecting airflow into the loft space. The ability to install insulation up to this point reduces cold bridging the underside of the ceiling, wherein your room mould may appear by the eaves as the warm air hits this cold section and causes condensation.


Figure 2. Cold roof ventilation. Circulating a fresh air feed from the eaves, using the
ventilated ridge system as an exhaust for warm stale air. An air-permeable underlay will
also aid this process but is not sufficient for the process alone.

Dispelling myths about using “breathable underlay or “felt” to reduce condensation
While the terminology of products changes, we commonly refer to products as they’ve always been known. A pitched roof covering was traditionally weathered with slate or clay plain tiles, these had ‘torching’ or ‘back pointing’ to prevent any driving rain as a secondary defence system. Where the use of materials evolved so did the construction of roofs. The fitting of single lap concrete tiles commonly referred to as ’tiles’ or ‘Marley moderns’, introduced the need for protective sheeting to prevent the tiles from blowing off due to air pressure within the roof space. A bitumen-based felt was used for this reason, also acting as a secondary defence against driving rain. Because it was bitumen-based this product was referred to as ‘felt’ and was a non-permeable (non-breathable) product.

The introduction of a lighter-weight underlay came along with the addition of it being ‘breathable’. This underlay is air-permeable, allowing it to effectively breathe, however, this is not sufficient for ventilation to control condensation by itself. This is where ventilation products must be installed to prevent these problems.


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Freephone: 0800 955 1693

Freephone: 0800 955 1693


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