Building with BLOCKS
Masonry is working hard to shake off its image as a traditional walling system — with plenty of innovation and choice, says Jason Orme
According to the most recent survey – of 500 recently completed projects for Homebuilding & Renovating’s Market
Report – 46% of self-builders chose blocks as the primary construction system for their new home. That’s significantly lower than the wider housebuilding sector, where something like 75% of all new homes are built with blocks. We’re not here to work out why that is, but what it does suggest is that there is a greater propensity for self-builders to avoid masonry than housing developers. So what’s going on? And what are the big masonry suppliers doing to make their systems appealing in an age where self-builders seem increasingly willing to explore the options of timber frame, ICFs (insulating concrete formwork), SIPs (structural insulated panels) and more besides?
Let’s get one thing clear: there is undoubtedly a bit of a perception among some self-builders that masonry is a slow-moving behemoth, constantly outmanoeuvred when it comes to competing on the factors that matter (energy performance, speed of construction, and increasingly cost). In part, that’s because many suppliers selling alternative systems are pretty good at promoting themselves — something that masonry just hasn’t felt the need to, at least in recent years. Look around any Homebuilding & Renovating Show and you won’t see many people selling the benefits of blockwork.
So, as a self-builder or renovator looking to make the right decision for the long-term, what do you need to know? How can masonry stand up against the new systems and what does it have to offer? There has been a significant amount of variety brought to market over the past couple of decades, with a series of innovations that now give self-builders a real choice.
lightweight and aircrete blocks. All of the key differentials (as detailed below) are impacted by this difference, so understanding what these main block types look like and how they’re made is essential.
Dense blocks (also referred to by homeowners as ‘breeze blocks’) are the blocks that you might have seen on building sites for years — they tend to be a bit darker in colour than the alternatives. They are a simple form of building system, made with a mix of Portland cement, sand and fine gravel fired into shape. They’re the industry standard and if you don’t make any active choice, this is probably what you’ll get. They tend to come in the classic 440mm x 215mm format and they aren’t hugely high performing from an energy perspective, (with thermal conductivity of up to 1.33W/m2/K). However, they are cheaper than the alternatives, and highly durable. They are very strong (dense blocks measure from 2.9N/mm2 to 40N/mm2) and are best in class for thermal mass.
Lightweight blocks, mixing an expanding aggregate into the concrete mix, are a middle ground between dense blocks and the ‘new’ kid in town (if you call something invented in 1920 ‘new’) aerated blocks, known universally as aircrete.
Aircrete consists of a PFA (pulverised fuel ash) aggregate block with air bubbles, formed after a reaction of aluminium powder and lime. The steam curing process gives the blocks a lot of strength (not as strong as concrete blocks, but still up to 9N/mm2), which means they can be used for loadbearing walls. There are three big selling points, however:
l Easier handling — they are lighter, so are quicker to work with and easier to cut.
l Energy performance — the air bubbles provide a good insulative performance, with thermal conductivity of up to 0.11/m2/K.
l As a result of the lighter weights, blocks can be manufactured in larger formats, which means quicker laying times. This is especially true when combined with a thinjoint masonry system (more on which later).
has decades of experience in terms of what blocks to specify and where. “Because we tend to specify a concrete beam and block upper floor, and might well include some steelwork, our standard specification is for a 7N/mm2 aircrete internal skin. It gives you a bit more leeway from a structural point of view. Externally, of course, our clients can choose brickwork, but if they want a render finish we tend to avoid aerated blocks on the external skin as they can suck up the moisture from traditional renders, although that’s less of a problem with the new system renders.”
One thing to bear in mind is the old building industry adage of ensuring that the block is always stronger than the mortar around it. The danger is when bricklayers go off piste in the mortar mix, so it’s worth ensuring they are following the manufacturer’s recommended guidelines for the mix — it’s different depending on the type of blockwork you choose to use.
If you’ve ever tried to lift one, you’ll know the traditional dense concrete block was, as its name suggested, pretty heavy. The Health and Safety Executive (HSE) directs that the regular lifting of blocks that weigh above 20kg is potentially a risk, particularly at standard frequencies of 2030 an hour. Some of the heavier, denser specialist blocks that are occasionally specified for foundations tend to exceed this (typically 20-25kg) but most of the regular-format dense blocks tend to weigh in between 13-16kg at the standard 100mm thickness. The aerated blocks tend to come in at around half of this weight on comparative formats.
Don’t forget, useability is about more than carrying weight. Blocks will occasionally need cutting and, to put it simply, that’s much easier with the aerated blocks than the dense ones. It shouldn’t be the main factor in making a decision, but it is worth considering, particularly if the bricklayer has been used to aerated blocks for a while.
“I would expect an experienced bricklayer to be throwing down 30-35 blocks an hour if they had a labourer helping out with mixing,” says Dan Mutti from D&M Homes. Using a typical format at 10 blocks/m2, that can mean a typical day might see 20-25m2 of blockwork walling going up.
In terms of cost differentials between aerated and traditional blocks, Dan, who has been involved in the specifying and purchasing of blocks for over 20 years, allows an extra £5-£8/m2 (depending on
the site location) for the specification of aerated. Labour costs are, of course, identical.
With high energy performance standards required of our houses – standards which most self-builders tend to want to exceed – walls are expected to do much more than hold the house up. The walls are also on the front line when it comes to reducing heat loss, and in this case the aerated block has a significant performance advantage. One of the key insulative tactics is to introduce air into the layer, and as such, aerated blocks, which consist of thousands of tiny air pockets, outperform dense blocks by a factor of up to 10.
The key to performance is allowing a greater cavity for insulation, says Dan Mutti. “We’re achieving a standard U value of 0.15 on our system, which is simply an external skin (usually brick), a 150mm cavity fully filled with fibreglass insulation, and an internal 100mm aerated block. If we want to improve (reduce) the U value to 0.1, we change the insulation to something like Xtratherm’s CavityTherm. It’s that simple. One of the other reasons we like masonry for energy performance is that there is no studwork, so you get a completely uninterrupted layer of insulation around the wall.”
U value performance of wall systems is the main aspect of overall energy performance, but air leakage is another major factor in achieving high levels of efficiency – and of course, it is now
measured under the Building Regulations with a minimum requirement of 10m3/h/ m2. Panellised timber construction has traditionally been heavily favoured in this regard, with the factory-controlled nature
of the construction considered to be a huge
advantage — as well as carefully detailed junctions and joints.
Masonry blockwork doesn’t enjoy these natural advantages, and so it has been working hard in recent years to improve habits. Tom De Saulles, building physicist at The Concrete Centre and executive director at Modern Masonry, explains: “The average air permeability of new concrete and masonry dwellings has steadily decreased in recent years, a consequence of knowledge gained since the introduction of mandatory pressure testing, which has fed back into the design and construction process. An air permeability rate of around 5m3/h/m2 is now fairly standard, with lower values easily achieved when required.
“In fact, pressure testing statistics have shown that, of the housing achieving an air permeability of 3m3/h/m2 or better, the highest proportion were of masonry construction with a wet plaster finish. A highly effective alternative is to parge the blockwork inner leaf prior to drylining the wall. Proprietary products are also available such as Blowerproof polymer liquid that can be spray applied or brushed on to create a flexible sealing membrane behind plasterboard and other interior finishes.”