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Self Build - The Process & Considerations

Welcome to the replay for our webinar, Self Build: The Process & The Considerations. The presentation lasts for approximately 30 minutes, and is followed by a live Zoom Q & A session with the audience, hosted by our Managing Director, Keira Proctor.

The webinar covers the following topics:

  • Introduction to building physics
  • Building regulations & guidance
  • Types of building structure
  • Construction membrane types & functions
  • Controlling condensation in roofs
  • Effects of Airtightness on Energy Performance

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Transcript

Today we're going to take a look at self-building, and cover some of the basics of construction practices and terminology across the entire building envelope. I completed my own self build project two years ago, and even working in the industry, there's still a good learning curve to negotiate undertaking any building project yourself.

So we're going to start today by looking into the various ways you can undertake a self-build project, and how hands on it's necessary to be as a client across these approaches.

We'll then discuss some of the factors involved in the building warrant processes, and introduce some of the basic building physics involved in houses.

From there we'll go through the anatomy of a house from foundations to roofing, and look at some of the key decisions required at each stage and what the implications of those are.

Types of self-build

There are as many different ways to self-build as there are self-builders, so throughout today's presentation we'll have generalise a bit, and aspects such as financing, plot choice and planning are very individual, so a little outside our scope.

We'll therefore start with how to manage the delivery of the project, and here there are a few options.

The least common option, but by far the simplest is appoint a design and build contractor, who will undertake the entire process from start to finish. This simplifies the process by limiting the processes the client has to undertake, but can have higher costs.

In theory however these costs are less likely to vary, and any issues that arise along the way are the contractors problem to deal with.

Most self-builders however want to have a greater degree of involvement in the process, as this is often a large part of the reason to undertake a self-build project in the first place.

In the majority of self-build homes therefore, clients take on at least part of the project management process, co-coordinating the design and build process themselves in partnership with architects and contractors.

Within this, there's a range of ways to manage the project. At the "most involved" end, clients manage the entire process dealing with planning, building control and architects on the design side, then appointing and managing individual trades directly.

This fully hands on approach is perhaps best undertaken by people already familiar with the construction industry, but offers the maximum possible flexibility in the process. For those unfamiliar with the process however, it can easily get out of control leading to higher costs and longer timescales.

A common middle ground approach therefore is to work with an architect on design and compliance, then appoint a main contractor to manage and deliver the project on site.

This does make it more difficult and expensive to changes specification and alter the project, but streamlines things a great deal as those delivering the project are familiar with the processes, regulations and technologies involved.

Finally, there is the option of a "kit house" produced by a timber frame or structural panel manufacturer. These are off the shelf design that can be supplied and erected on site with minimal modification.

Typically, these are supplied as a wind and watertight shell and will usually have various standard options to meet building regulations.

While this approach gives a degree of flexibility over aesthetics, it doesn't always allow a lot of scope to alter the fundamental layout of the home.

Balancing the flexibility given by managing the entire project against the time and expertise needed to do this effectively is the key to a successful self-build project.

Building Regulations

The building regulations exist to ensure not only that buildings are safe and robust, but also that the build environment contributes to wider policy objectives such as energy efficiency.

There are three sets of similar, but subtly different regulations across the UK covering England & Wales, Scotland and Northern Ireland. While some of the specifics vary, the general thrust of the regulation is similar, and they all reference a lot of the same research and guidance.

In the Republic of Ireland, the building standards also share some of this guidance, so the general principles apply there too.

Today we're going to be mainly considering the sections of regulations relating to energy performance and moisture control.

Energy performance, which includes both insulation levels and airtightness are address in Part L of the England and Wales regulations, Section 6 in Scotland, Part F in Northern Ireland, and different Part L in the republic of Ireland.

In all four cases, the energy use of the building is assessed holistically, with all factors such as air leakage, insulation levels heating efficiency considered together.

In all areas of the UK this is done via a SAP, or Standard Assessment Procedure calculation. In the Republic of Ireland a different process called the Dwelling Energy Assessment Procedure is used, but the concept is similar.

While the same procedure is used across the UK, there are some important differences in how the results are applied and what limits are set in certain areas, so a building or building type designed to Part L regulation may not comply with Section 6 and vice versa.

It's important therefore when choosing designers and suppliers that they are familiar with regulations in the specific part of the UK where the project is located.

Condensation and moisture control are covered by Part C for England and Wales, Section 3 in Scotland and Part F in the Republic of Ireland. Northern Ireland is also part C, but it's a different document from that applying to England and Wales.

As regards controlling condensation, the relevant clauses in all these regulations refer the specifier to BS5250, the Code of Practice for the Control of Condensation in buildings. This means that wherever in the UK or Ireland a building is, the applicable design principles in terms of minimising the effects of condensation are more or less the same.

Energy Performance & Heat/Air/Moisture

So we've seen the building regulations revolve around keeping buildings safe, warm and dry.

To put this into context it's important to understand some basic building physics relating to heat, air and moisture movement, starting with heat transfer.v

In terms of energy performance, managing heat transfer is the most obvious consideration in our design process.

There are three basic modes of heat transfer that are present in all buildings to some extent; conduction, convection and radiation.

The simplest way to illustrate all three, is heating a pan of water on a hob.

Conduction

Conduction happens where two objects are in contact with each other, like the base of the pan sitting on the hotplate. This direct contact lets the heat flow from the warmer hotplate to the colder pan, until either the temperatures equalise, or the pan is removed.

Most building insulation boards such as mineral fibre, or polyurethane, work to resist heat transfer by conduction.

Convection

Convection occurs when heat is transferred by a fluid medium, which in buildings is usually air, but in our example here, is the water in the pan.

As the water at the bottom of the pan heats up, its density reduces, and the warmer water will rise relative to the surrounding cooler water. The cooler water then sinks to take its place, setting up a circulation of water, known as a convection current.

As the temperature of the water becomes more uniform, the speed of this current will reduce.

In buildings, convective heat transfer can be minimised, by ensuring good air tightness is achieved, restricting this circulation from happening.

Radiation

Radiation is the transfer of heat from one material to another, which occurs by means of electro-magnetic waves.

Radiation does not require either physical contact of the materials, or a fluid medium. A warm surface such as our hotplate will emit infra-red radiation, which can travel across a space, even in a vacuum. When these Infra-red waves strike another object, they are converted back into heat.

The efficiency of this conversion process is governed by a property called surface emissivity. The lower this value, the less able a material is to absorb, or emit heat via radiation.

In construction, reflective foil membranes and foil faced insulation materials, can help limit heat transfer by radiation.

U-Values

To quantify the heat loss through parts of a building, we typically talk about u-values. These are derived from the thermal conductivity of each material in the elements, along with its thickness and a number of correction factors. We've gone into this in a lot more detail in previous webinars, so if it's of interest you can go back and take a look at those to learn more.

For now though, what we need to understand is that u-value measure the RATE at which heat flows through the elements, so the lower the u-value, the better the insulation performance.

The more insulation we add to a construction, the more we reduce the u-value, but this is not a linear process. As u-value get lower, the amount of insulation we need to add to improve them further increases exponentially, so it's not always practical to just keep piling in insulation.

We therefore need to think about what else we can do to optimise the energy performance of the building.

Cold Bridging

The first thing we can do is limit what's called cold bridging. This occurs where there is a discontinuity in the insulation layers, typically at structural members or junctions between walls, floors and roofs.

Thermal bridges have two principle negative effects. Firstly, they increase the overall heat loss by providing an easier pathway for heat to escape, and secondly they create cold spots internally. Condensation and mould growth can occur in these cold spots, which has a detrimental effect on the indoor environment.

The first of these, typically where structural elements like studs in a timber frame wall intersect the insulation is accounted for via an adjustment in the u-value calculation.

If we consider a 140mm thick panel of rigid foam insulation, if we used this in a typical timber frame wall, the u-value when adjusted to account for cold bridging at the timber stud will work out to around 0.22.

On the other hand, if we eliminated this cold bridging but using the same thickness of insulation, the u-value drops to 0.15, purely do to the lack of thermal bridging at the studs.

Thermal bridging at corners and junctions is accounted for using an additional value for linear thermal bridging, knows as a psi value, but in most cases, these are addressed using standardised pre-assessed details to limit heat loss.

These linear heat losses are combined and summarised into into a "Y-value" which is used in SAP assessments.

The easiest way to visualise the way thermal bridge free design works in practice is to simple imagine drawing a pencil line round a cross section of the entire heated envelope of the building.

Maintaining the continuity of insulation can pose challenges in some areas, such as window reveals, where space may be extremely limited. In these areas thin insulation systems like our spacetherm aerogel boards can help ensure this continuity without sacrificing thermal performance.

Airtightness

The other way we can optimise our energy performance is by paying careful attention to the airtightness of the building. By this we don't mean sealing up the inside entirely, but simply ensuring that the only air movement is air movement we can control via ventilation systems

Both the wind blowing, and heat rising by convection internally can drive air movement, and this can dramatically impact the overall heat loss, especially as levels of insulation increase.

Reducing this air movement is simple on paper, but can prove difficult in practice.

Traditionally air barrier membranes have been installed internally, on the warm side of insulation. Typically, these also function as vapour control layers to limit the ingress of moisture.

This positioning requires the air barrier to be sealed at all service, and structural penetrations, and remain sealed. If the membrane is damaged, either during installation, or by following trades or later refurbishments for example, then both air leakage and moisture vapour ingress problems can arise.

By moving the air barrier to the outside of the building, external systems such as our Wraptite, allow for an almost penetration-free airtight layer, which can be installed faster and more robustly.

It's important here to note that what we're talking about here is unplanned air movement. Planned air movement, provided by ventilation systems, is an important part of maintaining healthy air quality, so when we talk about air tightness, we really mean unintentional air openings and draughts.

If we can rely on achieving a good air leakage result, then that can be fed into the SAP assessment calculation, meaning this good performance can be traded off against, for example, thicker insulation or more expensive and harder to source windows.

For this to work though we need to rely on those design values, and system like Wraptite can help ensure the building meets those targets once it's finished on site.

In the case of my house here, the use of Wraptite externally delivered an air leakage rate of 0.64 m3/m2/hr, well below the Section 6 maximum recommended value of 7, and in line with best practice. This performance helps deliver a carbon dioxide emission rate over 30% better than required.

This also gives EPC ratings of 94 for Energy efficiency and 93 for Environmental impact.

Moisture

Controlling and managing the moisture flow in a building is fundamental to maintaining the durability of the building envelope, maximising energy efficiency and protecting the health and safety of the occupants.

There are a number of potential moisture sources affecting all building envelopes, the most obvious being the external environment, weather and rain.

Weather

While buildings are designed to be weather resistant when they're completed, they can still pick up a lot of moisture during construction.

The longer the internal fabric of the building is exposed during the build process and the more wet materials like concrete and mortar are used, the higher this moisture load will become.

Once the building is completed, weather-tight and heated, this moisture can begin to escape, and this initial phase is known as the drying out period. During this period this excess moisture must be able to escape without causing problems, so it's important a building is designed to accommodate this additional moisture transfer, over and above what might be expected when it's in use.

If this moisture cannot escape, it may lead to condensation problems in the building.

Condensation

Condensation is the formation of liquid water when warm humid air comes into contact with a cold surface.

The warmer the air in an environment is, the more water vapour it can retain. However, when this moisture laden air cools down it's capacity to retain water vapour reduces and this vapour will turn back into liquid.

This moisture can come from a range of sources like cooking, drying clothes, even form building occupants breathing.

The most common example of this is bathroom mirrors, where the warm air in the bathroom can support a lot of moisture, from, for example a shower, but when it hits the cold surface of the mirror it loses this ability, causing the mirror to "fog up".

The insulation in the building fabric help keeps the indoor temperature more or less constant, but as we move outward through the building element, the temperature will drop sharply across the insulation layer. As the temperature drops the air loses its ability to retain water vapour.

At a certain temperature called the "dew point" this air loses its ability to retain the moisture, and this moisture condenses.

In our bathroom mirror example this is inconvenient but pretty harmless, but in say, a roof-space, this liquid can cause timber to decay or damage any stored items.

Cold spots in the building either from poor insulation, cold bridging or draughts can also cause condensation in the living spaces, and this in turn leads to mould growth which can have adverse health effects on the occupants.

Taking these factors together, it's very important that the management of heat, air and moisture in the building is carefully balanced and the impacts of these are closely inter related.

We ran a webinar "introduction to building physics" a few weeks ago covering this in a lot more detail, so if it's of interest you can go back and check that out on our website, or YouTube channel.

So having looked at the regulations and physics, it's now time to think about the building itself, beginning at the very bottom.

Groundworks

The first part of work on site is the groundworks and foundations of the building. Groundworks covers the preparation of subsoil's and dealing with any contaminants, while the foundations are what supports the actual building structure itself.

Preparing the basic site is important, as not only does ground require to be levelled and cleared or vegetation or debris, but some sites can harbour contaminants, particularly if the site has previously been used for purposes other than housing, like former industrial sites.

As well as determining the load bearing capacity of the soil and rock on a site, care also has to be taken that the eventual structure is kept free of damp and water ingress from the soil, so an understanding of the water table on site is important also.

Generally speaking, these issues can be determined by conducting what is called a site investigation report, which looks at the underlying geology of the site, and any historical uses. It may also include borehole reading to measure the presence of any gasses.

There are four main contaminants we have to consider when assessing the measures necessary, Methane, Radon, Carbon Dioxide and VOCs, or volatile organic compounds.

Methane is a naturally occurring gas caused by the decomposition of organic materials, and it's most commonly associated with former landfill sites. If it's not correctly managed methane can cause headaches, dizziness and respiratory issues, but is most commonly associated with being explosive.

Carbon Dioxide or CO2 is an asphyxiant, and typically originates in former mine workings, where pockets of case can collect and be released by changes in ground water levels or atmospheric pressure.

Volatile Organic Compounds are things like hydrocarbon fuels or industrial chemicals, which may leach into soils and emit a range of harmful gasses.

These three types of contaminant are usually related to historical site usage, so its relatively straightforward to identify if they might be present by studying the site's history. If any former uses may cause a problem, boreholes and monitoring can be used to identify any contaminants, and classify the risk profile of the site.

Radon is a little different in that it's associated with underlying site geology rather than historical usage. The necessity of testing for radon is therefore determined by radon maps, which detail where in the country radon releasing geology is likely to be present.

Once the contaminants are identified and the associated risk quantified, remedial measure can be specified. At lower levels of contamination, a simple barrier membrane is sufficient, but at higher levels passive or active ventilation systems may be needed.

It can also be possible to remove or chemically neutralise some forms of contaminants, an approach mostly taken with VOCs, such as for example on the site of a former petrol station where fuel contaminated soil can be removed.

While it's not all that common to come across these types of sites in self build, a membrane under the floor is still necessary to prevent damp and moisture rising through the foundations, this is called a damp proof membrane or DPM. Most quality DPMs will also provide a degree of gas protection at low levels of contamination.

Foundations

This is the foundations of my own house, where the DPM is being laid under the floor slab. This protects against moisture from the ground, as basic concrete is not necessarily waterproof. This DPM can be upgraded with foils or specialist films if required to deal with any contaminants on site, but in this case that wasn't required.

Here, the floor slab is the poured on top if this membrane, and this type of foundation is known as a ground bearing in-situ slab as the slab is fully in contact with the ground and is poured into place directly.

Another type of floor is a suspended slab, where beams are laid onto the supporting walls and blocks or precast units are used to infill to give a solid surface. A thin layer of concrete, called a screed is then used to provide a finished floor surface. Suspended floors can also be made up of timber joists and flooring.

Suspended floors give a subfloor void which can be useful for service runs or providing ventilation, which can be important on sites with a gas protection requirement. If the floor is a solid ground bearing slab, it may be necessary to introduce an additional void forming layer to provide ventilation. In suspended floors however, vents can simply be introduced at the perimeters to permit airflow through this space.

Which type of foundation is used is determined by a range of factors such as geology, and the loadings from the building itself, the construction of which we'll look at now.

Superstructure

The parts of the building above the foundations are referred to as the superstructure of the building.

In "traditional" build the upper parts of the house are made from brickwork and concrete blocks, with floors either being beams and blocks, or timber joists.

Homes of this type are constructed folly on site, and this can be a relatively slow process as brickwork and blockwork takes time to lay, and concrete and mortar have to dry out as harden before additional works can be undertaken.

As we discussed earlier this slower process and higher level of wet trades means moisture loads may be a lot higher, and must be carefully managed.

Traditional build is however very good in terms of thermal mass, which is the building capacity to store heat. This means it takes this type of building longer to respond to changes int he heating or cooling conditions.

If the structure is heated to a constant temperature continuously, this can be of benefit to it's energy efficiency, but if not, it can mean the building takes longer to warm up, or cool down.

Timber Frame

Another type of structure used is timber frame, and this is the most common method used int he construction of kit homes. Homes of this type use timber panels and floor cassettes constructed in factories, which are delivered to site and then can be erected very quickly.

This gives a basic wind and watertight structure comparatively quickly, and the building can then be fitted out with services and finishes.

Things like insulation are usually added after the building structure is up, in what is called "open panel" construction, but sometimes the insulation is factory fitted too, in a "closed panel" frame.

The studs in this type of panel are the main loadbearing members, transmitting vertical structural loads, which the sheathing boards prevent the studs from twisting and moving.

These studs can however act as cold bridges, so panels like this tend to be thicker as this effect must be counteracted.

The main advantage to this kind of construction is the ready availability of suppliers and contractors, giving a competitive market and wide choice of systems.

SIPs

The last of the most common methods of housebuilding is a structural insulated panel, which is the type of construction my house used. SIP homes can be more or less any size and shape, with the panel design and layout adjusted accordingly to give a flexible and highly efficient method of construction.

A typical insulated panel comprises timber sheets, usually OSB, sandwiching a core of rigid foam insulation.

Because the panels are not bridged by any less-insulating structural material such as studwork, they can match or exceed the thermal performance of traditional timber frame walls with substantially less thickness.

There is however scope for problems to arise if project requirements are not fixed and tolerances on site are not achieved. SIP homes must be designed and manufactured well in advance of their arrival on site, and it's very difficult o account for any changes during the build process.

SIP systems also rely on the use of blown foams and manufactured timber products, so may not always align with project requirements if a more ecologically focused build method is required.

SIP systems are typically built by specialist contractors, in the case of my house here, by SIP Scotland, utilising Kingpan TEK panels to achieve both very low u-values, and an almost completely thermal bridge free design.

The thermal bridging heat loss, or Y-value was only 0.029, dramatically reduced from the 0.15 "standard" value.

The roof and wall panels here achieved u-values of 0.14 and 0.17 w/m2K respectively, which combined with the very low air leakage rates helped deliver the EPC A-ratings.

The thermal bridging heat loss, or Y-value was only 0.029, dramatically reduced from the 0.15 "standard" value.

Construction Membranes

Timber frame and SIP constructions typically use a range different membrane to protect and enhance the performance of the base constructions. Usually there is a membrane fitted either side of the wall, a vapour permeable or "breather" membrane on the cold side of the wall and a vapour control layer on the warm internal side.

The basic function of a breather membrane is to keep the timber in the structure dry during transportation to site and until the final weather protection layers of the building are added.

This not only reduces the moisture loads on the building, but helps ensure effective drying can take place later on.

This can be thought of like a Gore-Tex jacket for the building, it keeps the rain out while allowing vapour from the building to escape and dry out. Our Frameshield membrane fulfills this purpose, and has been used in timber frame for over 30 years.

The basic membrane can be upgraded to perform various other functions however.

The first of these is to add a reflective layer to the membrane to improve the level of insulation in the building. This reflective, or "low emissivity" layer restricts the flow of heat energy by radiation, and enhances the u-value of the wall element. Our Reflectashield-TF membrane is the highest performing such membrane available today.

The second of these makes the membrane airtight, which reduces the buildings air leakage rate, giving further enhancements to energy performance. Our Wraptite membrane, which combines this airtightness with a simple and fast self adhesive installation process, make this possible my sealing the building from the outside, well away from any switches, pipes or cables which might otherwise have to pass through the airtight layer.

On the other side of the wall, vapour control layer membranes help limit the ingress of vapour from the warmer and more humid areas to the colder outer parts of the wall, reducing the risk that condensation will occur.

A basic vapour control layer such as our range of Procheck membranes will perform this function while also, provided it's fully sealed, limiting air movement. As with vapour permeable membranes however, VCLs can be upgraded in various ways.

Our Reflectatherm Plus material adds a reflective coating, similar to that on the Reflectashield-TF,

in order to boost the thermal performance of the wall. To do this effectively it also requires an airspace adjacent to the membrane, and this space also provides a zone to run services, helping to reduce the amount of sealing required around sockets, switches and other building services.

The final type of vapour control layer is a variable resistance VCL, sometimes called an intelligent membrane, such as our Procheck Adapt. This material varies it's resistance according to the environmental conditions at any given time.

This membrane becomes more resistant to vapour in winter when the condensation risk is highest, and less resistant tin summer, which allows the warmer external temperature to dry out the building fabric effectively, offsetting any buildup of moisture that may have occurred over the winter.

My house used both Wraptite externally, and Reflectatherm Plus on the inside to limit the air leakage are far as possible. This not only assisted with meeting the targets in the building regulations for overall energy performance, but also ensured the mechanical ventilation and heat recovery systems was able to operate optimally, ensuring good indoor air quality while minimise space heating costs.

Roofing

The final part of the building envelope we must address is the roof, and there are several common types of roof, and even more factors to consider when designing one.

Flat roofs, despite the name are not strictly flat, but at pitched at 10 degrees or lower. The must have some degree of pitch in order to ensure rainwater drains adequately.

Most flat roofs today are of warm deck construction, which is where the insulation and waterproofing sits over the structure. The roof joists and structural decking are therefore kept well above the dew point temperature making this kind of roof far less prone to condensation issues.

Because the insulation is usually unbridged, they also maximise the thermal insulation performance.

Although flat roofs and low-pitched roofs are a common feature of contemporary designs, more commonly homes will have pitched roofs, and these fall loosely into two types.

Warm pitched roofs, in which the insulation follows the slope of the roof itself, and cold pitched roofs, where the insulation rests on the horizontal ceiling, with a cold loft space above.

Traditional UK practice has been to follow the latter model, but it's becoming increasing common to use warm roof construction.

The common structural elements of a pitched roof consist of trusses or rafters, over which are laid an underlay membrane, then battens and tiles or slates as the final waterproofing layer.

Some types of roof, and most roofs in Scotland, may also include a sarking board over the rafters, with the membrane laid on top of this. This stiffens the structure making it more resistant to wind loads, and also facilitate the traditional Scottish roof practice of fixing slates directly to the sarking boards.

If battens are used on the roof, there must be a gap to allow drainage under the tiling battens. This is achieved by either draping the underlay between the rafters, or by using a counterbatten to lift the battens up.

Whichever type of construction is used, ensuring the roof remains free of condensation is critical to long-lasting trouble-free roof.

Historically this was achieved by adding ventilation to the roof, to flush out the moisture before condensation could occur, but the is increasing being replaced by vapour permeable underlay membranes.

These membranes allow moisture to escape through the entire surface area of the roof, meaning there's less chance of ventilation opening being blocked, or incorrectly fitted and simplifying the overall design.

In a warm roof construction, our Wraptite membrane is ideal to ensure vapour can escape which limiting air leakage. The roofing membrane can simple be wrapped around onto the walls and sealed to provide an effective air barrier around the entire building.

In cold roofs, our Roofshield has a proven record of over 25 years in keeping roofs trouble free by not only allowing a high level of moisture transfer, but permitting air movement through the unheated loft space, making the formation of condensation almost impossible under normal conditions.

SUMMARY

So hopefully that's given a good overview of the basics design considerations in building a house, and there's a lot more detail on all of these factors available in our webinar series at www.proctorgroup.com or here on our YouTube channel.

Even being involved in the industry, building my own house was a learning process as to how all the systems and trades involved interact, and how best to manage to the processes.

Developing both a good understanding of the issues and compromises inherent in the construction projects, and a good relationship with all the parties involved in project delivery, really is the key to self-build success.

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