nakedgreenbuilders.com - Home extension

Home extension

Rural home extension & new-build holiday cottage. Renewable energy systems are becoming more popular, but what is most appropriate for you? This rural home illustrates how best practice can be achieved in different types of development across one site. Wednesday, 4^th October, 2006

We were to inspect the property with reference to the proposed development of a conservatory extension, solar hot water, photovoltaic installation, straw bale construction and reed bed water treatment. The visit enabled me to develop a good understanding of the building’s construction and the function of the construction elements. This report explains the issues identified during the site visit, sets out our recommendations and gives advice on how to proceed.

The client also requested further advice regarding the conversion of the garage into a home office and ceramics workshop, superinsulating the attic and converting it into useable space to expand their home, constructing a stand-alone holiday cottage, and the feasibility of micro hydro power generation.

During our visit we discussed your food miles and transport as well as other global environmental considerations. You have a good understanding of how your home and lifestyle fit into the broader context and seem willing to further improve these aspects of your life while improving the performance of your home.

During the general overview of the existing building’s thermal performance, the main areas I identified for improvement, in priority order, were:

- Improved air tightness – this would bring the greatest benefits
- Increased loft insulation – maximum return on investment, not disruptive
- Cavity wall insulation, and
- Winter window shutters

Conservatory extension

The proposed location for the extension has a good solar aspect, as the building could benefit from increasing its south-facing glazed area. Correctly orientated and operated conservatories can perform well as solar collectors. However, as they are made from glass they have poor thermal performance and lose large amounts of heat to the night sky. As such, they must be closed off from the rest of the house when they are cold. As this extension is liable to be used throughout the heating season, it should not be fully glazed.

I would therefore recommend:
A single-storey art deco-styled extension with accessible flat roofed terrace. This could include a small conservatory to the southern end.

Solar hot water (SHW)

The property has good solar potentional and this technology is already well advanced, offering good energy and financial return, probably better than double glazing. SHW works as a water pre-heater with other systems, contributing at the most energy intensive part of the heating process due to the latent heat effect. It works well with wood-fuelled heating as it can provide hot water in the summer when space heating isn’t required. Grants are available for the installation of SHW systems.

I would therefore recommend:
Installing a solar hot water system as part of the development to the roof.

Photovoltaic cells (solar electric)

This technology is still in its infancy and, as such, offers poor payback on investment. However as the industry expands, photovoltaic cells (PVs) are expected to fall in price and become a major energy source in the future. Power generated during the day is used to provide lighting at night, therefore the power must be stored until required. The unit cost of electricity is much higher when storage is required, because of the cost of batteries and their maintenance. It is better to use a grid connection if available. Where a grid connection may be expensive, for example for your holiday cottage, a stand-alone system may be more feasible. Electric lighting provides a safe way to light the holiday cottage and does not have a high electricity demand, so a large system would not be required. If you are investing in a stand alone system, PVs could be used in conjunction with a wind turbine. These compliment each other well since the weather is usually either windy or sunny. A PV system and wind turbine could be a iconic selling point for the cottage.

I would therefore recommend:
For the main house, other energy-saving measures should be addressed first as investment in PVs could be made later once the associated costs have fallen.
For the holiday cottage, a stand-alone system, with a small array of batteries, could incorporate PVs and/or a wind turbine. When the cottage is unoccupied, surplus electricity would require an outlet and could be used to pump water from a rain-water havesting holding tank to a header tank for flushing toilets.

Micro Hydro

I do not believe that you have sufficient height difference across your property to create a big enough head of pressure for an efficient system. A reservoir would be needed to store water and this would, therefore, need to be on neighbouring land. With a long distance between the reservoir and your property, a hydro scheme is likely to be uneconomical. Your property is, however, probably suitable for a wind turbine as it is in open countryside. Wind technology has developed considerably in recent years and has become more affordable.

I would therefore recommend:
Further investigation into the potential for wind, if you are keen on renewable energy technologies, particularly as wind compliments PVs.

Straw bale construction

Straw bales are affordable, easy to build with and produce interesting buildings. However they do not have as good a U-value (a measure of heat transfer) as other building materials so you need much deeper walls to achieve the same thermal performance. Having thick walls compromises the size of rooms unless you have space to build bigger and the opportunity to include straw bales in a building’s design from the outset.

I would therefore recommend:
Straw bale construction could be suitable for the holiday cottage, but probably not for the extension.

Reed bed water treatment

Your property currently uses a septic tank, which works by allowing solids to settle out and liquids (‘black water’) to overflow into a soakaway where pathogens can be treated naturally. Liquid leaving the soakaway is still high in nitrates, which are a plant food. High nitrate levels should not enter watercourses directly but should pass through vegetated land first. In your case, this seems to be working fine, although care should be taken not to let the septic tank overflow as this could cause black water to enter the watercourse directly. Septic tanks should be emptied regularly, ideally annually.

The quantity of water entering the septic tank should be minimised as a large inflow can inundate the system and unsettle the solids. At the moment, all waste water (‘grey water’) from your house and run-off from your roof is being directed into the septic tank. Run-off should be clean enough to go straight into the river or into butts for the garden. Nutrients in grey water from sinks and the washing machine could be recovered for use on the garden, although grey water cannot be stored.

Reed beds only treat liquids. That means that to treat toilet waste, liquids and solids must be separated, either with a settlement tank like your septic tank or a water separating toilet. As your soakaway is functioning adequately, a reed bed system for toilet waste from your current house seems unnecessary.

For your holiday cottage, reed bed water treatment would be appropriate and is looked upon favourably by local planning authorities. Alternatively, a dry ‘composting’ toilet could be used, although inside the house, residents would probably prefer a more conventional design. A water-separating toilet can satisfy the ‘porcelain standard’ and would work well as an alternative to a settling tank in conjunction with the reed bed system.

I would therefore recommend:
Reducing the quantity of water entering your septic tank by allowing run off to soak away and by reducing water consumption.
Providing your holiday cottage with a reed bed water treatment system, composting toilet and water separating toilet to treat its black and grey water. I have a Building Research Establishment (BRE) document on reed beds for you, to follow.

Garage conversion

Installing a home office and ceramics workshop in the garage provides opportunities to work at home, which would reduce fuel consumption. The building should be well insulated, with some ‘thermal mass’ to absorb heat from the kiln and store it.

I would therefore recommend:
A further site visit to discuss this in detail, measure up and produce designs.

Building science principles

Thermal mass

Heavy weight materials (e.g. brick and stone) absorb heat from their surroundings and release it when the air cools down. Buildings with a large amount of thermal mass therefore take a longer time to heat up, but store heat for longer, than lightweight (e.g. timber framed or straw bale) buildings. A combination of heavy and lightweight materials works well. Because heat rises, thermal mass is best positioned high in a room, for example as walls, chimneys or even ceilings. Thermal mass in the floor can be used to retain heat from an under floor heating system, from which heat rises to warm a whole room. With warm feet, a lower room temperature is required to produce the same level of comfort. Radiators, which heat by convection, should be positioned on internal walls in order to store the heat in the thermal mass, rather than beneath windows (particularly if they’re single glazed).

Solar Orientation & Solar Gain

The south-facing side of a building benefits from solar ‘gain’ (capture of the sun’s heat) for most of the day. Conversely, the north-facing side loses heat through the walls and windows since these rarely feel the sun, especially in the winter when the sun is low. To make the most of a building’s orientation, a larger proportion of glazing should face south, with increased thermal resistance to the north. Conservatories act as a solar collector if on the south side of a building. They can be opened to the building when the heat is available and closed off when the conservatory is cold, for example at night. Some people try to use conservatories at night and throughout the year and heat them to do so. This is an enormous waste of energy as glass offers no insulation and the heat is lost.

Daylighting

A balance must be struck between letting light in and letting heat in or out. With thick walls, window reveals can be splayed and painted white to increase the amount of light entering a room. To make the most of the light coming through a window, smooth uncluttered pale-coloured surfaces can bounce light around inside a room. Artex and other textured surfaces prevent light from travelling far into a room.

Breathability & Ventilation

Traditional building construction allowed materials to breathe so that moisture could be transferred to the outside. Some modern materials such as gloss paint and cement are not breathable. If applied to materials that need to breathe, moisture can become trapped inside a building or within walls. This causes damp, which people often mistakenly believe is due to adverse weather and apply further non-breathable layers in an attempt to ‘weather proof’. Moisture can be removed from a building by removing moist air directly, as well as through breathable walls.

During the summer, a building can be cross-ventilated by opening windows, allowing rapid air exchange through the building. However, during the winter, high levels of air exchange are undesirable because of the subsequent heat loss. Maximum moisture can be removed with minimal air exchange by removing warm air, since this holds most moisture. Letting the warmest air out at the top of a building in a controlled way draws fresh air in at the bottom. This provides a slower, more efficient air exchange than cross-ventilating, and can be shut off when it is too cold. Unwanted draughts should be replaced with controlled trickle ventilation to remove unwanted moisture. Controlled ventilation should be possible without compromising the security of a building so that it can continue to be ventilated whilst unoccupied.

Draught-Proofing & Insulation

Making a building more air-tight is the easiest, most cost-effective method of reducing heat loss from a building. When air is warmed it expands, rises and escapes though gaps around doors, windows, loft hatches etc., or through hairline cracks at junctures in the building fabric. As wind passes over a building, low pressure is created on the leeward side and air is sucked out through these same gaps. As warm air escapes, cold air is drawn in to fill the void and heat is lost. For example: a draught felt on the floor will be reduced by fitting a draught excluder around a loft hatch by preventing warm air escaping, as well as under the front door by preventing cold air from entering. All windows, doors, and loft hatches should be fitted with a flexible seal; letterboxes can be fitted with brushes and key holes fitted with cover plates. When decorating, a flexible decorator’s caulk should be used to prevent air loss through hairline cracks. A porch can act as an airlock, preventing unwanted air exchange and as a buffer, particularly effective on the north side of a building. When draught-proofing, it is important to remember air exchange also removes moisture from the building. Controlled ventilation must be installed (see breathability and ventilation).

Insulation prevents heat from being conducted through the fabric of a building to the outside. The rate of conduction depends on the thermal conductivity of materials used. Trapped air has the highest insulation value (lowest thermal conductivity). However, if a convection current is allowed, air can efficiently transfer heat over large distances. Insulation therefore needs to be fitted snugly, with no gaps around it. Windows, particularly single glazed, have very high thermal conductivity and can represent a large external area of a house. However, heat rises, so insulating the roof of a building is very important. As it is easier to insulate some parts of a building than others, insulation should be prioritised accordingly. Cavity wall insulation is an easier and cheaper way to improve the thermal performance of walls than replacing all windows, for example. If your case, it would be very easy to super insulate your roof (to a maximum of 600mm). Cellulose and sheep wool are natural materials, but if their expense means compromising on insulation depth, it would be better to use more mineral wool. If you are going to further insulate the loft, you should decide now whether you are likely to convert your loft in the future.

If you are interested in pursuing any development ideas further, for example your garage, I would like to propose carrying out a further site visit in order to discuss possibilities with you, illustrate my ideas with some sketches and measure up for accurate design drawings. Detailed drawings would then facilitate accurate costings.

If there is anything in this report which you require further details or clarification on, please do not hesitate to contact me.

Simon Lewis

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