The sun has made a welcome return this week. After spending the winter battling with the moisture produced from drying clothes inside, I was finally able to dry some clothes on the washing line.
Key to retrofitting homes is the management of moisture. As architects we often worry about the unseen consequences of an imbalance in insulation, especially internal wall insulation and the appearance of interstitial condensation between the layers of the construction. There are also far more visible consequences of high moisture within the volume of the home and drying clothes inside will add to this moisture load.
The Design Guide for Healthy and Low Energy Home Laundering looks at the health problems associated with the passive drying of clothes in the home. In particular, high moisture, mould growth, off-gassing from fabric softeners and dust mites, and the effect that these have on wellbeing and health. It considers what alternatives are available to passive drying and what other implications may arise out of their use.
In the context of choosing what energy we use and how, it is also important to think about the amount of energy used to dry laundry. If you dry your clothes on the radiator, is that radiator on purely to dry the clothes? Is the temperature turned up in order to dry the clothes faster, or do you open the window to help things along (even in winter)? Perhaps you prefer a heated clothes rail or have a rail over the bath? Alternatively perhaps you use a tumble dryer due to the quantity of laundry or speed of drying?
The Design Guide found that passive indoor drying adds significantly to space heating demand during the heating season. It must be remembered that moisture laden air takes more energy to heat than drier air and that the temperature of the heating needs to be raised to offset the cooling produced by the evaporation of moisture-laden washing. Moreover, in the context of rising energy prices, drying clothes with a tumble dryer accounts for 4.3% of total domestic power consumption [Ibid, p11]. So what are the alternatives?
The 1963 edition of the Building Standards (Scotland) Regulations had requirements for internal drying rooms in dwellings in addition to outdoor spaces. It acknowledged the importance of providing a separate, heated and ventilated space for drying clothes that restricted moisture ingress to the rest of the dwelling. The requirement for drying rooms in Scotland continued until the 1986 edition of the regulations, but the provision of these spaces gradually became lost with the advent of tumble dryers, which obviously have a much smaller footprint when energy constraints are put to one side. Also, space is clearly an important factor to consider when there is often limited overall area for habitable rooms in the dwelling, especially when the perception is that other methods of drying are so much more effective.
The 2020 Scottish Building Standards specifically say the “occupants should have the opportunity to dry washing other than by a tumble dryer which uses a considerable amount of energy.” Whilst it acknowledges that a utility room or bathroom with mechanical ventilation would normally be used for drying washing, it also gives a minimum ventilation extraction figure, with humidistat control, for other spaces. An indoor drying space is described as wall or ceiling mounted or indeed as floor space to set out a clothes horse, but it should also have a volume of at least 1m3 with no dimension less that 700mm. Disappointingly, the Building Regulations in England only mentions clothes drying in the context of other wet rooms and does not have a specific space requirement for drying clothes.
The aforementioned design guide promotes a designated drying cupboard as the better method for providing a dedicated drying area but it also recognises the opportunity to link passive drying requirements to improved standards of ventilation.
A method of mechanically reducing moisture within a house would be with balanced whole house ventilation with heat recovery (MVHR) and this supplies continuous fresh air to the habitable rooms, extracting that air from moisture-laden wet rooms, such as kitchens and bathroom. Extracted air passes through a heat exchanger and the recovered heat is used to warm the incoming fresh air. The water vapour from the cooled humid air condensates into the unit and drains away. It has the advantage that it filters all the air entering the building – usually with an F7 grade pollen filter combined with active charcoal for traffic fumes; and it can deliver fresh air and extract stale/humid air from specific areas. In contrast to passive window ventilation which is only effective with human control, it is effective without occupant intervention. Unlike continuous window trickle vents the system can respond to changes in humidity.
When we think about low-energy retrofit we have to weigh the benefit of a low-energy measure to the amount of disruption to the building and its occupants caused by its installation. If you are considering upgrading windows and insulation it is imperative that this is undertaken after or at least in conjunction with reducing uncontrolled ventilation through the building fabric whilst introducing controlled continuous ventilation. The installation of MVHR can be hugely disruptive within an existing building when joist runs and crossovers have to be considered along with access to fresh air and the location of the unit itself. Sometimes a step-by-step approach that takes into account the maintenance cycle of the component parts can be more appropriate for cost, but the sequencing of the work should eliminate abortive works.
Dedicated drying cupboards or rooms can of course be added to a balanced whole house system, but also the addition of both a supply and extract into a habitable room (rather than just limiting it to the wetrooms) can give flexibility to the location of more general rail or clothes horse drying solutions.
Recently the Passivhaus Trust published a document entitled “The case for MVHR” where they challenged the rule of thumb that MVHR systems are only effective in more airtight buildings. The paper certainly challenged my preconceptions and it raises the question of whether this should be the first installed measure for a low-energy retrofit?