Low energy timber frame buildings Preview by BM TRADA, Element and Warringtonfire

is an authoritative and inspirational design guide for clients and designers

covers the whole design process from site conditions and planning for better thermal performance to provision for alternative energy sources

describes how to optimise the layout to achieve ‘low energy’ designs, together with guidance on envelope performance and details for ventilation, airsealing and insulation

LOW-ENERGY TIMBER FRAME BUILDINGS

Low energy timber frame buildings

Matthew Hoad of HTA Architects, London: “This book is refreshingly simple to understand and follows logical arguments on where to best deploy timber construction (and other materials) in high performance buildings.”

Low-energy timber frame buildings Designing for high performance 2nd Edition Geoffrey Pitts with Robin Lancashire

Firstly the author describes influential factors such as external climate, desirable internal environmental conditions and aspects of site planning. He then focuses on residential building forms, the significance of internal planning and the construction system in designing low-energy timber frame buildings for high performance.

Pitts / Lancashire

The energy consumption associated with buildings has consistently been estimated to be up to 50% of the total national consumption in industrialised countries and, of this proportion, residential buildings account for more than half. Timber frame systems are readily adaptable to the increasingly tough energy-saving regulations needed to cut energy use in buildings.

low-energy timber frame buildings offers guidance on assessing the thermal resistance of the building envelope, together with a rational approach to thermal capacity and response of timber frame buildings. This is developed in detail across the various building elements. The book includes practical advice on choosing fuels and space heating, as well as exploiting renewable energy. Geoffrey C Pitts (author) was formerly a housing innovation consultant specialising in timber frame. Robin Lancashire (technical editor) is senior timber frame consultant at BM TRADA with particular expertise in timber frame construction.

BM TRADA, part of the Element Group, provides a comprehensive range of independent testing, inspection, certification, technical and training services. We help organisations to demonstrate their business and product credentials, and to improve performance and compliance. We help our customers to make certain that the management systems, supply chain and product certification schemes they operate are compliant and fit for purpose.

BM TRADA

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Low-energy timber frame buildings Designing for high performance

Although materials technology is now much improved, the principles outlined in this book have changed little since the first edition was published more than 20 years ago. Meanwhile, concerns about climate change and rising energy prices have re-focused attention on designing for high thermal performance. This revised edition shows how timber frame systems are readily adaptable to the increasingly tough regulations needed to cut energy use in buildings. New illustrations provide clear guidance to designers throughout. Building on influential factors such as external climate, desirable internal environmental conditions and aspects of site planning, the book focuses on residential building forms, the significance of internal planning and the construction system in designing low-energy timber frame buildings for high performance. It offers guidance on assessing the thermal resistance of the building envelope, together with a rational approach to thermal capacity and response of timber frame buildings. This is developed in detail across the various building elements. Practical advice on choosing energy sources and space heating, as well as exploiting renewable energy, is included. Geoffrey C Pitts (author) was formerly a housing innovation consultant specialising in timber frame. Robin Lancashire (technical editor) is head of the Building Performance section at BM TRADA with particular expertise in timber frame construction.

1 Introduction

2 Building design and use

3 Site conditions and planning

4 Building design and construction

5 Building envelope performance

6 Building envelope elements

7 Airsealing and ventilation

8 Passive solar heating potential

9 Fuels and services

10 Energy matrix and design checklist

Appendix: A user operation manual

References

1 Introduction Although energy efficient buildings are desirable, energy efficiency comes at a cost. Energy is another word for fuel, and in buildings this is most commonly electricity, gas, oil or solid (coal) fuel. All fuel costs money in some form, and the more fuel that can be saved with energy efficient buildings, the lower the running costs are for building users and owners. Saving money is generally considered to be a good thing, but is money saving a good enough reason for designing energy-efficient buildings with higher construction costs? Hence the question; energy-efficient buildings: why? There is no shortage of energy in the world; the sun shines, the wind blows, the waves remain in motion, tides continually cycle and plant-based (biomass) fuel can always grow year on year; all of which are potential inexhaustible sources of energy (Figure 1.1). All these sources are currently utilised to some degree and this is expected to increase in the future, so why be concerned with energy efficiency? The reasons can be many and varied, but there are six which stand out: 1. The easily exploitable energy sources (i.e. ancient fossil fuels – gas, oil and coal – have a limited life and the price to the consumer always increases relative to the reducing availability. New fuel fields are continually being found, but extraction processes are more complex and the route to market is increasingly difficult, meaning the price continues to increase. There has always been “political” and “market” pricing for fossil fuels, but the certainty is that as resources diminish, costs will increase. Energy-efficient buildings can reduce consumer spending and prolong the life of the ancient fossil fuels. 2. All energy generation and use carries with it an associated environmental impact, albeit of widely varying degrees (eg land devastation by mining and drilling; water pollution by oil spillage; chemical and thermal pollution in generation and end use). The most important environmental impact is associated with the most commonly used fossil fuels. All consumption of fossil fuel results in carbon dioxide (CO2) emissions and it is widely recognised that CO2 is a major greenhouse gas which is the most significant contributor to climate change. Energy-efficient buildings can reduce all of the environmental impact associated with the consumption of fossil fuels and, in particular, can reduce CO2 emissions.

Figure 1.1 Sources of renewable energy

3. Biomass is a renewable energy source, which is commonly plant waste or matter grown to generate electricity or produce heat. 7

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Low-energy timber frame buildings: designing for high performance

In general terms, the higher the mass of these materials, the greater the thermal mass of the component. The thermal response of a construction is the time taken for the surface temperature and the associated room air temperature to reach a given level. Typically, the response time will be shortest for constructions with insulation at, or close to, the room surface; ie low thermal mass constructions which warm quickly to comfortable temperature due to heat input. Conversely, a high thermal mass construction warms up slowly as a proportion of the heat input is required to warm the high mass material inside the insulation, as well as the air; therefore, such constructions have a long response time. Thermal resistance does not affect the thermal mass or response time, but the position of the insulation does affect these properties. Figure 5.1 defines a broad classification of medium and low thermal mass for timber frame construction.

Figure 5.1 Classification of thermal mass

In cool temperate climates any level of thermal mass and response time can, in conjunction with any heating system or occupancy pattern, provide comfortable temperatures. However, not all combinations are energy efficient. For energy efficiency the level of thermal mass and response time should be considered in conjunction with the heating pattern and orientation, building form and internal planning. Heating patterns are normally intermittent to some degree. For residential buildings, heating patterns typically range from morning and evening only (for example, single person, key workers or working couples) to continuous for most of the day and evening, but off overnight (for example, parents with young children or elderly people). In offices, schools, public buildings or shops, the heating is normally on all day in the heating season but turned off at night. The degree of intermittency is one of the factors that determines the desired level of thermal mass and response time. A very heavy-weight construction is inappropriate for the use of very intermittent heating, as heating is required for a long period before occupation to enable the temperature to reach a comfortable level at a given time. 30

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Figures 7.2 and 7.3 illustrate examples of airsealing methods for these leakage points. An additional advantage of sealing these gaps and cracks is that mass transfer of water vapour into the construction is considerably reduced and the risk of interstitial condensation is further minimised. The effectiveness of all air leakage measures for proper energy efficient buildings should be checked on site using a “blower door” pressurisation test. If the design airtightness is not achieved, a “smoke gun” can be used to find where the unexpected infiltration is occurring. Draught lobbies are considered essential as part of infiltration control, and the landscape wind shelter techniques shown in Figure 3.7 will always reduce infiltration heat loss. All improvements in terms of sealing the vapour control layer help in reducing infiltration heat loss; a model vapour control layer specification is given in Table 6.3. A “tight”, well-sealed house has energy advantages in terms of infiltration reduction, but for the comfort and safety of occupants and for condensation control, planned controllable ventilation must be provided. The ventilation rate is normally expressed as whole-house air changes per hour (ACH), but this rate for a given building cannot realistically be calculated as it requires data for localised wind speed and direction, the position and flow characteristics of all openings, the detailed mean surface pressure coefficient distribution for the given wind direction and the internal and external temperatures. These factors are highly variable over a heating season, but even for average winter conditions, localised wind patterns due to topography, vegetation and other buildings are largely unknown. In fact, the average air change rate can only be determined retrospectively by test and then only for the particular conditions occurring on the test day. In practical terms, designers cannot plan for a specific ventilation rate and should ensure: first, that the house is well sealed to minimise infiltration heat loss; and second, that the planned ventilation has sufficiently fine control to enable occupants to find their own level of ventilation for economy and comfort (guidance to occupants should be provided by the designer; see Appendix). Fine control can be defined as the ability to provide a ventilation rate ranging from almost zero to the desired level in small increments. Window opening lights do not normally provide fine control, as the increment from zero (window closed) to the minimum opening is too large, and even at this minimum opening, heat loss can be excessive and draughts may be caused, as shown in Figure 7.4. Slot ventilators can provide an almost continuous range of apertures from zero upwards, and because of their position at the head of windows, they do not cause draughts. Experimental evidence indicates that a well-sealed house with controllable ventilation will have an average winter ventilation rate of 0.5–1.0 ACH. Heat loss calculations can be made using an ACH figure within this range. BS EN ISO 13790 Energy Performance of Buildings – Calculation of Energy Use for Space Heating and Cooling (5) can be used for ventilation heat loss calculations. Planned, fine-controlled ventilation can be provided by: natural means under regular control of occupants; a combination of natural and mechanical means still largely under regular occupant control; or a full mechanical

Figure 7.3 Air sealing: mastics and tapes

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