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Fabric Energy Efficiency

Some of the critique leveled at building regs in the UK is the lack of heed paid to fabric energy efficient that is why from 2013, part L of the building regulations will introduce a minimum standard for Fabric Energy Efficiency. Fabric Energy efficiency is a key concept in energy efficiency, after all before installing solar panels etc. we should consider energy efficiency. This is the norm on the continent with standards such as Passive House (Passivhaus). Employing extreme levels of fabric energy efficiency. To the point where they do not even need heating!

Elements of achieving high fabric energy efficiency;


Insulation has been added to houses for decades, in loft spaces from the 70s onwards and started to appear in walls and floors in the 90s.

Door and Windows (Openings)

Door and windows often prove to be chink in the insulative armor of a building. A typical wall might have a U value of around 0.3, whilst a fairly high performance window will still only achieve around 1.5 U value. Code for sustainable homes requires excellent fabric energy efficiency and also good daylight lighting. So they is a balance to be had here . . . huge windows lead to poor overall insulative qualities, and possible reduction of fabric energy efficiency. There are windows that offer very high performance such as those designed to the passive house standard. Passive house windows can have U-value of 0.58 W/m²K.

Air Tightness

Even a well insulated house will not stay very warm if it is drafty. This is why for high fabric energy efficiency we must work to achieve very low air permeability in a house.

It is important to provide some flow of air where it is needed, but eliminate air flow altogether where it is not. This is quite tricky to do in some cases, and it is worth checking that recommendations in reports, are followed through to construction crews.

The classic example of this is use of timber joists set in walls, where at the time of building when the joists are moist (very most if treated) there are no air gaps around the joists. But after the house has been completed and heating switched on, the joists shrinks and all of sudden there are hole everywhere. A simple and virtually zero cost solution is to cut the joists around 5mm short and spread mortar over the ends of the joists, keyed in to block work at either end. Whilst the joist will still sjrink the motar layer will not, and stop air flow around joists.

Other Thermal Properties

Thermal mass should also be considered. Many modern timber frame homes have a very low thermal mass. A high thermal mass is good provided the mass is insulated from the outside elements. An insulated cavity wall with brick exterior and block exterior has a higher thermal mass.