I was recently working with a client who wants to build a new, energy efficient home that presented an interesting problem. At a meeting with the clients and their architect everyone agreed that minimising the energy needs of the house had to be the first priority. Also that it is the cheapest option. The payback on achieving even super-insulation levels will be in the region of 4 to 5 years. The sort of figure that solar panel manufacturers can only dream of.
In short, insulation is king. But insulation is only part of the problem. The majority of heat is lost through the movement of air in and out of the house. Wherein lies the problem. The client’s lifestyle is one of outside activity, gardening and so forth. The go in and out a lot and when they are in, they like having the windows open. So how do we design an energy efficient house with potentially huge and unpredictable air movement?
In addition the architect had designed a large sun room on the south facing elevation and wanted to install a heat recovery system to capture and redistribute the passive solar heat that it would generate. We quickly realised that heat recovery might not work as specified when the windows and doors are open and that it could be an expensive failure.
Air Tightness
It is mandatory for all houses gaining planning consent after April 2006 to meet the Building Regulations Part LA standard on air tightness. That standard is a “leakage” of not more than 10m3/h/m²@50pa. That is, 10 cubic metres of air per hour escaping for every square metre of the envelope surface area with the air at a pressure of 50 Pascals. On completion the house must be tested to ensure it meets the standard. If it fails there are expensive ramifications in terms of remedial work. But there is a get out clause. It is permissible to specify 15m³/hr and NOT be tested – so long as the property, as design, still gets a good SAP rating.
What this means is that if you install renewable energy systems to provide the space heating and hot water, you will get the SAP rating, and don’t have to worry about air-tightness.
Insulation – How much is too much?
The answer is, of course, it depends. Heat loss can never (in practical terms) be reduced to zero, even the best insulation can only slow it down. How close you want to get to zero is the issue.
To give a couple of examples: Building regs call for a minimum 270mm mineral wool insulation in the loft. That equates to a U-value of 0.16W/m². Increasing that to 450mm will reduce the U-value to 0.8W/m². But increasing to 600mm will only reduce the U-value to 0.6W/m² - there is a law of diminishing returns.
Building regs call for a U-value for walls of no more than 0.33W/m², which is generally achieved with 90mm rigid foam insulation in the cavity. Passiv Haus construction will have 350mm of wood fibre or mineral wool insulation in the wall giving a U-value of 0.1W/m² - a 70% saving on heat loss. And 35% of the heat escaping from a house goes through the walls. The construction cost of either option will be largely the same.
The Answer
In this case we case we opted for the lower 15m³/hr air-tightness standard and increased insulation levels to around twice building regs requirements. In addition we designed in a passive ventilation system that distributes the heat generated in the sun room. That meant that a solar thermal system and an air source heat pump would comfortably meet the heating and hot water demand.
While air-tight houses are very thermally efficient, they are difficult and potentially very expensive to achieve. The lifestyle issue may be more important and it is quite possible to design an energy efficient house that you actually like living in.