Energy efficiency should be thought about from the earliest stages of design — trying to retrofit it later is not only expensive but often compromises the end result.
This is particularly important in ROI, where major renovations or extensions (affecting more than 25 per cent of the existing building) must achieve a Building Energy Rating (BER) of at least B2. In NI, an Energy Performance Certificate (EPC) is only required if the work creates a separate dwelling.
Of course, building regulations are a minimum standard and most self-builders who are building their family home for the long term future will want it to be as energy efficient as possible.
How you answer the following questions will have a very large effect on the design, layout, specification and ultimately the cost of your home:
- How important to you is having very low heating bills and a low household running carbon footprint?
- How environmentally friendly and healthy do you wish your home to be e.g. non-toxic/more natural materials, low embodied energy/carbon building materials.
- Do you have the budget and are you willing to invest in higher specification items?
- Are you prepared to go without some items that might traditionally have been standard e.g. open fires or oil-fired range cookers?
It is now possible to build a house that will not require a central heating system – a passive house – however to achieve this the house must attain a high level of airtightness, be highly insulated (better U-value) and use high specification windows and doors as well a centralised ventilation system with heat recovery.
This may, for example, impact on the amount of glazing you are able to have (walls are much more efficient at keeping the heat in than even the best windows on the market) and will require your architect to pay particular attention to the detailing to avoid thermal bridges.
There are also downsides to having a highly insulated and airtight home if it is not designed correctly with solar gain causing overheating of rooms in summer time. This can be avoided/minimised with good design (orientation and window placement), shading and purge ventilation.
Airtightness is central to achieving energy efficiency and it cannot be compensated for by adding more insulation. Indeed, without airtightness, insulation cannot perform its role effectively. Planning for airtightness from the start, with a strategy for how the tradesmen will run their services without breaching the building envelope, is essential. Specifying the right windows, doors and sealing products will also be key.
Remember too that there comes a point where adding more insulation will not be worth the extra money, as it will not lead to a big gain in energy savings. If going very low energy or even zero energy, i.e. if you are building an extension or renovating your house so that it requires no net energy (heat or electricity) input to run, the insulation and airtightness specification may limit the choice of materials you can use.
Each project will throw up its own set of questions to answer. For instance for cavity walls will you fill them with boards, beads or foam; or will you leave a gap in the cavity? Will you use insulated plasterboard internally? Your energy assessor will run through all the options with you, comparing cost, energy efficiency and ease of installation.
Most energy assessors agree that with airtightness comes the need for a mechanised ventilation system, to guarantee minimum air flow rates, which must be designed into the plans from day one (for ducting and unit).
