HPH233 : How to choose a construction system for a house build – with Mike Hardwick
Self build consultant, Mike Hardwick, explains the pros and cons of different construction systems.
Interview with Mike Hardwick
Mike hasn't followed a typical path into the industry. He actually worked in the RAF for 26 years, and with all the different postings came many different house moves for him and his family. Deciding to settle in one place they selected a location (based in the middle of where possible future postings could potentially be), bought a plot of land and built their own home.
Mike often found himself propping up the bar in the Officers' Mess answering questions about how to build your own home. When he came out of the Services in 2005, he realised that what he would really like to do was get more people interested in building their own homes, and at that point decided to make a business from it.
The first series of Grand Designs – copyright Channel 4
Mike's house build coincided with the first series of Grand Designs
They bought the land for their self build in 1999, got planning permission in 2000 and in 2002 were able to move in. Doing the build in parallel with the first episodes of Grand Designs often left them able to empathise with the self-builders, pick up a few tips or wonder why they were doing things a certain way.
Mike credits Grand Designs for redefining the scope of self build, making people realise that it doesn't have to just be for the very wealthy. Although he does concede that the TV programme “Sensible Couple with Reasonable Budget Build Modest House On-time and On-budget with a Competent Builder” is never going to get commissioned!
A build system is the structural element of the house
That is, what it is built from and held up by. “And the confusing bit with a build system is that just because what’s on the outside of a house looks like maybe a brick, it doesn’t necessarily mean that it’s a house made out of bricks structurally.” Most, but not all, houses in the UK are built with a twin-leaf construction, where the inner leaf does the supporting work and the outer leaf makes it look pretty, with timber, brick, tiles etc.
As well as holding up the structure of the house and being the main material used in the construction, it is what the services run through and are bolted to.
As such, getting the right system for you is going to be an important decision to make, and generally it's entirely your choice. Because the building system you use doesn't usually dictate the external appearance, the planners aren't interested. So as long as Building Control are happy that it will comply with regulations then you are free to choose.
Price comparisons aren't straight forward
Mike explains that people will always ask him which system is the most cost effective, but drawing a price comparison is actually quite difficult. He does say that there is a fairly narrow band of cost when it comes to the different build systems, and what influences the price most is actually the detailing and the finishes that go on inside.
What really hikes the price is when you trade in the materials for speed of construction, such as with houses that are pre-fabricated and then erected on site in a matter of days.
Masonry construction
Mortgage companies for example will typically ask whether the house you're buying is of masonry construction with a tiled roof. They like it because it has substance to it, it's a safe thing to invest against, and will be appealing to potential future purchasers.
“It’s the way that we’ve trained our builders, it’s the way we’ve been conditioned to think about how houses work, and it’s what we fall back to. So, they’re easier to insure, they’re easier to mortgage against and they’re easier to sell. Which is why people still do it.”
If you're working to a modest budget, then using brick and block is generally at the lower end of the cost spectrum, so is likely to be a baseline for constructing your own home. They are quick and easy to build, relatively cheap to buy, and are easy to handle and cut up. They tick the boxes for mortgage companies, insurers and builders.
Other positives are the strength they provide, that you can put beam and block floors in. The thermal mass provides a stable temperature throughout the house, and the masonry affords you flexibility of design. You can build just about anything you want in masonry, and combined with steels can give you large, open spans.
Traditional breeze blocks were made out of cement covered cinder. Modern blocks are lightweight and made from pulverised fuel ash, so the detritus from a coal-fired power station mixed with resin and turned into a block. The aerated middle traps air and provides insulation, and also makes them lighter. In the UK there is currently a shortage as there aren't many coal-fired power stations remaining, so the raw materials have to be imported.
Timber frame
Timber frame basically replaces that breeze block element with a structural framework of timber. It is associated with being environmentally friendly, high quality, accurate off-site manufacture. It is efficient, effective and with the speed of construction it makes for an attractive proposition.
Whereas masonry has high embodied carbon in the manufacture, timber frame is carbon neutral in that you plant a tree, chop it down and plant another tree. Timber is also a flexible material in that you can build the walls, ceilings, floors, windows, doors, stairs etc from it.
Mike is keen to point out to people who worry that building a timber frame system might suffer from problems such as termites or rot, that probably around 65% of what we would call a masonry house is actually made from timber anyway, being the roof trusses, stairs, skirtings and so on.
Mike concedes that, if pressed, he would say timber frame is probably the cheapest way of building the structure of the house. But that would only be if you were doing the most economical and basic timber frame with no frills, and without doing anything to attenuate the acoustics. Most self-builders would want to go above and beyond building regulations and put in good sound-deadening insulation to minimise noise transmitting through the house. When taking into account the extra money on the sound-proofing, you are probably then looking at a more expensive path than masonry.
“Now, if you want the environmental stuff but you don’t want a lot of noise transmission, timber frame with noise attenuation is a good thing but it’s going to cost you more than it will for a masonry house straight out of the box. This is where you need to start doing your homework and making your calculations about what you actually want, how you’re going to live your life, and what you’re trying to get from the home that you’re building.”
The thing to remember with timber frame is that it will last well as long as the moisture content in the structure is kept to a minimum, and allowed to escape so that it doesn't build up. On the inside of the house behind the plasterboard there will be a non-breathable, vapour control layer. This stops any moisture occurring inside the property from getting into the structure. On the external wall, underneath the render or brickwork, is a breathable membrane, allowing any moisture that does get inside to find a way out.
Insulated Concrete Formwork (ICF)
These are a type of fire retardant polystyrene foam block. They fit together to create a solid outer wall and a solid inner wall, with the gap in the middle being filled with poured concrete. When the concrete sets and cures, you end up with a waterproof, insulated, monolithic wall which is highly insulated, highly airtight and with the strength and density of a masonry wall. This makes it popular for people building to the Passivhaus standard.
It also benefits from being very quick to construct, you don't need to be too skilled to put it together, and being waterproof is a good choice for flood-prone areas.
Some people aren't keen on this method because of the perceived embodied carbon being quite high. In reality, Mike says, they are mostly made of air with very little actual structure to them. There are other alternatives, such as Durisol blocks which are much the same in principle but use chopped up timber instead of the polystyrene foam as the insulant.
Although there is a lot of concrete used, Mike suggests thinking about the amount of energy that the structure saves straight away with its high levels of airtightness and thermal performance.
It is worth thinking about future use, as putting in more doors or windows for example at a later date is going to be very difficult. Also perhaps a marine ply or a Thermacell type board is advisable for indoors when there might be considerable weight such as cupboards or TVs to be hung from them.
Structurally Insulated Panels (SIPs)
These are manufactured off-site and built of two oriented strand board (OSB) sheets that are filled with expanded urethane insulation foam. The panels can be cut, shaped and fixed to other panels to make a complete housing structure. They tend to be accurate, airtight, watertight and put up on site very quickly.
If you were thinking of using this building method, you would take your designs to a SIPs company whose CAD systems would create the plans to feed into the machines that produce the panels to millimetre perfection. These panels work with the other panels around them, and can even have steelwork in there for covering large spans, to give the structural integrity to the house.
Mike explains how the success of a masonry constructed home is very dependent on the quality and skill of the bricklayer. With ICF it's harder to go wrong, as the panels are designed to fit together at 90 degrees, meaning the walls are going to be straight and it will be easier for fitting out afterwards.
One drawback is that you can't later decide to move things about or add doors or windows because the structural integrity will be compromised.
“The advice I would give to anyone contemplating building in SIPs is to spend a really long time thinking about what you want and what’s going to go into your house, and the design of your house, and then say, ‘that’s it, no more changes. We’re happy with this. Go and provide this house.’ Because once it goes into the computer and they pull the handle, then that’s what you’re going to get. If you suddenly say, ‘about that wall there, can we have another one here or another window here? Can we have that door moved to the left?’ that’s going to be expensive, it’s time consuming, you’ve got to start again and recalculate the whole thing in the worst cases.”
In terms of cost, Mike says ICF and SIPs used to have roughly a 10% premium on them because there were new factories to be paid for, but as they are becoming more common-place the prices are more competitive. Some manufacturers are even assuring that they can produce a house for about the same cost as brick and block.
Straw bale
Previously seen as something you wouldn't consider for mainstream building, nowadays the increased focus on energy efficiency, natural materials and environmentally conscious building has given straw a greater appeal. Mike explains that structural straw bales, as opposed to ones you would buy direct from a farmer, are processed and compacted to form dense blocks.
“When you encapsulate this within a lime water and plaster that you would normally be using with this build, you’ve got something which has got no oxygen. So, it doesn’t catch fire. It doesn’t. It’s just a really good carbon neutral method, environmentally friendly way of getting all that insulated wall which looks really good as well.”
Straw bales do mean that you end up with thick walls, but for some people that is a positive feature. The deep window and door reveals combined with the lime plaster make it look and feel like a solid house with a natural, rustic feel.
Mike concedes that some mainstream lenders and insurance companies might not be keen to deal with these, but others like the Ecology Building Society see the benefits and will lend against them.
Prefabricated system builds
Most of these factories tend to be in continental Europe where the prefabricated kit is made and then brought to your plot to be assembled. This method probably won't suit those on a smaller budget but for projects on a grander scale, or at least a grander budget, they will be rewarded with superb, quality finishes.
“And for those people who want to be hands-off with the self-build rather than actually getting involved and really worrying about how the bricks go and how the timber frame is constructed, they can sit back and basically say, ‘here’s the house I want. Build me one of those. Here’s where I want it.’ And somebody else goes and does it for you. You pay for the privilege of doing that, but the stress levels are going to be a lot lower than somebody who’s actually project managing their own build all the way through.”
Again, this is a system where decisions need to be made up front as any changes later are going to be extremely costly.
Steel frame
A steel frame self build house wouldn't be commonplace in the UK. They tend to be more popular in warmer climates. If you have a steel structured house and are trying to stop cold-bridging then you're starting from a losing position.
However, recent changes in planning regulations, where permitted development allows people to take farm buildings and convert them into residential dwellings with less of the hassle, has made extensions and renovations of steel-framed farm buildings more common.
Thin joint
The masonry industry has recognised some of the problems of building with blocks (weather dependent, speed of construction, reliant on quality of the builder) and come up with a variation on the masonry theme called Thin Join. They use over-size versions of the cinder blocks and instead of using a 10mm sand and cement mortar base, they are held together with a type of glue adhesive.
“But by taking out things like the mortar bed, you’re taking out two things. One is a passage for air to get through, and when air can get through, noise can get through. So, by taking that mortar bed out, you’re putting better airtightness into it, putting more insulation into the wall and you’re taking away some of the need for skilled construction to put it together”.
It's also a cleaner system, using a bucket of adhesive instead of having piles of sand and cement spreading around the site.
Which system to use depends on what is right for your circumstances
“So, if low cost is your priority then you need to be looking at the traditional masonry or the timber frame setup. If you’ve got family and you want a bit of peace and quiet, you want some separation of noise, acoustic attenuation, then the masonry end is probably the way you want to be looking at.
If you want energy efficient, you’re environmentally aware, then you want high efficiency and low running costs. Then timber frame is probably where I’d be looking at. That’s at the lower end of the spectrum on there.
And as you go up the performance scale, that’s when I’d start looking at the ICFs and the SIPs panels. They might cost you a little bit more to build but again, there’s a trade-off there against the cost of the materials and you need less labour to put it all together.
At the very top end, if you really are a hands-off builder and you want the sort of quality which just comes with that continental standard, then these system builds that we spoke about, hand over the money and let these guys come and build it. You’ll have a house which will be good enough to feature in the magazines. But you will pay for that privilege.”
Additional resources
National Self Build & Renovation Centre
Transcript
Download a transcript of the interview with Mike Hardwick.
