( MORE: An Expert Guide to How to Stop Condensation)Īdding insulation to a wall will change the place where the dew point occurs. In most cases, it is slightly inside the external surface. Ideally the dew point will occur either on the external surface of the wall, where moisture can evaporate away, or in the ventilated cavity (if the wall has a cavity), where the same thing happens. The temperature will change through a wall as it moves from the outside ambient temperature to the internal temperature. The dew point is the point where air meets a temperature that causes the moisture to condense out of the air as water. However, they do not stop moisture reaching the internal surface of the wall from people and what they do - and that can mean that more home ventilation is needed to remove the moist air before it gets to the walls. Many modern insulation materials are usually non-permeable and will be rendered or clad to prevent rainwater penetration. The cavity wall by comparison is an impermeable wall in that the cavity is intended to form a ‘barrier’ to prevent moisture penetration any rainwater entering the wall is evaporated away by air movement in the cavity. A solid brick wall will tend to be the former and a stone wall will tend to be the latter. Prior to the widespread introduction of the cavity wall, the housebuilder generally had a choice to build either an impermeable wall that stops moisture penetration to both surfaces, or to build a breathing wall that allows moisture to penetrate (to a degree) and be evaporated away. The potential exists for moisture to enter a wall from both sides: rain on the external skin and moisture from people and the activities they undertake (cooking, drying clothes, washing, breathing, etc.) on the internal skin. 110mm mineral wool, wood fibre, hemp batts gives 0.30W/m².When it comes to using external wall insulation to insulate a solid wall, you'll be looking at the following figures: The requirement under Building Regulations is to reduce that to no more than 0.30W/m².Ī 50mm injected foam cavity-fill plus 20mm PUR external gives 0.28W/m² A 450mm stone wall will be virtually the same, and a brick cavity wall about 1.50W/m². Need more advice or inspiration for your project? Get two free tickets to the Homebuilding & Renovating ShowĪ 225mm solid brick wall will have a U value of around 1.20W/m². How Does External Wall Insulation Affect U-Values? The thermal performance of the insulated wall must have a U value of no more than 0.30. Under Building Regulations, if 25 per cent or more of a wall is to be insulated externally, it is typically necessary to bring the entire wall up to current standards - which makes sense in that, if you’re going to go to the efforts and expense of insulating externally, then you may as well do it well. Building Regulations and External Wall Insulation For homes in Conservation Areas and for listed buildings it is quite possible that consent will not be forthcoming. In most cases that will mean gaining planning permission before undertaking the work, so do check with your local authority from the outset. Whatever else external wall insulation does, it will change the external appearance of the house. Will I Need Planning Permission for External Wall Insulation? Whichever is used, there will be a cost involved in erecting scaffolding and removing/replacing all of the pipes and cables that are fixed to the wall that cannot be avoided. A three-bedroom semi is likely to cost £5,000-9,000, with a larger detached home in the region of £8,000-15,000. There are proprietary systems that require specialist installation, but there are also materials from builders’ merchants that are a lot cheaper. The cost will be higher than internal wall insulation. How Much Does External Wall Insulation Cost? External wall insulation is thick, which can cause issues around windows, eaves and sills.May not be suitable for listed buildings or those in Conservation Areas.Likely to require planning permission if changing the appearance of your home.Potential to cause issues with condensation, which could lead to damp problems.What are the Drawbacks of External Wall Insulation? It reduces condensation on internal walls.It improves not only weatherproofing, but sound resistance too.It allows walls to contribute to thermal mass (the ‘tea cosy’ effect).This type of insulation does not reduce internal floor area.It does not disrupt the house while being installed.Fewer draughts and increased the sense of comfort.What are the Advantages of External Wall Insulation?
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