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.

Photos

One happy hen, good to see the sunshine again. Although freezing at night time. One house with vapour membrane complete. Imagine the condensation and damp here without a ventilation system in place. No draughts though.
One gorgeous view from our bedroom window.
One selection of electrical tagliatelle feeding into the utility room.One snapshot of infrastructure to the master bedroom en-suite. Heat recovery ventilation, hot and cold water feeds and returns, underfloor heating and soil pipe.

Good weather = Good Progress

Afterwhat has seemed a long spell of dreich weather, the sun finally came out today and the slaters made significant progress on the roof, now about 75% slated. Inside the joiners completed the poythen vapour membrane work on the 1st floor such that we now have a near airtight house.

As I have mentioned before airtightness is an essential element in low energy housing; it allows the insulation to work properly, prevents draughts and ensures that ventilation can be properly controlled. The old adage 'insulate tight - ventilate right' has to be the watchword for the eco-selfbuilder, and if planned and implemented properly, will go a long way to achieving significant savings in energy even before considering renewable energy systems and high efficiency boilers.

Brochure management..!

Now we're on a mission to make sure we're ahead of the game on 'client supply items', we're starting to get rid of the accumulation of brochures as we actually bite the bullet and start buying stuff!
So far we've bought the sanitaryware for the three bathrooms, external insulaton system, chosen the wood stove and heat pump, and ordered and fitted all windows and doors - all of which we separated from the main build contract to retain maximum freedom of choice. Today our fridge freezer was delivered (Liebherr A+ rated) marking the first delivery of kitchen items.
The focus now is on the overall kitchen as Debs mentioned in her last blog, with Magnet and John Lewis very much in the running and we should have costed designs from both by the end of this week.
With regard to the overall build program, the joiners we're back last Friday fitting the internal airtightness/vapour membrane and tomorrow we should have a full crew including the slaters who probably have around two weeks of work to do in order to complete the roof. That said, we have snow and gales forecast tonight which might make roofing work a little tricky in the morning!

Wind and watertight for Christmas!

Yesterday we reached a major milestone with the delivery of the patio doors and main entrance door - the building was made wind and watertight and we now need a key to get in! This means that first-fixes and roughings can continue whatever the weather and the building is drying out nicely inside.


I've been working away from 'home' the last few days which has been frustrating, not least as there are many questions on detail items to deal with. I caught up after a half hour tour round this morning and it's clear that things are really moving on.

First fix electrics and plumbing are nearly all complete, with a few adjustments to make to the underfloor heating in the first floor bathroom and en-suite. Electrics are also nearly there with a few outstanding decisions to make on light switch locations and cooker load (ie. what cooker are we going to install!). The other first fixes remaining are the Heat Recovery Ventilation system ducting, TV/Satellite, phone, DAB (weak signal area for digital radio so need an aerial) and Cat 5 cabling.

Last but not least the slaters are making good progress and the stainless steel chimney for the wood stove looks superb, especially as the installation has been reinforced from the inside thus avoiding the need for external stays which might have spoilt the roofline.

Wall plates installed

On Tuesday of this week the builders spent the morning fitting the wall plates (also called 'sole plates'). In timber frame construction these set out the plan of the ground floor layout and form the interface between the walls (both external and internal) and the ground floor slab/foundations.

The wall plates comprise timber of the same width dimension of the wall panels they support (ie. 140mm and 89mm), and are simply cut to length and laid on a bed of mortar (and damp proof course at the outside walls).

Whilst an apparently straightforward process, locating the wall plates in the correct positions is critical to the ultimate accuracy of the frame installation.

As an aside, this process also introduced a simple but effective airtightness detail to the building, in that the overhanging damp proof membrane (from under the slab) is folded back over the slab at the edges for later trimming and connection to the vapour membrane to be installed on the inside of the wall panels. Whilst not an essential detail (and usually omitted), this ensures an airtight and flexible linear floor/wall joint that should last through the first few years of the building 'settling' and drying out, a process often responsible for significant degradation of airtighness as cracks open between differing materials (eg wood/mortar) and thus worsening energy efficiency. Polythene membrane, being a flexible material, withstands this movement.

As an aside, airtightness is a key component to low energy building design alongside insulation, the minimisation of thermal bridges, orientation and glazing. Coupled with this it is essential to have an effective ventilation system to ensure good air quality inside - more on this and other airtighness details as the house takes shape.