A BUILD BESIDE THE SEASIDE, PART 3 – Building the shell and making it airtight

This article continues my interview with John Hardiment about his selfbuild at Gurnard on the Isle of Wight. He and his wife had originally purchased a shack by the seaside to get on the property ladder, but when it became clear that the shack was beyond repair, they decided to replace it with a small, very energy efficient, chalet bungalow.

Me:How did you select your timber frame supplier?

John:I did the drawings and specifications for my timber frame.

We went out to seven contractors for the supply of the frame and the additional cost to erect it, but we didn't get any satisfactory responses. So we went out to tender again, to two local companies and to four others on the mainland. There were some really weird and wonderful prices. They wanted to do their own thing, and they moreor less ignored the spec. It was difficult to get the prices in for the same thing.

This was about three years ago when the housebuilding industry was depressed.

So it wasn't because they were too busy?

That's right. They were very badly organised. I was keen to have one package without divided responsibility [ie, for supply and

erect]. In the end I negotiated with a local contractor, Gibbs, for supply only. I could go along to see Gibbs to do a lot of the alterations. They were local, and if we had problems, they would send someone over straight-away.

I did a lot of the detailed drawings. They hated me because I did all my drawings with a pencil, and they were all on CAD [Computer Aided Design].

I wanted to change the specification. I didn't want OSB [Orientated Strand Board – for the wall and roof panels]. I had read that if you have OSB on the outside of thick insulation, OSB is not breathable enough. So the moisture doesn't escape fast enough.

Eventually I went for Huntron's Bitroc sheathing at an extra cost. Bitroc is Norwegian; it's made of waste sawdust with a moisture resistant skin of bitumen and reconstituted newspaper. It's like a sort of fibreboard really. It's better insulation than OSB, and much more breathable. The frame maker had to put in twice as many nails to get the right racking resistance.

Gibbs would not release their design calculations prior to full payment. They sent them to building control direct. Payment terms were 25% with order, then 50% of thebalance on delivery of the ground floor frame, and the rest on delivery of the remaining: first floor and roof. The two stage payments were made by BACS on the day before delivery.

How long did it take to put the frame up?

It was about three weeks. I think my carpenter, John, took about a week longer than Gibbs would have done. He had his son-in-law helping him. It was appalling weatherwith snow [January].

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We had a tele-handler to put the far steel frame up, but it couldn't reach. So I said “Right. We'll take down the scaffold that is in the way”– the tele-handler was then just able to reach.

They subsequently found that one of the steel frames had not been put in the right place, but the carpenter and his son-in-law were just able to crow-bar it into the right position.

Why did you need the steel frames?

Because of the windows. The three gables are glass right up to the ridge. The ridge beams for the roofs needed to be supported by steel frames, not by the windows.

The ridge beams support the rafters, and opposing rafters are fixed to a ridge beam with steel connectors. What we were worried about was the possibility of the walls being pushed apart. [There are none of the usual ceiling ties because of the sloping ceilings.]

Frame under construction

The ridge beam in the foreground is for the kitchen roof. At its far end will be a window shaped like '˄'.

Note the reddish steel frame that reinforces the timber frame. The blackish sheathing is Bitroc.

How well insulated is the frame?

My structural engineer had taken me to have a look at a SIP building that was being constructed. The client there said he was having terrible trouble because the insulation wasn't fitting the timber frame. He was having to go round foaming the gaps.

Then I was talking to John, my carpenter, about the insulation, and he suggested that I put polyurethane sheeting on the inside – and that's what we did.

So I went for 140 mm Rockwool, which is 600 mm wide and therefore slightly oversize – it fits snugly between the 140 mm studs at 600 mm centres. The 25 mm thick polyurethane boards have a foil finish, and I taped all the joints [for airtightness]. Between that and the internal match-boarding finish, there's a 25 mm service void. There's no thermal bridge through the frame.

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The polyurethane boards helped the insulation, and also gave a very good base forthe vapour barrier. In many builds the vapour barrier is just a polythene membrane onthe inside, but I wasn't happy about that at all. I felt it was too susceptible to damage.

The polyurethane is sandwiched to the wall with 2 x 1 inch battens, which are vertical or horizontal to take the internal match-boarding. The sloping ceilings are similar, though with 200 mm thick insulation between the rafters.

On the inside, we must have used nearly half a kilometre of match boarding [to line the walls and ceilings]. The walls and ceiling of the ground floor have plasterboard beneath the matchboarding for fire protection.

On the outside, the Bitroc sheathing is quite a good insulator – much better than OSB. Outside that there's a 25 mm gap, and then Siberian larch weatherboarding.

Where pipework has gone through the blockwork upstand, I've both sleeved and foamed it, for airtighness.

What roof covering have you used?

We were going to have a tin roof [ie, corrugated iron]. Nicole [John's wife] wanted atin roof, and there's lots of tin roofs around here. We got planning permission for a tinroof. I was slightly uneasy about it. We've got sea air here, so when you cut a sheet, what happens to the exposed edge? There's a tin roof up the road which was seven years old at the time. I was walking past one day and I could see through the sheets atthe eaves. The sea air had eaten through it.

Then my carpenter said he could slate the roof for us. By then I knew that if he said he could do it, he would make an excellent job of it. We got Spanish slates for £1 each. In fact, the material costs were not that much more than for corrugated iron.

I did the lead flashing myself – I learnt to lead burn with the help of YouTube.

You used Siberian larch for the cladding?

Yes, but there was a load of hassle with that. One side wall is very close to the neighbouring plot – between a metre and a metre-and-a-half away. The timber for cladding had to be fire-proofed by impregnation – the local suppliers of the cladding [Jewson] did the impregnation.

The cladding at the far end is less than a metre away from the boundary, and there we not only had to have the cladding fire-proofed, we also needed a separate fire barrier on the inside of the cladding. That was expensive. We used Glasroc 'F' Multiboard.

All the match-boarding on the internal structural walls had to be fire-proofed as well, which we did by painting on HR Prof fire retardant.

Can you tell me about your foam gun?

There were inevitably some places in the shell where the joints were not airtight. But by using expanding foam, I did get a very airtight joint. [Expanding polyurethane foam guns cost £10 – £50.]

What other airtightness measures did you take?

What we did when we were laying the first-floor joists was important. We laid polythene membrane on top of the wall panels and taped it to the wall. We laid the ends of the top chord of the Posi-Joists on the membrane, and then brought the

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membrane round on top, so the upper wall panels were laid on top of the membrane, which was taped again (to the upper wall). This made an airtight barrier at the ends of the joists – otherwise this is very difficult achieve. [This is a variation of the 'TonyTray' applied to timberframe construction. See Further Info.]

The Tony Tray

The black polythene aids airtightness where the floor trusses meet the wall. (The greenish board in the stairwell is temporary.)

For the Bitroc sheathing on the outside, I sealed all the joints with silicone. On theinside, we had polyurethane sheathing, and we taped all those joints. That was my vapour barrier.

Where there were odd gaps, I filled those up with foam, and taped them.I got a lot of ideas from Natural Building Technologies, especially about

airtightness. The NBT manual was good, very good. It showed how to deal with cold bridging and all sorts of things. NBT suggested to go for either Passive House or Silver standard. [Silver – an AECB standard for low energy building.] I would say we have achieved almost Passive House Standard.

How did you get an airtight joint around a window?

We had some stainless steel straps holding a window in place on some spacers. We foamed the window in with some specially strong, structural foam [expanding polyurethane]. Then I pulled out the spacers and foamed those gaps. We trimmed off the surplus, then taped the outside and taped the inside. For this we used some tape that is guaranteed for 60 years. If you put it on in the wrong place, there's no way youcan get it off! [Pro Clima Tescon Vana multi-purpose adhesive tape.]

What airtightness figure did you finally achieve?

It was the best reading the tester had ever had. [1.4 m3/m2h at 50 pascal pressure.]

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The porthole window

John the carpenter and John the selfbuilder prepare for the porthole window

The circular plastic lining was custom made. (It is the only cold bridge in thebungalow.)

FURTHER INFO:

Tony's HouseA very low energy house with an inter-seasonal thermal store.The website includes details of the 'Tony Tray' – an airtight way of fitting the ends ofjoists into a wall. ('Tony Apron' might be a more descriptive term.)www.tonyshouse.info.

Natural Building TechnologiesTechnical manuals for both new build and retrofit. Download as free pdf's.www.natural-building.co.uk.

Words: 1780.

© Copyright article by Robert Matthews in SelfBuild & Design magazine, June, 2016.

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