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Airtightness: Reducing Energy Demand for Heating and Cooling
The core aim of sustainable refurbishment is to reduce the demand for energy for heating and cooling while improving the health and comfort of occupants. The first step to achieving this is to reduce unwanted draughts or air infiltration and leakage. Uncontrolled airflow in and out isn’t what we need. Instead, homes must breathe in a controlled way that permits the occupiers to regulate air quality – humidity, cleanliness, temperature and so forth. This will ensure huge savings over the building’s lifetime.
If I’m wearing a wool jumper on a windy day, the wind will blow right through it. If I put on a wind or rain jacket and zip it up, then the wool insulation can do its job properly. It’s the same with a building. You might have insulation in your house, but you’ll still be cold if the wind is penetrating. The idea is to keep all the warm air inside the building and all the cold air outside, to prevent draughts and high heating bills.
Andreas Schmidt, SIGA
The aim of airtightness is to ‘build tight, ventilate right’. This means that there are no breaks or gaps in the envelope of the building fabric, to maintain a controllable interior temperature at low operating cost regardless of the conditions outside – a climate-controlled home can keep out hot air during hot spells, as well as banish cold air streams in wintry weather. Airtightness can be measured with pressure testing. All openings and ventilation systems are sealed and a fan is used to put the building under a standard pressure difference of 50 Pascals. You can then measure how much air is leaking out.
Where does Air come in?
Air finds its way in through all kinds of cracks and gaps. The following non-exhaustive list will not apply to every single dwelling – for example flats do not have lofts – but some will apply to all.
Airtightness
Airtightness is measured in air changes per hour (ACH) – the number of times the home’s air is replaced by outside air in an hour. In a typical unrefurbished UK home air change rates average between one and two volumes per hour but can be much higher. Newer construction tends to be tighter – below one. Tightly constructed homes may reach an ACH of 0.6 to 0.5 or less.
Airtightness is also described in terms of building envelope permeability – in cubic metres of air leakage per square metre of external area of the building per hour – m3/m2 h at 50 Pascals. It is defined in BS EN 13829. Minimum targets apply as follows:
| Target | m3/m2h |
Part L of the UK Building Regulations | 10a |
Energy Saving Trust Best Practice | 5b |
Code for Sustainable Homes (CSH) level 3–5 | 3c |
Higher CSH levels (they go up to 6) | 1 |
Passivhaus | 0.6 |
Notes
a Source: Amendments to SAP 2005 (version 9.81) April 2008.
b Upper limit under Part L Source: ‘Proposed Building Regulations Part L1a 2010’: NHER Summary of the Consultation, National Energy Services Ltd, June 2009; Source: EST CE83.GPG155 ‘Energy-efficient refurbishment of existing housing’ (Nov 2007 edition).
c The Code itself stipulates no figures for airtightness or permeability. Instead it discusses the building fabric in terms of heat loss parameters. But the Energy Saving Trust has produced helpful documents to show strategies for achieving the targets, which take for granted a target of 3m3/h m2 at 50 Pascals. Sources: ‘Energy efficiency and the Code for Sustainable Homes Level 3 EST CE290’ (June 2008 edition); ‘Energy efficiency and the Code for Sustainable Homes Level 4 EST CE291’ (May 2008 edition); ‘Energy efficiency and the Code for Sustainable Homes – Levels 5 and 6 EST CE292’ (May 2008 edition).
In other words, the lower the figure, the better the performance of the building.
The way you go about dealing with these gaps depends on whether the home is being renovated piecemeal fashion, room by room, whether an extension like a loft or conservatory is being added, or if it is a whole building refurbishment. If it is any of the last three options it’s much better to install interior or exterior insulation with a vapour-permeable membrane (see Chapter 3).
This chapter deals with measures to take if you are draughtproofing on a less comprehensive scale. Proceed methodically from room to room applying the measures below in each category. When you’ve fixed all the obvious gaps, check whether you’ve been successful by using the smoke from a candle or burning incense held near a suspect area on a windy day, or a day when there is a temperature contrast between inside and outside, to see whether the smoke direction reveals a draught. After attending to any further leaks found, a pressure test could be conducted as a final check.
Figure 1.1 Brush seals on the inside of a letterbox
Depending on where you are starting from, some of these measures will be zero net cost and quick wins (highlighted below by a ‘Q’ next to the measures).
Letterboxes (Q)
If the flap doesn’t close properly a brisk draught can invade. Fit a good quality outside flap with a strong sprung return mechanism to ensure that it closes properly. Fit a second flap or a brush unit inside.
Figure 1.2 Brush strip
Source: © Energy Saving Trust
Key Holes (Q)
A mortice lock is a hole. Fit a flap over the outside hole that pivots out of the way when the lock is used. Fit a second one on the inside.
Cat Flaps (Q)
Cat flaps are a bad idea, but if one is essential, choose an airtight, insulated one with a close-fitting flap and strong return mechanism. (Some even come with a coded transponder that is fitted to your cat’s collar so only yours can come in!)
Figure 1.3 Wiper seal
Source: © Energy Saving Trust
Doorframes and Doors (Q)
Badly fitting doors are major sources of draughts. Draughtstripping is inexpensive, simple to install and can greatly improve comfort as well as reducing fuel bills. Exterior doors should be fixed first, then (unless you’re installing whole house ventilation) the interior doors, to stop air travelling from unheated (or uncooled) areas of the building to others. Typical payback for doors and windows is three to four years.
Various types of seals are available depending on the door and the side to be sealed:
- Compression seals: for external doors. Draughtstrips with a range of 6mm and a compression allowance of 3mm will allow for a seasonal variation in gap size of up to 3mm. The seal is maintained if the gap expands and when it shrinks the door will still close. Synthetic rubbers (including EPDM and silicone seals), sheathed foam or nylon brush, all perform well, with rigid PVC-U or aluminium carriers nailed or screwed to the frame of the door. Don’t over-paint them.
- Low-friction or wiper seals: suitable for most doors and window types. Commonly made of nylon brush pile, they are self-adhesive and available in a variety of heights for different gaps; especially good on sliding windows and doors. Rubber blade types are good for wooden doors and casement windows and sliding applications.
- Gun-applied sealants and fillers: larger gaps, including those at the heads of windows, can be filled using silicone or polyurethane sealants. These expand, set and harden to fill the gap permanently. Do not apply to dusty or wet surfaces and check that the filler has fully filled the gap and bonded.
Figure 1.4 Compression ...
