A Low Powered Home

 As we draw near to completion of this project, to the final finishing touches before the results of the as built testing are in, there are still some important things to talk about. A truly energy efficient building should also be a low powered one. Throughout the house we have specified low energy lighting, even the lowest level under the code for sustainable homes requires that low energy lighting accounts for at least 75% of the total. It would also make sense that part of the overall effort to save energy should include the installation of low energy appliances and devices to at least an 'A' rating.
   

The solar heated hot water and central heating plant.

The internal fit out is fully underway.

The external finishes are almost complete.

The all important MVHR unit in the attic.

The almost complete north elevation.

The house is very nearly finished, next comes the as built testing.

Saving Your Water

Another aspect of creating an efficient home is water usage management. Within the home, water saving fittings can be installed such as low-flush or dual flush toilets and flow restricting shower heads. Efficient appliances such as washing machines that use less than 50litres per wash and dishwashers that use less than 15litres per wash can all be employed, even existing fittings can be modified or adapted to consume less water. Such  fittings within the home will reduce overall water usage and of course if properly metered will mean lower water bills. Another way to reduce mains water consumption is to install a means of rainwater harvesting. Many of us now have free standing water butts or in-line water butts, these collect rainwater that we can use to water our flower or vegetable patches without having to connect a hose to the mains supply. At the site in south Shropshire we have installed a 'Graf Platin 5000ltr Flat Tank' which was easily laid in a shallow excavation for connection to the storm drainage pipes.

Above: The 'Graf' 5000ltr tank on site before installation.

Rainwater will be filtered & stored within the tank, then pumped as required to a separate header tank as a non-potable supply to appliances and toilets. As the tank becomes full, excess water is drained via an overflow to an on-site soakaway for natural percolation into the ground.

Photo Update: South Facing Elevation with solar collector in place and connected.

Self Sufficient Efficiency

 

  To meet our clients brief and provide them with an energy efficient home means more than just providing an air tight, highly insulated building. In line with the requirements of the code for sustainable homes, our design scheme includes the provision of supplementary technologies which will further increase overall energy savings and make better use of resources. 

Harnessing the power of the Sun.

 

  One of the most costly areas of fuel consumption in the home comes from heating water for all our daily needs. It takes about 4 times the amount of energy to heat a given volume of water compared to an equivalent volume of air heated to the same temperature, and of course the water we use to wash up for example is heated to a much higher temperature than our living rooms, this obviously means that a good proportion of those ever rising fuel bills is down to us needing a hot bath!

  So what can we do?, there are several different options available for providing a percentage of your own hot water and heating without firing up the boiler. Ground source or air source heat pumps can be utilized to heat both water and air, the first harvests heat stored below ground through an exchange system, and air source heating extracts heat from outside air, this system works a little like a fridge in reverse. In each case one system may be more suitable than another, for example, installing a ground source heat pump is more costly and may require more land than is available while an air source heat exchange system yields the most benefit in an air tight construction. Another option is to install solar thermal collectors, which in this case is what our clients have chosen to do, making good use of the south facing roof space. Solar thermal collectors are generally either flat plate or evacuated tube arrays, the latter being the type of installation planned for this energy efficient home. Solar thermal collectors should not be confused with P.V (photovoltaic) panels which generate electricity.

Above: The mounting fixed on the south facing roof ready to receive the solar collector.

 

  An evacuated tube solar collector is based on array of air vacated glass tubes, each containing a heat pipe connected to a heat absorber plate. Heat from the hot end of each heat pipe is collected and transferred to the heating coil of a domestic hot water tank inside a heat exchange manifold. The manifold is insulated and encased to protect it from the elements. Our clients intend to have this system connected to supply both the hot water tank and the central heating system. During summer months this system could potentially supply most of the hot water, though in winter months secondary heating may be required.

Above: Viesmann flat plate & evacuated tube solar collectors, image courtesy of viesmann.com

Above: Descriptions & schematics for  daily hot water & central heating systems using
'Viesmann' solar thermal collectors. Images courtesy of viesmann.com 

Much of the external render has been painted.

 

Natural Light & The Heat Loss Fight

 

  The greatest potential for heat loss is through glazed windows and doors, fortunately modern triple glazed units  can now achieve a  transmittance or ‘U’ value as low as 0.7 W/m2K which is far more effective than a traditional single glazed unit with a ‘U’ value of around 4.5 W/m2K. Both triple and double glazed units of the highest rating will be utilized in the new house. 

Double glazed windows will be used in the south facing elevation as the area of glazing is nearly half that of the north elevation, this will take advantage of the extra sunlight a south facing window receives and will help to balance the overall solar gain. The larger north facing glazed units afford the best view from the property but will also suffer the most from wind chill, because of this, all windows and doors facing north will be triple glazed. Both the double and triple glazed units will be Argon filled and manufactured with low emissivity glazing. The glass within low 'E' glazed units has special reflective coatings which reflect heat back into the room, the coating has very little impact on the transparency of the window unless tinted by design, a very slight tint may be visible from outside under certain conditions.    

MVHR. Mechanical Ventilation & Heat Recovery

 

At the heart of the MVHR system is a compact air handling unit that has a highly efficient heat exchanger within, the unit draws fresh air from outside which is heated by the thermal energy taken from the used air it extracts. When designed and installed correctly MVHR units are very quiet and should achieve a heat recovery of up to 95%. The efficient heat recovery of an MVHR system means that there is very little compromise of the buildings thermal envelope. While such systems do require energy to run, the amount of energy saved far outweighs what would otherwise be lost through a poorly insulated, ‘leaky’ construction.

Above & Below shows the flexible ducting for the MVHR installed between the floor joists.

The roof is battened ready for tiling as the first layer of render is applied to the exterior.

Ducting for the MVHR system is installed along with the other main services.

The layers of PIR insulation are fitted between the roof trusses and made air tight.

First floor decking is laid.

The first layer of external render is finished and tiling to the roof continues.

The lattice of the posi-joists allow for easy installation of service pipework and ducting.

The first floor studwork partitioning is erected.

The MVHR ducting is insulated before the voids are filled with mineral wool.

The second layer of external render has been applied and the roofing tiling is complete.

Painting of the external render begins.

Clearing The Air

Through design and construction we have ensured that our building has plenty of highly efficient insulation in order to greatly reduce the amount of heat lost through the fabric of the    building, however, the effectiveness of our ‘thermal envelope’, also relies on the building being airtight, therefore steps are taken to seal any areas where air leakage may occur, some of which are highlighted below.

Above:Polyurethane expanding  foam has been applied to seal around posi-joists at each end. 

 Below: Sealant has been applied around insulation at window and door reveals, and to full depth

               around each frame. The excess foam will later be trimmed flush before  rendering.

Of course an air tight building will very quickly become a ‘sick’ building unless adequate air circulation and ventilation is provided. We can immediately see conflicting requirements, on one hand we have the need to prevent heat from escaping, and on the other a building and occupants that we must keep healthy by preventing the build up of moisture and stale air. In short, we need to keep the heat in, the moisture out and provide fresh air for everyone inside to breathe. In order to achieve all the above requirements and tick this seemingly contradictory set of boxes, our clients new home will have an MVHR system designed and installed. MVHR stands for  Mechanical Ventilation and Heat Recovery, this will allow the building to be properly ventilated with very little loss of heat.

The first roof trusses arrive on site.

The roof trusses are craned into position.

The diminishing truss set is put in place.

The roof trusses are braced and tied as the gable walls are completed.

The main structure is ready for rendering and the roof ready to be covered and tiled.

Internal Insulation

We do not only insulate the exposed areas of the building, we also use insulation within the internal floors and partitions. Here we have specified 200mm thick mineral wool between the floor joists and also 100mm mineral wool between the members of any stud partions. Insulating what would otherwise be structural voids in this way, not only reduces the risk of fire spread, but also offers better sound insulation between rooms and floors.

  Good insulation between rooms and floors also increases the potential to save energy by providing greater control over the dispersal of heat throughout the building. This means that energy is not wasted trying to keep the temperature in one room higher while heat otherwise escapes through the walls and floors into another.

Roof Insulation

  There are several different methods we could use to insulate the house at roof level. Here  the trussed rafters are 200mm deep which accommodate 175mm of PIR board insulation, laid in two layers. The insulation is installed between each rafter and flush with the lower edge of the rafters. A 25mm air gap has been maintained above. Rafter level insulation is typically used in ‘warm’ roof construction. A degree of thermal or cold bridging occurs through the rafters because the heat conductivity of any timber is much higher than PIR board  meaning that heat will be transmitted through 200mm thick timber much more quickly than through PIR board of equal thickness. Potential cold bridging can be reduced by having a third continuous layer of insulating material either below or above the rafters. Any joints between the PIR boards are taped as within the cavity wall.

   Insulation within the rafters is only continued up to ceiling height. Above and between the ceiling ties 300mm of mineral wool insulation will be laid, again in multiple layers, with the thickest being applied as a continuous layer over the top. Although mineral wool is not as effective as PIR board, it still offers high level of insulation at this thickness. Where the two types of insulation meet it is important to try and eliminate any gaps, either with staggered taped joints or by further packing of mineral wool.

   Any remaining attic voids above the insulation are classed as ‘cold’ roof areas and are ventilated to eliminate risk of condensation. In this case ample ventilation is afforded via special tiles that are installed in the roof to maintain access for bats into a protected bat loft.

The inner structure reaches the ground floor ceiling height.

The first floor joists can now be fixed in place.

With the first floor underway, work on the cavity walls continues.

 

Cavity Wall Insulation

Because we are using a traditional cavity wall construction and not something like SIPs (Sandwiched Insulated Panels), we have used an extra wide cavity, this is in order to accommodate a more effective thickness of insulation. With a cavity width of 200mm we are able to install 150mm of PIR insulation, retaining a 50mm air gap. The effectiveness of any rigid insulation is reduced at joints, so two 75mm layers have been used rather than a single layer. Using two layers allows us to stagger the vertical joints where each PIR board abuts with another within the cavity.  All remaining exposed joints are then taped, this also greatly  minimizes any air leakage.

We have


calculated that this cavity wall specification will achieve a ‘U’ value much lower than required by legislation, being almost twice as effective at reducing heat loss. PIR board insulation can also be used to line walls internaly or externaly. This may be used as a solution for insulating solid walls or timber frame structures.

A 200mm cavity requires the use of extra long wall ties which tie the inner and outer block skins together. Various types of extra long wall tie are now available. Here we are using basalt fibre based ties, which have very low thermal conductivity, this further reduces potential cold bridging.

Ground Floor Insulation

To insulate the ground floor we have used 150mm thick Polyisocyanurate insulation, this reduces heat loss through the floor by almost double the mandatory target. Polyisocyanurate or (PIR) insulation has an almost unmatched level of thermal resistance and today is commonly specified in a wide variety of applications. Although ‘man made’, much of the raw component is recycled content, PIR board has zero ozone depletion potential and virtualy no global warming potential.

Highlighted in the detail above and shown in the photograph is a load bearing insulating element which is laid at low level as part of the cavity wall inner skin, this is used to prevent  ‘cold bridging’, essentialy this  greatly reduces potential heat loss through the internal blockwork to maintain a very low overall ‘U’ value. The ‘U’ value is the rate of thermal transmittance and when calculated takes into account the gauge or thickness of  the material. The load bearing insulating element we have used is similar to PIR board but with a much higher compressive strength.

Anatomy of Thermal Efficiency

 

In order to maintain a very low level of heat loss we form a near unbroken barrier or envelope of  insulation around the building. Insulation is laid at ground floor level, applied to or set within the external walls and installed at roof level. We also apply insulation between floors and within internal partitions. Below is one of the typical sections from our detailed drawings. Our detailed drawings show sections through the building at different points to show detailed elements of the structure. The details are accompanied by notes which specify the materials, dimensions and other relevant information for both building control and contractors. 

  In the following posts we will give an overview of some of the main areas of importance with regard to providing solutions for whole house insulation, and also highlight the materials and techniques we will be employing to achieve a high standard of thermal efficiency.     

With foul drains pre-laid the internal walls are built up to floor level. 

A sand blinding layer is laid over the level fill of compressed hardcore.

With the damp proof membrane laid the ground floor insulation is cut and fitted to suit.

Another protective layer of  D.P.M is laid before the floor slab is poured and leveled.

With the slab now dry, work begins on the inner block skin.

Work continues on the ground floor block partitioning.

All joints between the thick slabs of insulation are taped and sealed. 

The build climbs towards the first floor.