It’s not just heating and ventilation … retrofit can and should do so much more.
Energy and carbon savings can often dominate discussions on retrofit.
However, we can also design for and build in thermal comfort and health benefits.
In lesson 1.3, we mentioned comfort and health alongside other potential benefits of successful retrofit.
Now we look at comfort and health in more detail.
By the end of this lesson you will have learned
- Why are comfort and health important?
- How can retrofit improve comfort and health?
- Example of retrofits where comfort was monitored
- What affects comfort levels?
- Keeping warm in winter
- Keeping cool in summer
- An example from a real retrofit
- Ways of defining or measuring comfort
1. Why are comfort and health important?
One of the most obvious components of comfort in the home is a pleasant temperature. For much of the year in the UK, the temperature outside is colder than we would find comfortable in indoor clothing for normal indoor activities, so the first priority is warmth.
Many people who retrofit their homes state that comfort is one of the key drivers.
High levels of comfort may also be desirable for other reasons too, for example:
- If a home feels uncomfortable (due to draughts or cold surfaces), the occupants may turn up the heating. This increases their energy bills and carbon emissions.
There is a great deal more to comfort than this of course and being comfortable is an important part of being healthy.
- Cold worsens respiratory and cardiovascular conditions particularly in people who are unwell, elderly, or very young.
- Overheating can lead to serious complications including heart problems, coma and even death, particularly in people who are unwell, elderly, or very young
- Better indoor air quality, (Lesson 1.5) affects health in a number of ways:
– Lower levels of VOCs and particulates: These pollutants can worsen respiratory problems and a range of allergic conditions, and have also been implicated in causing cancer. (See also ‘external air quality’)
– Reducing damp, mould and house dust mites, elimination of soil gases: Mould spores and dust mites cause allergic reactions, worsen respiratory conditions, and can also lead to other ailments e.g. stomach problems.
– Healthy humidity levels: Both damp and very dry environments can increase the incidence of infections and aggravate allergic reactions.
A good retrofit will simultaneously improve comfort and various physical determinants of health. These improvements include those directly impacting on perceived comfort, and also ‘invisible’ improvements which are nonetheless important to health.
Additional improvements that we won’t explore in detail although are worth a mention and consideration are:
- Adequate and flexible control over indoor conditions.
- Quieter indoor environment: May help lower stress and improve sleep.
- More secure: May help lower stress and improve sleep.
The relationships between particular indoor conditions and changes in health are quite complicated to measure and predict. The health problems mentioned above may be long term and are likely to have multiple causes – and people living in unhealthy homes may be subject to a cluster of both home-related and other health risks together.
However a couple of aspects of the indoor environment – temperature and air quality – have been studied in enough detail to be able to generate recommendations for optimally healthy conditions. We deal with these items in the following sub sections below: “Keeping Warm in Winter”, “Overheating” and in Lesson 1.5, “What level of indoor air humidity is desirable?”
As we will learn in later modules, poor levels of thermal comfort may also be indicators of possible health issues for the building.
How many examples of potential health and comfort issues can you think of for someone living in a thermally inefficient home?
2. How can retrofit improve comfort and health?
Looking at the list above…
What opportunities might a retrofit provide to improve thermal comfort and health?
If you wish, base this on your own home or other buildings that you know well.
We go on to look at these in more detail later in this lesson and lesson 1.5. In the meantime, here are a couple of examples:
Acceptable air temperatures are necessary, but there is more to thermal comfort.
- For example, issues like thermal bridges and draughts can impact on comfort in a room whose air temperature is apparently ‘warm enough’.
Similarly, adequate ventilation is necessary, but on its own is not sufficient to ensure good indoor air quality.
- For example, if the ventilation air is mixing with musty air from a damp cellar, it won’t be healthy at all. Air quality is covered in Lesson 1.5.
Thermal comfort, good indoor air quality and occupant health can and should be addressed by a comprehensive retrofit. If they are not, the dwelling might be warmer and more energy efficient, but it may not be much more comfortable and it may not be healthier.
In worst case scenarios, it might even be less healthy, so an integrated approach focusing on energy, carbon savings, comfort and health is very important.
3. Example of retrofits where comfort was monitored
It is certainly the case that a good retrofit can make a cold and draughty home more comfortable.
Let’s start with an example from the Retrofit for the Future project. 76 of the retrofitted homes in this UK-wide project were assessed for perceived comfort levels before and after retrofit. (see green box below)
Note that these retrofits were very varied, with almost as many architects and builders involved as individual homes retrofitted.
Example – Retrofit for the Future projects
Occupants completed a survey on comfort in their homes beforehand and again after retrofit. (The “before” situation was sometimes the same home as “after” retrofit, and sometimes a different home.)
In the graph below: Looking at the “after” situation on the right, there is a clear increase in the number of households where comfort was perceived as “good” or “excellent”.
The Technology Strategy Board focused on 23 properties where there were both energy performance data and surveys recording occupants’ reported levels of comfort before and after retrofit.
In 22 (out of 23) properties, occupant comfort had either improved or remained the same after retrofit.
Of the 13 properties with the lowest CO₂ emissions in the Retrofit for the Future analysis, 12 reported either good or excellent levels of comfort. This suggests that comfort does not need to be compromised by delivering low-carbon retrofits.
In general, the properties are delivering significant CO₂ emissions reductions with no loss of comfort. That is to say, they rarely stray outside comfortable temperatures or humidity levels, or suffer from poor air quality.
4. What affects comfort levels?
3 key factors affect comfort levels:
o Essentially our metabolic rate – or how much heat we are giving off
- Clothing level
o What we wear clearly influences how much heat we retain
o Both air and surface temperatures affect how comfortable we feel
o Air movement also plays a role
o Humidity will also influence comfort levels
Below, we will focus most on “Surroundings” as this is the aspect we can influence when retrofitting a building.
The more extreme the environment in a building, the more stress will be placed upon occupants and their regulatory functions to attempt to restore a balance with the surroundings. This stress can be particularly dangerous for those suffering ill health or who are very young or elderly.
Generally, unless we are considering the retrofit for a leisure centre, hospital, or some other more specialised building, we would assume a moderate level of activity – somewhere around 100W of heat output (for an adult), for example. The list below gives a range of heat outputs for an adult undertaking different activities. It is also worth noting that occupant based heat output is a significant contributor to internal heat gains and should be taken into consideration when addressing overheating.
Activity and associated rate of heat loss:
- Sleeping ~74 W
- Seated work ~104 W
- Machine work ~209 W
- Exercise ~414 W
The type and amount of clothing will also contribute to regulating our body temperature and our perceived level of thermal comfort. As with insulation in a building, the more surface area that is covered and also the more layers doing the covering, the lower the rate of heat loss from occupants. Clothing controls heat loss by minimising the impact of mostly convection on the body’s surface.
For residential retrofit design, we would generally design based on a typical level of indoor clothing: underwear, trousers, shirt, shoes and socks. However, there are situations where higher temperatures are desired, e.g. in bathrooms & showers where our clothing levels are at their lowest and we require higher temperatures to maintain our comfort levels.
How do temperature and air movement affect thermal comfort?
Thermal comfort or our perception of thermal comfort is the same in any building. We have complete control of conditions in new build whereas in retrofit, particularly partial retrofit, our ability to improve the thermal comfort may be hampered by inconsistent improvement measures.
- there may be minimal or no thermal improvement of certain elements, such as the ground floor in a recently renovated kitchen. or
- existing windows are left in place during the addition of external wall insulation.
Both of these decisions will mean that the elements left untreated will be colder than adjacent roof, wall or floor elements that were upgraded. This difference in surface temperatures may seem minor, however, we experience temperature indoors as a combination of:
- surface temperatures
- air temperature
High air temperatures alone will not ensure thermal comfort, particularly if one or more room surfaces are several degrees cooler than the air temperature.
It is worth knowing that 2/3 of the heat we feel comes from radiant temperature of surrounding surfaces and only 1/3 comes from the air temperature.
Example: Think about the sensation of sitting in a heated room near a single glazed window in mid-winter. Although the air temperature may be 18-20°C, the surface of the single glazing may be closer to 13-16°C, which will mean that our body will radiate (lose) heat more quickly to the cold window and therefore we feel cold.
The human body is very sensitive to even small changes in temperature and air movement. Therefore we should seek to minimize the uncontrolled movement of air and the range of temperatures experienced indoors if we aim to provide a thermally comfortable environment. Below are some basic guidelines to consider with regards to temperature and ventilation when seeking to create a comfortable environment:
- Occupants may experience discomfort if one part of their body is 2°C warmer or cooler than another. Think about the feeling of a cold draught at your feet in an otherwise warm room.
- If there is a perceived temperature variation from place to place in a room of 0.8°C or more this can be uncomfortable.
- Discomfort arises when the temperatures of different surfaces vary by more than 4.2°C
- Air movement of 0.8 m/s or higher will be typically experienced as a draught
Optional extra detail:
The main point to be taken from the graph below is: air speeds above 0.1 m/s require an increase in temperature to maintain the same level of comfort.
Guidelines like the ones above are the result of experimental studies undertaken by Ole Fanger in the 1970s in steady state climate chambers. The graph below is from one such experimental study by Fanger that demonstrates the relationship of surface and air temperatures and air speeds on the perception of comfort. The experiment is a useful tool to help improve our understanding of how energy input into an environment is related to our activity and clothing levels, air and surface temperatures and air movement.
As you can see in the upper right corner of the graph grid, the activity of the test subjects is defined as medium and their clothing is described as “light” (0.5 clo – where clo is an unit of how much insulation clothing provides).
The x-axis displays the air temperature
The y-axis displays the mean radiant temperature (i.e. the average of surrounding surface temperatures)
The level of air movement is shown by the 5 lines sloping from the upper left corner down to the middle of the x-axis. Each of these lines represents the velocity of the air, in 0.1 m/s increments, starting at 0.1 m/s for the leftmost line and ending in 1.5 m/s for the line at the far right.
If surface temperatures are low, then the air temperature will need to increase to maintain the same level of comfort.
For example, look at the red dot in the graph above – shown at the position of 20°C mean radiant temperature and 20°C air temp.
If the surface temperature drops to 15°C, indicated by the yellow dot, then the air temperature will need to increase from 20°C to 23°C (following the diagonal line which shows the velocity of the air flow in this particular example).
As air speed increases (moving from the leftmost to rightmost diagonal lines), both the mean radiant and air temperatures would need to increase to maintain the same level of comfort. If the rate of air movement increases, the average surface temperatures will need to increase to maintain the same air temperature. If the air temperature drops, there will need to be an increase in the average surface temperatures to maintain the same level of comfort.
This study highlights the importance of a close relationship between surface temperatures and air temperature in creating and maintaining comfort.
Creating the condition for even surface temperatures will only be achieved where insulation and air tightness measures have been applied in a robust and consistent manner.
Therefore to improve our comfort we should design to:
- Insulate as uniformly as is reasonably possible, minimising thermal bridges
- Improve the airtightness of our buildings to reduce uncontrolled air movement
- Ensure that the rate of supply and extract of ventilation air in each room is not perceived as a draught
If you have an infra red thermometer (or access to a thermal camera), look around your house for the coldest points (on walls, floors, ceilings, windows, junctions between these elements).
Of course, this is best done at the coldest time of day (early morning or late evening perhaps).
Which cold spots do you think are due to draughts? Which could be draughts behind plasterboard? Which could be missing insulation? Which would be cold bridges? And which are likely to be a combination of these?
5. Keeping warm in winter
It is widely known that colder weather brings an increased risk of health issues (heart attack, stroke, seasonal illnesses) and death. According to Public Health England, each winter (December to March) around 25,000 deaths that are in excess of what it typically expected during the rest of the year and this is largely down to the poor condition of our existing domestic building stock:
“The high prevalence of cold, damp, poorly energy efficient households in the UK is considered one of the main reasons why the UK continues to have higher excess deaths over the winter period when compared with other European countries.”
Public Health England, Minimum Home Temperature Thresholds for Health in Winter – A Systematic Literature Review (http://www.aecb.net/knowledgebase/minimum-home-temperature-thresholds-health-winter-systematic-literature-review/)
Clearly retrofit has a role to play in improving conditions across the UK by providing solutions that bring increased comfort and health, though just how warm is warm enough? Based on a recent literature review and previous work from Public Health England, the recommendation is to provide temperatures of at least 18°C for those who are considered at risk (through illness or age), or slightly higher, up to 21°C.
Public Health England (October 2014) – Recommended indoor temperatures for homes in winter
Heating homes to at least 18°C (65F) in winter is recommended
The 18°C (65F) threshold is particularly important for people 65 years and over or with pre-existing medical conditions; having temperatures slightly above this threshold may be beneficial for health.
The 18°C (65F) threshold also applies to healthy people (1 to 64 years)*; if they are wearing appropriate clothing and are active, they may wish to heat their homes to slightly less than 18°C (65F).
Maintaining the 18°C (65F) threshold overnight may be beneficial to protect the health of those 65 years and over or with pre-existing medical conditions; they should continue to use sufficient bedding, clothing and thermal blankets, or heating aids as appropriate.
Overnight, the 18°C (65F) threshold may be less important for healthy people (1 – 64)* if they have sufficient bedding, clothing and use thermal blankets or heating aids as appropriate.
* Advice is that rooms in which infants sleep should be heated to between 16 to 20°C (61 – 68F).
6. Keeping cool in summer
Although overheating is well documented, it is a subjective experience and not easy to precisely define.
There are around 2,000-7,000 deaths each year that are considered heat-related and “excess” to the number of deaths expected during other times of the year.
In a recent (2015) study on overheating, titled “Impacts of Overheating”, the Zero Carbon Hub (ZCH) undertook a literature review with the aim of providing advice on the point at which overheating occurs. Ultimately, the ZCH concluded that:
“the evidence base is considered to be insufficient to define an indoor overheating threshold for health risk”
Additionally, there is currently no statutory definition for overheating.
In place of clearly defined thresholds, we are left to rely on guidance from documents like
- CIBSE Guide A and
- energy standards such as Passivhaus.
CIBSE Guide A states that temperatures above 25°C (in living spaces) and above 23°C (in bedrooms) significantly increases discomfort amongst occupants. Guide A provides further recommendations: in living spaces, temperatures shouldn’t rise above 28°C for more than 1% of the occupied time; this drops to 26°C for bedrooms.
One of the certification criterion for the Passivhaus standard is to ensure (through modelling in the Passivhaus Planning Program, PHPP) that the average internal temperature doesn’t exceed 25°C for more than 10% of the year.
In terms of building physics and comfort, overheating is not strictly to do with high temperature. For example, if occupants are not able to provide adequate ventilation or are exposed to several days of high temperatures, a combination of the factors below can all contribute to overheating.
Many of these factors can be influenced by the way we build and retrofit our buildings:
- Single days of high temperatures can be problematic though longer heat waves profoundly increase the risk of overheating.
High levels of relative humidity
- The more humid the air, the less effective evaporation / sweating is at cooling us down.
Lack of air movement / insufficient ventilation
- Poorly designed or maintained mechanical ventilation and un-openable or inadequately sized or poorly placed windows restrict the amount of convective cooling available
Little or no shading
- Keeping the heat of a hot summer’s day out of buildings is one of the most effective means of reducing the risk of overheating. Buildings that have little in the way of natural or artificial shading will be at greater risk of overheating.
Thermal capacity of building and surroundings
- Thermal mass (either in buildings or their surroundings) can store heat – helping to reduce peak temperatures, though mass can also be problematic if the heat stored is not fully dissipated (through ventilation) or is released at the wrong times (e.g. in the middle of the night)
Urban heat island effect
- Generally, temperatures can be several degrees warmer in built up, urban areas when compared to suburban and rural locations. This places buildings in cities at increased risk of overheating compared with those in less built-up areas.
Occupant behaviour / health
- If a person is unwell or their body’s ability to regulate their metabolic heat output is compromised, the chances of heat-related illness or death will increase.
Although guidance on preventing overheating primarily focuses on new build, it is accepted that overheating is an issue in existing buildings as well. New building advice to reduce glazing areas is not directly applicable, though the existing glazing areas and orientations in a proposed retrofit may be problematic.
In the next few paragraphs, we look at overheating from the perspective of the CLR Certification system and highlight what designers of prospective CLR Certified projects should consider when addressing overheating.
If little or no shading exists, assess the risk of the proposed glazed areas (are they excessively large and south facing; east or west facing?) and design shading to suit. Remember that overhanging shading works well on south facing elevations and that east and west elevations will require vertical shading elements.
The wider site context is also important to assess as a shading strategy that relies on a row of trees is more risky than one that is designed into the project (because the trees may be removed in time).
If mass is a part of the design, ensure that it is exposed as much as possible to increase its effectiveness. Also consider how the storage and release of heat through ventilation will be managed by the occupants.
Regardless of the general ventilation strategy in use, it is important to work up a natural cross ventilation strategy that can work without compromising security or comfort (in terms of excess noise when the window is open etc.). This is especially important for project with large amounts of thermal mass.
Internal heat gains
Make realistic assumptions for actual heat gains – this includes the number of occupants; type and number of appliances and electrical equipment. Where possible, include the actual internal heat gains (IHGs) in any assessment of overheating risks and don’t forget to pay attention to reducing the IHGs through compact and efficient design of the domestic hot water system.
Other general tips to consider when developing and modelling designs to minimise overheating are listed below:
When using MVHR in summer – be aware that even on bypass, an MVHR may recover up to 25% of the heat in the ventilated air – counter this by reducing the modelled ach by 25%
If using PHPP to model your design, consider using the following “stress test” from Warm: Low Energy Building Practice:
- Minimum user operated summer shading
- Set mechanical ventilation at half the typical rate
- Assume half of the achievable night-time rate of cooling
In PHPP, it is also possible to import projected climate data (as long as it is properly formatted) to assess the future risk of overheating due to climate change.
What opportunities might a retrofit provide to improve thermal comfort and health? (Think about your own home or a project you are working on, if this is useful.)
In your head or on paper, list at least 5 building features or technologies that you would want to include in a retrofit, and how they would enhance comfort and health for the future occupants.
Or if you prefer, list comfort and health situations you would want to prevent in your retrofit, and a very brief indication of how you might do this.
7. An example from a real retrofit
Bringing it all together
Below we have a real-world example of a low-energy retrofit that highlights the impact of thermal bridging and ventilation flow rates on occupants perception of comfort. In the image one can see three different rooms highlighted:
- a bathroom (1),
- a study / home office (2) and
- a living room (3).
Notes on the likely clothing and activity levels are also shown on the image for each room.
(click the image to enlarge)
[edited 27.08.18 by AS]
Thermal bridges: the bathroom in this house works well and is comfortable, but the living room has minor thermal bridges along the base of 3 externally insulated masonry walls, leading to surface temperatures of 17-19°C at the wall –floor edges. Despite the room air temperature often being 22-23°C, and with the entire family in the room, it can occasionally feel ‘cold’ to some of the more ’sensitive or tuned in’ occupants!
Ventilation draughts: The air supply flow rate for this room has been designed to be quite high relative to what is a fairly small room and also the supply terminal fitted was the wrong kind. As a result the terminal does not ‘push’ the supply air (typically at say, 21C) into the room at ceiling level where it can mix with the room air before interacting with occupants. The net result is that the air supply flow can be felt by occupants, compounded by being at a lower temperature than the room air may be – and this can create a feeling of the room being ‘chilly’ at times for some.
Interestingly, despite these issues, one of the owners makes it clear that to guests the room is felt to be very comfortable, whilst the other does not. However, the residents have become attuned to a considerably higher standard of comfort post retrofit and may have developed a lower threshold for environmental conditions that many others would not typically notice.
Clearly there are lessons to be learned here and in other similar situations. Correct design, installation and commissioning of MVHR units is important to ensure flow rates are within acceptable limits. Additionally, it is clear that neglecting thermal bridges in a room that will be occupied a great deal could mean that warm air temperatures may not be enough to ensure comfort. If such an issue were present in a bathroom, it would likely cause discomfort as well as pose a risk for condensation due to high humidity for example after showering.
Optional extra information
8. Ways of defining or measuring comfort
There are different models and indices used to understand and describe thermal comfort. Below are the two most widely known thermal comfort models and related indices:
This model is based on steady-state, climate chamber experimental data. It assumes a fixed internal temperature year-round and is generally applied to buildings with mechanical ventilation.
It uses the following indices:
- Predicted Mean Vote (PMV)
- Predicted Percentage Dissatisfied (PPD)
It takes into account :
- Air speed
- Surface temperatures (aims for consistency across different surfaces – to avoid creation of discomfort through variation in surface temperatures)
- Air temperature stratification
For further detail, see: CIBSE Guide A, Section 1.4
This model is based on hundreds of field studies and assumes that occupants are active participants in managing the environmental controls. There are three categories of adaptation:
Generally this model is applied to buildings that are entirely naturally ventilated.
This lesson has covered:
What makes a building feel thermally comfortable to its occupants?
What steps can a retrofitter take to make a building easy to keep warm in winter and cool in summer?
- WHO Guidelines for Indoor Air Quality: Dampness and Mould http://www.aecb.net/knowledgebase/guidelines-indoor-air-quality-dampness-mould/
- Dampness in Buildings and Health – Nordic Interdisciplinary Review of the Scientific Evidence on Associations between Exposure to ‘‘Dampness’’ in Buildings and Health Effects (NORDDAMP):
- Public Health England, Minimum Home Temperature Thresholds for Health in Winter – A Systematic Literature Review (http://www.aecb.net/knowledgebase/minimum-home-temperature-thresholds-health-winter-systematic-literature-review/)
- advanced level/checklists:
http://iaq.supportportal.com/link/portal/23002/23007/ArticleFolder/974/Indoor-Air-Pollutants (portal disabled)