Most properties (built after around 1930), are essentially designed and constructed as tightly sealed boxes with the aim of keeping all moisture out. This is typically achieved with the use of impermeable membranes at ground level (such as a damp proof course) to prevent moisture rising up from the ground below, and cavity walls - designed with a gap (cavity) between two skins of masonry to prevent moisture penetrating through from the outer skin to the internal parts of the property. Windows and doors are sealed tightly, and most modern houses are constructed without chimneys. For these types of property, where dampness is found internally it is generally due to a specific defect which will require repair.
Older properties, generally those of solid wall or similar type construction and built before around 1930, were constructed in a very different way and the design allowed for an amount of moisture to naturally enter the property. They often may not have a damp proof course (particularly those built before around 1875) or cavity walls and are not intended to be a sealed unit. These properties are designed to allow an amount of moisture to enter into the structure, which is then released naturally through evaporation. When designed and maintained correctly, this natural process will allow sufficient movement of moisture before it is able to negatively impact on health or the structure of the building. This is particularly important for any embedded structural timbers (of which there are many in older buildings) to prevent rot, decay and potentially structural failure.
At the time, it was normal for older properties to have various openings where moving air could enter in and out the building, such as fireplaces, around doors and windows, through exposed timber floorboards, in the loft space etc. As a result, natural ventilation levels were high, allowing moisture to evaporate readily in the moving air, and be carried away externally before it was able to cause issues internally. For example in a solid wall, where moisture is allowed to enter into the wall structure, although the wall may have elevated moisture levels, this would be negated by the rate of natural evaporation which would prevent deterioration and excessive dampness.
Often in an honest attempt to increase the thermal efficiency of the property, increase thermal comfort, or sometimes even to prevent rainwater penetration alterations to older buildings are now carried out which restrict air movement and natural evaporation such as:
· Use of impervious hard-cement renders, pointing and/or internal plasters
· Unnecessary chemical DPC injections
· Fitting modern underfelts below roof coverings
· Blocking and sealing fireplaces
· Adding secondary or double-glazing
· Draught-proofing door and window openings
· Fitted floor coverings laid over timber floorboards
· Replacement of suspended (ventilated) timber ground floors with solid concrete
· Raised external ground levels restricting or blocking sub-floor ventilation and natural evaporation of wall-bases.
As a result, moisture levels in the property can rise due to restricted evaporation and trapping of dampness as a result of modern alterations carried out without care as to how they affect the original design of the building. This then leads to dampness becoming visually evident causing deterioration to internal finishes and (more seriously) can lead to decay/rot of structural timbers within the property. Modern lifestyles also tend to generate far more moisture than would have been the case with occupants at the original time of design and construction, further increasing the need for adequate ventilation. It can be safe to carry out such alterations to an older building, but it is essential expert advice is obtained to ensure any alterations carefully account for the original function and design of the various building components.
Many older homes (around pre-1930) were built when lime mortars were the primary method of setting bricks and stones. Lime mortars/plasters/renders are both flexible and porous and allow natural moisture evaporation, unlike hard, inflexible and nonporous hard-cement products used in more modern construction. Therefore, it is essential lime-based products are used to repair and maintain older buildings to prevent deterioration. In modern times, we see many repairs carried out to older homes using hard-cement renders/plasters/mortars. This seals the gaps between the bricks and/or stones, trapping moisture in the wall and leading to failures. By reducing the amount of moisture that can evaporate through the wall to the outside, it increases dampness levels inside. Lime-based products, being more flexible, also allow for thermal and seasonal movement which older buildings were designed to accommodate, unlike hard cement-based products which are far more susceptible to cracking under such movement.
As a result of the actions described above, it is common today to find unacceptably high moisture levels in older properties. The consequences of this can cause significant defects within the property. In particular, high moisture levels (especially in suspended timber ground floors, roof spaces and cellars) can promote the development of timber rot/decay and wood boring insects such as Common Furniture Beetle, and Death Watch Beetle. To avoid these defects developing and becoming a serious threat to the building, it is important to be aware of the consequences of any actions which may have an impact on moisture management within the building.
For advice on how to best repair or maintain and older/historic building please get in contact. Our expert Surveyors will provide bespoke advice on any such moisture issues in older properties and the best recommended course of action.
It is important to remember that, if the property is Listed or in a Conservation Area, any works you wish to carry out may require Listed Building or Conservation Area Consent, and it is always best to check with the local authority Conservation Officer before undertaking any activities.
There are many useful resources for older and historic properties, some of which we have attached below for your convenience.