In this article we are going to figure out the effect of U-values on the heating demand of your home. By adding insulation to the walls of your home, you reduce the U-value and in turn you will save money on your heating energy bills. Let see how this works.

The heat demand of your house depends on a number of factors such as:

1. The insulating properties of the fabric of your house, known as U-values of the walls
2. The amount of heat carried out of your house by air movement, depending on how big your house is and the number of windows
3. The difference between the inside and outside temperatures – the heat requirement for a home varies from season to season (more heat is needed during the winter months for example).

A better way to estimate the annual space heat consumption is to divide the heating into periods for different outside temperatures, such as mid-season and winter. For example, during winter the average outside temperature is 6^oC while the inside temperature is set to 20^oC to stay comfortable. For a standard temperature difference and constant heat losses through ventilation, the amount of heating required depends on the insulation of the walls, roof and floor. In the next section we are going to examine how insulation affects the heat transfer through the walls.

How does U value change when cavity or solid walls are insulated?

Common cavity walls built after 1930s consist of two layers of brick of about 100mm each separated by an air cavity of 70mm in the middle as you see in the picture below.

The u-value of an Un-insulated Wall

The insulation commonly used in cavity walls like this is EPS (such as Polypearl Plus and Polypearl Platinum) – this is expanded polystyrene bead which helps to reduce the thermal transmittance of the external cavity walls. By adding 70mm of Polypearl Platinum insulation to your cavity wall you can reduce the U-value from 1.94W/m²K to 0.41W/m²K.

The u-value of a cavity wall insulated with Polypearl Platinum (EPS Insulation)

The u-value of an Uninsulated Solid Wall

Solid Walls commonly consist of a brick layer of 250mm thick. Most houses built before the 1930s (and many blocks of flats up to the 1950s) have solid walls without any insulation – these tend to suffer from large heat losses with U-values reaching as high as 2.58W/m²K.

Insulated Solid Wall

By adding 100mm of mineral wool or expanded polystyrene (EPS) in your solid walls you can reach a U-value of 0.31W/m²K and 0.28W/m²K, respectively.

Mineral wool insulation

EPS

What is the impact on U-value of adding even more insulation to walls?

In the table we have figured out the reduction of the U-values by adding different thicknesses and types of insulation in a solid brick wall and a cavity wall. As you can see there is no need to go for insulation thickness higher than 100mm as you can reach the U-values required to meet building regulations.

What practically a reduction of the U-value means?

Heat transfer between indoor and outdoor

This is an example to help you understand how U-values affect the heat losses of your walls.

The heat loss through the walls can be described by the equation:

Fabric Heat loss through the walls = Σ UxA x ΔΤ, where

Σ UxA is the sum of the products of the U values and areas of all parts of the external envelope of the building,

ΔΤ is the difference between external and internal temperatures

So, for 1m² of solid un-insulated wall and 20^oC – 6^oC = 14^oC = 14K  the heat loss would be 2.58 W/m²K x 14 K = 36.12W/m² as can be seen in the next table.

By adding insulation e.g.. mineral wool there would be a reduction of 2.58 – 0.31 = 2.27 W/m²K in the U-value of the solid wall. Smaller U-values practically means a reduction in heat loss of 31.78W/m².

We should also take into account the air leakage of your house to estimate the heat demand of your house. For example, for an average semi-detached house of 98 m² with a total volume of 245 m³ the heat loss coefficient is c_f = 304.02 W/K for un-insulated walls and standard U-values for the windows, floor and roof as you see in the next table. For the same house, the ventilation loss coefficient c_v = 65 W/K. By insulating the wall with mineral wool the fabric heat losses will be reduced to c_f = 202.03 W/K.

So, the new heat demand would be:

Heat demand/ m² = (202.03W/K + 65 W/K) / 14K = 19.07W/m²

If you heat your house with gas (4.36p per KWh), this practically means that you will have saved about 0.01907KWh/m² x £0.0436/KWh = £0.00083145/m² per hour. During the heating season, if you heat your house for 7 hours you will save about £0.0058/m² per day. So, for the whole heating period of approximately 3 months you will save £0.524/m². For an average house of 98m² this means that you save £51.35 to heat your house by adding mineral wool insulation to your solid walls.

In reality, in order to give a more accurate estimation of the heat demand we should take into account the heat gains from the lights and electrical appliances of your home, and the heat produced by the occupants and the solar heat gains mainly through windows. So, actually the heating loads will be a little bit less than calculated in this example – the difference between the heat losses and the heat gains.

The 2015 United Nations Climate Change Conference (COP 21 or CMP 11) was held in Paris, France, from 30 November to 12 December 2015. It was the 21st yearly session of the Conference of the Parties (COP) to the 1992 United Nations Framework Convention on Climate Change (UNFCCC) and the 11th session of the Meeting of the Parties to the 1997 Kyoto Protocol (CMP).

196 parties attended the conference to negotiate the Paris Agreement which is aiming to reduce climate change. Climate change seems to be the most important issue facing our planet today as stated by more than 150 world leaders that attended the conference including Presidents Obama, Putin and Xi.

Climate Change Prediction

Change in average surface temperature (1986−2005 to 2081−2100), Source: IPCC 2014, Synthesis Report

According to the IPCC’s 2014 Synthesis Report, if greenhouse gas emissions continue to rise at the same rate throughout the 21st century, the surface temperature is projected to rise up to 11 degrees Celsius. This will result to increased heat waves and precipitation events that will last longer and be more intense and severe. Sea levels will rise dramatically and cause the elimination of many species.

Goal of Paris 2015

The goal was to reach an agreement to limit the global temperature increase to 2 degrees Celsius compared to pre-industrial levels by reducing greenhouse gas emissions. The greenhouse gases are the main cause of global warming, two times more harmful than man-made aerosols, solar energy and volcanic activity.

To reach this goal the greenhouse gas emissions have to be limited to a net zero level sometime in the second half of the 21^st century. France takes the lead in this attempt being one of the few developed countries in the world that from 2012 have managed to generate over 90% of its electricity from zero carbon sources, including nuclear, hydroelectric and wind.

Top 3 Emitting Regions

The top 40 CO2 emitting countries across the world in 1990 and 2012, including per capita figures. The data is taken from the EU Edgar database

The role of China, U.S. and Europe is of great importance to reach this goal, as together they are the top 3 emitting regions in 2012, accounting for more than half of the total CO2 emissions globally. During the last two decades, from 1990 to 2012, China and the United States increased their CO2 emissions while Europe managed to decrease this percentage.

Suggested Commitments

Prior to the conference, the so-called Intended Nationally Determined Contributions (INDCs) were published in a series of proposals to be achieved during the next decades. The suggested commitments were to:

1. Limit global warming temperature increase to 2.7 degrees Celsius by 2100
2. Reduce greenhouse gases emitted by human activity by 40% by 2030 compared to 1990
3. To review each country’s contribution to cutting emissions every 5 years  beginning in 2023
4. To help poorer nations to adapt to climate change and switch to renewable energy

Outcome

On 12th December 2015, 195 of the participating countries agreed by consensus to the Paris Agreement to reduce emissions as part of the method for reducing greenhouse gas emissions. Τhe members of the conference agreed to take action in order to reduce their carbon output and to do their best to keep global warming below 2 degrees Celcius. France’s Foreign Minister Laurent Fabius said this “ambitious and balanced” plan was a “historic turning point” in the goal of reducing global warming.

The agreement will become legally binding if joined by at least 55 countries which together represent at least 55 percent of global greenhouse emissions. Such parties will need to sign the agreement in New York between 22 April 2016 to 21 April 2017, and also adopt it within their own legal systems. However, there remain doubts as to whether the good will of the past few weeks will carry over to the US next year.

Think we missed something? Do you have a different opinion?

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Estimating how much energy a home uses

Domestic buildings are responsible for 30-40% of primary energy use in the UK. When talking about primary energy use, we tend to be referring to electricity and gas, although some properties use oil or other means for heating. Electricity in the home tends to be used for lighting, cooling systems (AC unit) and electrical appliances while gas is normally used for heating and hot water generation. Energy is used 24/7, all year round, as most homes have background appliances running all the time.

It is said the average home in the UK has an average energy bill of £1,326 per year – which is split between electricity and gas.

This ‘average house’ phrase is used all the time in the energy industry – and more and more often we are subjected to it in the media too – but what exactly is an average house?

Perhaps more importantly we are going to look at how homes vary from the average – the housing stock in the UK is so diverse, not many of us are going to find ourselves living in an average home – so we hope having read this you can perhaps tailor the meaning of this average home to make it slightly more specific to your home!

So first then – what is an average home?

The best ways to save energy in your home

Read more

Determining the size of a typical home

The most common type of dwelling here in the UK is a three bed semi-detached property – with 2-3 occupants. The average home comprises of a living room, kitchen, bathroom and 2 or 3 bedrooms. The average home tends to have been built from solid brick 220mm walls (instead of cavity walls), double or single glazing with modest loft insulation and some form of central heating system.

According to RIBA (The Case for Space: the size of England’s new home, Royal Institute of British Architects, September 2011) a survey of a sample of 3,418 homes across 71 sites concludes that an average three-bedroom home has a internal floor area of approximately 88m².

Average energy cost for a home

In the period between November 2014 and October 2015, the annual average dual fuel customer bill in the U.K. was £1,326 as can be seen in the table below (source: Ofgem). According to the Department of Energy & Climate Change (DECC), in 2014 the average domestic electricity consumption per UK household was 4,000kWh and the average domestic gas consumption per UK household was 12,400kWh.

As we mentioned, most people don’t fall into this average home.  In order to make a more accurate estimation of the energy usage of your own home we need to take into account several different parameters – they are as follows:

• The type, size and age of your home
• The location and weather
• The heating and cooling type
• The levels of wall, floor and loft insulation
• The external wall area and windows area
• The efficiency of the appliances and lighting
• The occupancy
• The users’ status and lifestyle

In the next section we explore a little bit into the variables that determine the energy usage of a home. This will hopefully put into context some of the research published in this area and what we talked about at the top of the article.

The location and weather

To estimate the heating loads of a house we should take into account the difference between the inside and outside temperatures. So, the total heat losses of the house are therefore determined by the fabric and ventilation heat losses. The fabric heat losses can be calculated working out the surface of the exposed external walls and then calculating the u-values of each element. The ventilation losses depend on the cubic volume of each room and the air changes per hour through open windows and other forms of infiltration. 

For example, the fabric heat loss coefficient of an average semi-detached 2-storey house of medium size (Total floor area: 98m², Total window area 8m² South and 8m² North) can be given by the next equation:

c_f = Σ UxA (W/K) where,

c_f is the fabric heat loss coefficient,

Σ UxA is the sum of the products of the U-values and areas A of all parts of the external envelope of the building

So, external wall area: 3sides x 7m length x 2floors x 2.5m height = 105m²

If we remove the area of windows the total external wall area will be: 105m² – 16m² =89m²

Given the typical U-values of such a house we will have the next table:

The fabric heat loss coefficient is c_f ≈ 115 W/K

The ventilation loss coefficient will be:

c_v = (ac/h x Volume of building / 3) (W/K) where,

c_v is the ventilation loss coefficient,

ac/h is the air changes per hour (0.8 ac/h for single-glazed windows and old frames)

The volume of house is: 49 x 5 = 245 m³

So, c_v = 245 x 0.8 / 3 ≈ 65 W/K

To calculate annual space heat demands we have to add up daily heat demands over the whole of the space heating season. So, the annual space heat demand of a building is given by the next equation:

Annual space heat demand = (24/1000) x (cf + cv) x degree-days (kWh) where,

degree-days are the number of days when the average outside temperature is lower than a base temperature above which a building needs no heating.

The degree-day figures normally assume a balance temperature or degree-day base temperature of 15.5°C. As can be seen in the next map, the value for London is approximately 2100 Degree days, for Newcastle 2400 and for Plymouth 1900. This figure is supposed to be representative of most dwellings, but for very well insulated dwellings and many commercial buildings it is too high, and leads to significant overestimates of annual space heat demand.

Figure 5: Degree Days map of UK based on 15.5°C

The next table shows the space heating demand for 3 different UK locations. Notice that this is the price for space heating only and not for the total gas consumption that comes from the hot water and cooking too.

* Gas unit price is taken as 3.50p/kWh

The space heating type

The main sources of heating are gas and electricity, but some homes are heated by oil and even biomass pellets. Most homes have condensing gas boilers and a typical seasonal efficiency on those units is approximately 87% over a heating season (for condensing boiler types), but you may well have an air source heat pump or a biomass boiler so the efficiencies will vary a huge amount. Non-condensing gas boilers have efficiency between 0.65 and 0.70. We will now use the gas boiler example to demonstrate how to calculate the useful heating required for your home.

To calculate the space heating required take the gas consumption for the home in kWhs (you can look at your annual gas bill statement) and then divide this figure by the average efficiency of your heating system.

For example, for an average house like we described previously located in London, to reach the required space heating demand of 9,072 kWh the energy consumption is:

• For a condensing boiler: 9,072 kWhs of gas/ 0.87 SPF = 10,428 kWh for this particular house, assuming continuous heating throughout the heating season (the six months around Christmas).
• For a non-condensing boiler: 9,072kWhs of gas/ 0.65 SPF = 13,957

According to Sedbuk, the annual fuel costs for different boiler types is:

*These figures are based on a gas unit price of 4.36p per kWh (April 2013)

Quality of wall, floor and loft insulation

The average energy performance score (determined by the EPCs) of the UK is SAP60, which puts it into a band D. At SAP60 the insulation quality of a home can be described as moderate with areas that could certainly be improved. There are millions of homes though that have a far lower SAP energy score though – particularly homes built before 1930, when insulation quality wasn’t really a consideration.

Below we look at a home with some of the typical characteristics including the property fabric, quality of insulation and information on the heating system.

This is a typical semi-detached house with 4 habitable rooms, built between 1930 and 1949 with solid brick walls. There is no wall insulation and very little in the loft. The windows are double-glazed. The central heating system uses gas and the total area of a house is 100m² and is occupied by 4 people.

This type of house would have the following kind of SAP information

We ran some numbers through the RDSAP calcualtion method, and the typical household energy bill for such a house is £1,576, 65% of this spent for electricity and 35% for gas. Given the prices for electricity and gas, 12.5p/kWh and 3.5p/kWh respectively, this house consumes 4,413kWh for electricity and 29,269kWh for gas per year.

Figure 5: Façade of a common detached maisonette

Figure 6: Floor plans of a common detached maisonette

The external wall area and windows area

The type of glazing and the frame type are responsible for the heat losses through the windows. Most of the UK houses have already replaced the old single-glazed windows and frames and they are now double-glazed. The ventilation and infiltration rate through the glazing and the frames is calculated given the volume of the house and the air changes per hour.

So, for an average UK house with double-glazed windows the air changes per hour are 0.8-1.0 and for double-glazed windows are 0.3.

The efficiency of the appliances and lighting

A typical 3 bedroom semi-detached house has about 10 lights, each of them consuming 50W of electricity. So, each hour they spend 500W and the total energy for an average use of 4 hours per day is 2kWh. In the UK the cost of electricity is approximately 13p/kWh, therefore, the cost per day is £0.26 and the annual cost for 365 days is £95.0.

By replacing all of the incandescent light bulbs with LED lights the cost would be reduced to £9.5. So, you would have a reduction of 90% in your electricity consumption for lighting. Taking into account that lighting and home appliances tend to account for 20% of a typical energy bill this investment will save a significant amount of your money.

(See also 7 reasons why you should swap to LED lighting)

The occupancy

The heating and cooling loads of a home are influenced by the internal gains of the people who live at the house. At the same time, the more the occupants the more energy usage of the house appliances is being made.

The users’ status and lifestyle

Ofgem suggest the following for yearly energy consumption by user group:

Table 2: Energy consumption of each energy user group according to Ofgem

An average ‘medium user’ is defined in terms of energy use, as using 3,100 kWhs of electricity a year and 12,500kWhs of gas a year. As we stated previously, this kind of property describes a medium sized property, with three bedrooms occupied by 3 to 4 people (for example 2 adults, 2 children). During the day children are at school and parents at work, and everyone comes at home in the evening. They house appliances’ usage is a few times a week for the washing machine, regular heating, occasional dishwasher, TV and electrical appliances in the evenings, as can be seen in the next figure.

Figure 7: Three main categories of energy users according to Ofgem

To sum up, for different types of UK house the energy consumption is as follows, as calculated by using the Stroma RSAP program:

Table 3: Annual consumption for different house types

You can save money from your energy bills by either reducing your existing energy demand or using renewable energy, or with a combination of them. Applying one or more of the next methods in your property can reduce the domestic energy consumption:

• Wall and loft Insulation
• Double glazing
• Draught proofing
• More efficient boiler
• More efficient light bulbs
• Low consumption house appliances
• Thermostats
• Home automation systems

The Renewable energy systems include:

• Solar Pvs or Solar thermal
• Biomass
• heat pumps

How do we deal with hot weather in the UK or getting rid of latent warm air without spending huge amounts of energy and money on air-conditioning systems? Although natural ventilation is the commonly low-cost practice to cool a building efficiently during the summer, mechanical ventilation in theory appears to be more efficient due to several reasons that are explored in this article.

Mechanical Ventilation advantages over natural ventilation

Mechanical ventilation can be retrofitted to almost every house and a large number of commercial premises to promote fresh air into spaces and then remove any latent heat. As previously discussed (reference to natural ventilation article), natural ventilation is based providing fresh air to a building through openings such as windows and doors. The way it  works will depend on the type of the openings and the layout of the building.

Excessive solar gain and cooling with natural ventilation

The big issue with natural ventilation is that with glazing causes solar gain and in the summer months if this is excessive, it can cause local discomfort. In this case, mechanical ventilation would be more effective because it provides a more homogeneous effect due to a more even distribution of air.

The other issue with natural ventilation is that it can lead to excessive cooling capacity of rooms and again this may be worse / better depending on the building layout.

Mechanical ventilation reduces noise and air pollution

Mechanical ventilation is the only option when the building is located in noisy areas or in areas where the local air quality is poor, and therefore the use of openable windows for natural ventilation is not a practical solution. It is worth noting that security considerations may also lead to the use of mechanically assisted ventilation in many buildings as it allows the units to be securely locked.

However mechanical ventilation can be a costly solution

Mechanical ventilation definitely has merits over natural ventilation, but the issue comes to retrofitting a solution to a building, which could add quite a bit to the cost. This is why if possible mechanical ventilation is designed into the building plans, so that the costs to implement kept to a minimum.

Also, many companies lease the buildings that they occupy, so the decisions about the facilities management are taken out of their control. Landlord in turn would find it hard to invest in a system that would require significant funds but not so obvious quantifiable benefits.

Why is ventilation important?

Read more

The next section deals with some of the practical mechanical ventilation solutions and brief explanations on how they actually work.

Mechanical Ventilation Solutions

The simplest forms of mechanical ventilation systems simply circulate the air at the ceiling level, for example ceiling fans. They create air movement at the ceiling level to cool the space.

We may also have roof ventilators with louvers that help cold air enter the house and warm air to be exhausted as can be seen in Figure 1.

Figure 1: Example of roof ventilators (BSRIA BG 2/2005)

The roof ventilators can also be solar assisted to enhance the stack effect by increasing the air temperature in the top of the house. As mentioned before in natural ventilation the stack effect is based on temperature difference within the space and the outside air – the hotter it is at the top of the building the quicker the latent warm air is allowed to escape.

Ducts can be used to supply air to different levels of a multi-storey building, using underfloor or displacement ventilation that provide fresh air from floor-mounted or low-level wall mounted ventilators within the occupied rooms or zones.

Figure 2: Typical air flow pattern of displacement ventilation

In addition ventilated façade systems that are installed in the window frames such as those that can be seen in Figure 3 can also be classed in the mechanical ventilation family.

Figure 3: Ventilated facade system

Supply and extract systems

Fan-assisted extracts work by enhancing the air movement to exhaust latent heat when the internal temperature is high.

Mechanical ventilation systems can have both supplier and extract vents assisted by fans as can be seen in the next se of figures. These systems may also include filters to ensure a higher standard of indoor air quality, coupled with heating and/ or cooling coils. When installing these systems they need to be ideally positioned into spaced to take into account occupational density and well as tackling cold draughts.

Figure 4a: Supply and extract systems with re-circulation

Figure 4b: Supply and extract systems incorporating indirect heat recovery

Mechanical extract only systems are used mostly when the air becomes contaminated such as in kitchens, bathrooms etc – where there is a need for constant and predictable extraction of air. Supply only systems are suitable for houses and occupied offices that need to be supplied by fresh air when the air movement needs to be controlled.

Figure 5: Fan-coil unit with direct fresh air supply through external wall

Other mechanical ventilation systems

Another option for mechanical ventilation systems is to go for mixed-mode ventilation where the system works in conjunction with natural ventilation. It corresponds well to the operational performance of the occupied buildings providing indoor air quality and preventing summer overheating.

Mixed mode ventilation can reduce annual energy use by over 41% with 19% of this reduction is due reduced night cooling of the building. Night ventilation is enhanced by mechanical systems that ensure an adequate level of airflow to remove heat gains during night.

Figure 6: Mechanically assisted night ventilation

Summary of mechanical ventilation

To sum up, the main reasons why mechanical ventilation wins over natural ventilation:

• The layout buildings does not help natural ventilation because the spaces are too deep to be ventilated from the perimeter or the air paths are being mitigated by the internal partitions.
• Natural ventilation absorbs any poor local air quality if present whereas mechanical ventilation can filter these pollutants out.
• Mechanical ventilation will allow less noise pollution to enter the building via openings such as windows.
• Highly dense areas will not have enough wind velocity to pump enough air flow into a building.
• Mechanical ventilation increases security as the building unit can be more sealed.

Some of the mechanical ventilation solutions explored

• Roof ventilators
• Ducts to encourage underfloor or displacement ventilation
• Ventilated façade systems
• Supply and extract systems
• Mixed mode ventilation

Cost / benefit of mechanical ventilation

Mechanical ventilation if designed into the plans of the building is a great concept – however to retrofit a solution may be too expense to implement.

Reference: BSRIA, The Illustrated Guide to Mechanical Building Services