Pouring the Floors

The monster concrete pump pouring the floor slabs for the two adjacent cottages, this covers the underfloor heating which is attached to the steel mesh. With the heating encased in the concrete slab this provides a high level of thermal mass to stabilise the internal temperature of the properties. We used the same principle on the main house which we built 3 1/2 years ago and it really works well, alongwith the high levels of insulation and air-tightness.

For the most part since commencing the build the weather in this part of the world has been unseasonly poor with twice the usual rainfall and generally lower temperatures. As the concrete was poured on a warm, sunny, breezy day the builders then had to spray water onto the surface to reduce the temperature as it set in order to avoid cracking.

Proof of the pudding...

It's December, and time I blogged again. The last seven months of occupancy have been intense at ECF, not only with moving in but also just catching up with 'normal' family and work life. Despite the festive season being just round the corner, we at least feel we're getting there!

On the energy front, seven months of occupancy means we've got some data on energy consumption to compare with what we specified and predicted...and the results are quite a relief!

The bottom line is that we're using 25% less energy than we expected; and that's in a house already designed to use less than a quarter of the energy of the average new housing stock. In general hot water and space heating energy use are on target, although these account for less than half of our predicted energy use. The big savings have thus been made in other energy use, namely lighting and appliances. In particular the use of LED and CFL light fittings, coupled with A and A+ energy rated kitchen appliances has paid dividends.

Whilst in most houses heating and hot water use accounts for up 80% of domestic energy demand, our experience just shows that real savings can be made with little effort by replacing lighting and appliances (when worn out of course!) with more energy efficient equivalents.

The ideal low energy house?

What we've set out to do at ECF is to build a house which will use 70% less energy than one built to current building regulations. Its timber frame construction detailing hasn't wavered too far from the 'norm' to present any major problems for a timber frame kit manufacturer, decent building contractor or building control, and the costs of going this 'extra mile' haven't been excessive to the extent that they will be paid back within a decade in terms of reduced energy costs.

In essence, we have built to a standard which is accessable now by most contractors and self builders which is around 5-6 yrs before its time if the UK Government is to fully implement its Code for Sustainable Homes (CSH) strategy, on which this building should come in at around 3-4 on a scale of 1-6. There is clearly some distance to go from this to 'zero carbon', so what represents the 'optimum' for 'Level 6 living', the magical zero carbon level required by 2016 for all new homes?

Well, the same principles apply, namely...

  • building orientation to maximise solar gain in winter, spring and autumn whilst avoiding overheating in summer

  • high levels of insulation around the entire building envelope

  • high levels of airtightness coupled with heat recovery ventilation

  • highly insulated and well sealed doors and windows

...but for CSH level 6 the wall insulation would need to be increased to >300mm (from our 200mm), roof insulation to >450mm (from our 350mm) and windows to be triple glazed krypton filled units with insulated frames and glazing spacers (vs. our argon filled double glazed units). This would result in a building which could feasibly rely on the heat given off by its occupants and collected though its windows to keep it at a comfortable temperature without having to introduce a heating system. Construction might rely on internal masonry/concrete walls to store heat and keep a steady internal temperature, with the insulation fixed to the outside of this.

On the face of it quite simple, but a seriously long way from what the UK housebuilding industry is used to. Roll on 2016.....there is a lot of catching up to do and mindsets to be re-programmed.

Ventilation strategies

As for living comfort at the levels of airtightness we are building to, it becomes necessary to use a whole house ventilation system. For this there are two main options; Mechanical Extract Ventilation (MEV) and Mechanical Ventilation with Heat Recovery (MVHR).

MEV uses a constantly operating fan which extracts warm moist air from the warm moist rooms (ie. bathrooms, kitchen etc) via ductwork, with fresh air effectively being sucked in via trickle vents and gaps in the structure. MEV is fairly economic to install, especially as it eliminates the need for dedicated extractors in the bathrooms. However, for very airtight buildings additional openings in the structure need to be introduced and warm stale air is replaced with fresh but cold air, thus driving heat out of the building and reducing its efficiency. As an aside to this, an MEV option is available for our heat pump which actually uses the heat from the outgoing stale warm air to pre-heat the ‘brine’ before it goes into the heat pump, thus recovering some of that energy. This is a great idea and an option well worth considering for self builders opting for a heat pump, but we eliminated it on the basis that that our first floor is largely unheated and such a system might lead to the cooling of that area via the trickle vents in the Velux windows which would need to be open for this system to work properly.

MVHR combines MEV with an intake system which supplies the ‘dry’ rooms with fresh air, preheated via a heat exchanger which takes heat from the extracted air. This is the system we have chosen, a (claimed) 95% efficient unit made by Dutch company Renovent and supplied by Ubbink in the UK. The unit has three settings and is virtually silent in operation. At the lowest setting (normal operation) it uses just half the power of a 60W light bulb and should be adequate to ‘heat’ the three first floor bedrooms alongside the heat convected from the ground floor. Other advantages are good air quality by using fresh, filtered air from outside, and the ability to use the unit to provide cooling in summer by bringing in cool air at night into a solar heated building.

Airtightness details

Having specified and designed a house to meet the AECB’s Silver Standard, airtightness plays a key role alongside high levels of insulation to achieve a low energy house. In our case we have followed the AECB’s Silver Standard construction details for timber frame buildings which advises the use of a continuous air/vapour control layer inside the building with all joints lapped, sealed and mechanically trapped.

This has probably been one of the most difficult aspects to achieve on site for our construction team which, in common with most UK builders, is simply not used to working to such a tight specification. With this in mind we opted for a solution which has largely avoided the need for specialist tapes and sealants, and in the main relies on the mechanical trapping of taped and lapped joints to provide a positive seal along with silicone sealant. We won’t really know how well this has worked until we do an airtightness test on the building, but on the basis that opening the front door feels somewhat akin to opening the door of a luxury car (ie. that air suction noise!) gives me some confidence. Also when it’s blowing a gale outside, there are no obvious draughts entering the building apart from the open trickle vents (shutters not yet fitted) and the yet to be connected stove flue.

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