Building Air Pressure Testing and MVHR Fitting

Case Study: 89 Culford Road

My sister and her partner asked me to be the builder for their no compromise refurbishment of a Georgian terraced house in Hackney London. As a result I was asked to write a “blog” of the construction. This was then published on a “zero-champion builder” e-zine along with articles by the architect and the clients all of which seems to have disappeared but here is a transcript of my entries:

The site blog: builder’s side of the story.

Around November/December 2007 my sister, Bronwen, and her partner Rob bought 89 Culford Road, and I was told I was going to do the building work on the house — once they had appointed an architect, decided just what they wanted to do with the house, decided how far to take the green refurbishment and sorted out the planning permission. In due time and without any real great haste as far as I could gather Bob Prewett was appointed as the architect and Bron and Rob instructed him on a “no compromise” refurbishment.

Sitting on the sidelines, getting the occasional update from my sister and from my 30+ year’s experience of building works in and around London I knew that the proposed plans were never go to be an easy planning application to get approved by Hackney (or any other council) and that the De Beauvoir owners’ association would object to a change of even the letter box. Also I did find “the clients’” slowness of decision making worrying for future “build decisions” and their surprise at the missed deadlines by Hackney a little perplexing. So I did not drop all my other work in readiness for the refurbishment — but I did worry about having to tell my sister that there would be a month of no work whilst I went off to row a raft down the Grand Canyon with a group of friends in June 2008.

grand canyon 074

Rowing the 20ft raft through Granite rapid.

Fast forward to November 2008, and after a very enjoyable Grand Canyon trip I start work on some trial pits and demolition of the old rear extensions whilst waiting for the party wall agreement to be signed “imminently”. Imminently obviously has different meaning in surveyors’ language and so I close the site down from 13th November till 5th January.

5th January comes, and the party wall agreements still are not signed, and I have another week off work

12th January and finally, finally we can start real work.

The build specification requires steels to be mounted on spreader plates in the party walls, to hold the new ceiling and floor joists. This way, there are no joists mounted in the external front wall and a continuous wall of insulation with no air leakage or cold bridges can be inserted between the steels and the wall. I told you it was a “no compromise” energy-saving design. Also, the new steels have to be placed in the middle and rear of the building to hold the new roof and roof-room. Fitting these top-floor steels enables me to take the house down while leaving the old roof in place, so we can work in the rain and the party walls are kept dry. And so from the middle of January till the middle of March we demolished the interior and the rear wall of the house, storing the old floor boards for re-use in the rafters as we came down floor by floor.


Cracks in the brickwork revealed once the plaster was removed.

Then we discovered the state of the old brickwork. If anyone says to me “Oh but they don’t build houses like they used to”, implying that modern building standards and methods are poor, then they have never seen old houses stripped back. Victorian and Georgian builders built terribly, and the good ones are the ones that are still standing! 89 Culford Road turns out to be the worst built house that I have ever worked on; funny given the value of the property in the neighbourhood. The rear wall was built in two skins: the interior built using all the broken bits of brick lying around; and the exterior built using face bricks, but with very few header bricks — just snap headers to make it look like a proper build. There is a 70mm gap between the outer and inner leaves of the rear wall where it meets 87 Culford Road! This requires a sudden re-think of the demolition process. Once the inner rear portal steel frame is in place, we strap the wall together using 150x50mm timbers inside and out, bolted together using threaded rods. We then work above our “clamps” and gradually move these timbers down, taking the broken-brick wall down VERY carefully. Also we fit stainless steel “helio-bars” across the cracks and sand cement render the party walls in an attempt to hold the building together. To make matters worse the middle rear wall has bowed outwards and the bow has been taken out with around 50mm of render. This means cutting the wall up with angel grinders, as attacking it with hammer drills is just too risky. Apparently the speculative builder was going bankrupt as he was building the De Beauvoir estate and hence he used up of all the dross and brick waste as his brick suppliers would not sell him any more materials. Another boom and bust building cycle!

By the first week in March the demolition is finished, the steels are in position, and the rear portal frames are in place and “chem-fixed” to the party walls where possible. Quite whether the frames are holding the walls or the walls are holding the frames is a moot point. In fact there are no solid bricks to fasten the outer frame to, so we cast concrete to hold the steel and the wall together. When this is all done, I breathe a sigh of relief – it has been a worrying couple of weeks and the scariest demolition in my 30+ years of building.


Rob Cohen proudly (?) standing in the rear garden of his demolished house. The old roof is sitting on the new steels and the temporary sheeting hanging in place to protect the property.

18th March and finally we all change our jobs and become builders rather than demolition workers. From now on we will be carrying materials into the house rather than bagging up rubble and carrying it out and dumping it in skips!

At the “before the build starts” party everyone was saying to me when I was introduced as “Bron’s brother the builder”, that I must be very excited by this project – but I wasn’t; it was just another build. To me it was going to be just the same: hammering nails, driving screws, laying bricks and the endless moving of materials either out to a skip or back into the house, same as all jobs. And it is the same: bags get filled with rubble and dumped in skip after skip after skip, and materials have to be carried back in to the job (but at my age that is why you employ younger workers). As the job proceeds, however, there are new problems that are thrown up. These range from the simple – 200mm cavity walls need bigger ties but Magmatech provided them; to the more complicated – how do we avoid the implied cold bridge if we build as one would normally; to the still-to-be-solved, do we have to worry about condensation on the 9” brick front wall, can we put the insulation against the brickwork without a ventilation? I think you can; Bob the Architect thinks we need 25mm vent space. I say that the rear cavity wall has no ventilation; Bob says that English Heritage say there is a problem; I say that if you put a ventilation gap you then have moving air and Newton’s Law of Cooling specifically says that the rate of cooling of a body is directly proportional to the difference in temperature if the body is in a draught (I knew I paid attention to those physics lessons for a reason). Others talk about Dew Points. The biggest reason I can see for NOT putting a ventilation gap is that we are going to great lengths to make the house air tight and then we introduce four air bricks – we might as well put in a cat flap and letter box whilst we are at it! The reality is that neither of us knows and we are making guesses based on hypothetical conjecture! The answer will come from Knauf, who are providing the insulation, when Phil Rigby their insulation expert visits the site.


SO we get to the end of another building job – well it will never be the end because it is my sister’s house and I will always be “on call” to maintain the house as it gets older. Mind you she will always be on call for when I need a place to stay.

So how did the job go? When I last wrote for the blog we were still at the change-over stage from demolishing the old building and starting building the new. So the build starts and the challenge of making an eco-friendly house begins.

As I said in my original blog entry the build was not just another renovation but a renovation with added problems. Some of the problems had been thought about in advance and solutions found and others had been realised but left to be solved on-site, others just appeared.

200 mm cavities for the rear wall we have spoken about before but then there were the added problems of trying to eliminate cold bridges at the joins between the old and new walls and other transitions that had either been thought of before or became apparent as we progressed and solved using special expensive insulating bricks. Problems also arrived at the top of the extension where we had to close the cavity and cap the parapet – they don’t make coping stones that are 600 mm wide. We had to cast these and carry them into place. Here it was a bit of a disaster as one casting after another cracked as we tried to lift it into place. It would have been better to cast these in place but this too was a problem as there was no practical way of holding the form-work in place on the inside where the insulation for the flat roof rose up the side walls to eliminate the cold bridge. Also clambering all over the newly laid “green roof” carrying large heavy coping stones has done nothing for the well being of the sphagnum mosses.

Anyway here is a “time lapse” series of photos of the progression of the demolition and re-build of the house.

Culford Rd collage(Click on it and it will enlarge!)

Other problems we encountered during the build that are specific to eco side of the build:

  1. Making a building air tight to a ratio of below 1 is hard – very hard and you have to be thinking of air tightness all the time. If you go near the air tightness line with a tool – think! Of all the things I have learnt during this build this is the most important. The architects did come and lecture us about this and we listened (sort of – I did feel that being told how to put tape on to OSB was a little rich as I was the master of duct-taping leaking glass-fibre kayaks in damp Welsh car parks before canoe slalom runs). But we, the builders, felt that because we were spending huge money on air-tight windows and doors and all the effort of removing the joists from the external walls then the major sources of leaks were “designed-out”.
    Also I feel the goal posts were moved during the build, but others disagree. I thought we were aiming at a figure of “3” and since current building regulations stipulate “7” (having just been reduced from “10”) I thought “3” reasonably achievable, especially after talking to friends who work on “new builds” where they had achieved “5.5” occasionally when aiming for “7”. However, when aiming for “below 1”, the laws of diminishing returns are stacked heavily against you and the gradient of the reward curve is steeper than a hockey stick. Halving the figure from “6” to “3” is a lot easier than halving it from “3” to “1.5” and reductions from here on in get harder and harder. We ended up with a ratio of around “1.1” which is a disappointment since we were aiming at anything below “1”. I am still not sure why “1” – no one gave me a reason why “1” is the holy grail – I sort of feel it is a convenient number to aim for even referred to as the “iconic 1”! Whether the effort involved in reducing the ratio by a further 0.1 is worth the increases in efficiency of the MVHR unit is a job for someone else looking for with a PhD project.
    To give an idea of how small the leaks are that you are chasing when you are trying to get from “1.1” to below “1”; on our final air test we pressured the building took a reading and investigated the drafts – it appeared that our special air tight windows were leaking a little – we cleaned the seals and wiped down the joins and when we re-pressured the building the reading was slightly worse! Removing general London grime had reduced their air tightness. We achieved a reading of “1.12” before plastering the house and we felt that once plastered-boarded and plastered the ratio would reduce further: it did not. The problems were not the taping of the OSB but somewhere else. Still all in all “1.1” is darn good. The world of fluid dynamics never was an exact science from what I remember of my university civil engineering course.

  2. AIR TIGHTNESS is all about attention to detail and everyone has to learn about air tightness. For instance designers of boiler flues and ducts need to think about this, when sealing round pipes for boiler flues and exhaust and intake pipes for the MVHR unit as they pass through the “air-tightness” plane with tape in three dimensions is very difficult. Someone needs to build a circular flange that can be taped to the pipe/flue and to the flat external wall separately, just filling the hole with foam is not really a solution! You cannot put light switches and socket boxes on external walls unless you put the back box in a airtight box and seal the cable as it enters the box. Letter boxes have to become just that “letter boxes” outside the air-tightness line. Cat flaps – forget it train the beasts to live in a kennel!

  3. The “Eco-studs” which Knauf supplied are a great solution to the problem of thermal bridging on insulating the external front wall. However they compress very easily when fastened which can result in an undulating wall. You cannot just tighten the screw to lose the screw heads – you have to drill and countersink the OSB strips on the Eco-stud – not a problem but a labour intensive. A lesson learnt. Maybe the studs need a form of insert/stiffener to stop the compression. A very good product that needs a little refining.

  4. Builders are going to have to become geeks or at least friends with geeks. Computers are becoming more and more important and require technical knowledge that most builders don’t have. Luckily I know people who know these things but to be told “there is a software glitch in the serial bus connection” means little to me. (Maybe, as with computers, this will sort itself out as interface designers get better, but at present it is a bit like connecting a printer to a laptop ten years ago: you plugged in the printer, loaded the driver and pressed “print” only to be told there was no printer connected; this was blatantly not true, as you could see the printer and it was connected (well visually at least). I hope “plug and play” will arrive in heating controls soon but it ain’t there yet)! Someone should get all the central heating control makers in one room and not let them out till they have agreed on a SINGLE system that is compatible with all products. Valiant and Honeywell and Heatmiser and the rest – lets get all their brains in one room and sort it before we have VHS verses Beta-max again where the worse system won!

  5. Us builders are going to talk more and more about temperature gradients and dew points to clients as insulation levels increase. We should also know what this means and what the implications are; if we slap some polystyrene-backed plasterboard onto a cold north-facing wall when renovating a utility room will we have dry rot forming in the floor joists from the resulting condensation! Also how much condescension (sic) will we get in the uninsulated conservancy (sic) when the rest of the building is air tight? This is all going to become more and more of a problem. If the education is not there in several years time we are going to have problems – a bit like when flat roofs arrived in the 1960s and no one had thought to publicise the condensation problems that would arrive above bathrooms and kitchens. “Cold” or “Warm” roof theory was not appreciated by the general builder in the early days of slapping on three layers of bitumen felt.

  6. There needs to be an evaluation of how much technical management of heating controls is necessary and which are relevant to what system. There has to be a point where the increase in efficiency is not worth the increase in complexity of the system. For instance, if your heating source for the house is conventional wet underfloor heating system where the water is blended at the manifold then, is there a need for a central heating return to be thermostatically controlled and recirculated rather than returning to the boiler before recirculating? Obviously not (I think but I may be wrong). For normal radiators I can see the point of recirculating the return until it drops below a certain temperature and then re-heating it but I would be interested to know how effective this is. For how large a system would this be impractical. There must be a point when the recirculated un-reheated water got half way round the system it is now cooling the radiators. In a research lab I can see the theory being conceived and in test houses I can see it producing results but in practice – I am not so sure. (At some day I will have to deal with the even more complicated issues thrown up by the heat pumps and the like!) External temperature compensator – again the theory sounds to me as if it works in the computer model but not for the client when she gets out of bed in the morning!

Enough of boiler controls! Does 89 Culford Road work as a eco-friendly house? You will have to ask the client in a few months time when the heating controls have buried their differences with the internet and have started talking to each other – at present Palestine and Israel are on more amicable terms. However before the boiler was even switched on I had two painters living on site when all the snow and ice was around and they did not complain about the cold, in fact during this time I went up to see how the plumber was doing connecting up the boiler to find him with his overalls tied around his waist and the windows open. Yes the insulation works!

A good question to ask is “what would I have done differently if I was starting again?” On the Eco side of this question the answer is “pay more attention to the rendering of the party walls which was our “air-tightness” line. I think our biggest leak is in the party wall between 89 and 91 but this is not necessarily so; it could be this is where the leak shows and the actual breach is somewhere else! So actually the answer should be “pay more attention to the air-tightness line at all times”.

As with all builds there were errors made and they had to be corrected.

The client being my sister and her partner made for a sometime easier, sometimes harder relationship between client/builder than normal. Also the brief was different; usually you have a more trade off between budget and desire, here we had more of a trade off between environmental impact and pretty well nothing. At times I can make decisions for my sister and, because I know her, get it right at other times it is amazing how wrong I could get it! I am a builder everyday it is the first and probably only time she is going to have to make these decisions. My experience, her and her partner’s taste combined with an eye on the budget and two eyes on the eco side of the build made for complicated equations!


Taking this project forward is the big challenge: to make future renovation projects environmentally more sustainable is a huge challenge that to building industry is going to be confronted by. Building regulations, already thought of by clients as a nuisance/interference with their “castle”, are going to get stricter and as a result more expensive. The temptation to avoid compliance will be directly proportional to the increase in costs. Tackling this side of the equation is the most important issue. The regulations can insist on stringent “u-values”, air-tightness ratios and the like but if the client does not want to pay for them and can circumvent the inspectorate then the carbon emissions will continue. It is easy to educate clients as to why they can’t have open tread stairs with no bar between the treads once you explain that it has nothing to do with your feet slipping through between the treads especially when the client is pregnant and you point out how easy it is for a baby to hang itself! It is harder to convince a client that these new eco-regulations are a good idea but are going to cost them 20% more especially as they are likely to move in 5 years time when baby No.2 arrives and so they will not recoup this cost in fuel savings.

Do you want the builder on the eco side? Here is a simple inducement: make that if you register with the local authority building control office you get a parking permit for the duration of that job on that street. With parking tickets at £60 in London a pop it is a strong financial inducement (but nobody in charge of making the regulations will come up with this idea especially as the councils love the hundreds of £60s they get every week!) OK lets look at this from not so far out of the box:

  • put V.A.T. back up to 20% on gas (and give the pensioners a rise to keep the Daily Mail quiet) and then it makes even more sense to insulate and cut down draughts etc.

  • make all rents inclusive of heating costs then landlords will have an incentive to either make improvements or sell and have to make the improvements anyway.

Apart from the parking ticket issue this does not effect the builder. Our problem is to make the passing of heat through the fabric of the building harder and to do this a cheaply as possible and to keep the planning authorities happy. From a builders point of view:-

  • We need an economical method of sealing joist that are mounted in the walls.

  • We need conservation zones and listed building to allow double glazed units.

  • We need better extractor fans that run for the prescribed over-run time and then when they stop seal properly so as to prevent heat loss. Maybe even make single room heat recovery fans compulsory.

  • Make traditional sash windows illegal and move over to spiral bound windows only – no if or buts or listed building exemptions. Sash cords and pulleys are just leaks in the fabric of a building. Spiral bound windows look the same as sash windows.

  • Limit the maximum temperature a central heating thermostat can go up to 21 degrees.

  • Someone has to design a letter box that is more air tight than is currently available to make the existing rubbish obsolete!

  • Development of more effective draught proofing systems for retro-fixing onto doors – current systems have a very limited lifespan before they are flattened into uselessness. Fitting a new door is not always an option.

If all this needs a financial inducement then make all approved “green” products 0% rated for V.A.T.