First airtightness test

Tomorrow (5th April) is a big day for the project. Paul Jennings from ALDAS, will be conducting our first airtightness test. We have been preparing for the day by going through the whole building to ensure there are no forgotten gaps or holes.

All the windows are now in and although there are still snags and issues to be resolved, they all close well enough for the airtightness test. In the refurbished part of the house, the windows are mounted on the ouside of the original walls with the external insulation wrapping around the window frames to minimise the thermal bridging around the window edge. This also minimises the area of window frame, which helps aesthetically and improves the energy performance of the window installation because window frames are generally the poorest performing part of the window. The photo below shows a section of a window with the first of four layers of insulation attached around it.

The next photo shows another section of window with all 180mm of phenolic foam in place. The work to cut and attach the insulation was time-consuming and really unpleasant for the team -  the stuff makes you really itchy. The job was made more difficult because Kingspan would not supply their product in broad sheets of 100mm and 80mm depths. Instead we had thinner (40mm and 50mm) and smaller area sheets. This meant much more glueing and cutting. We have also had to use more of the fixings than would have been needed with the broader sheets. Thank you Kingspan.

In the photo below, the airtightness tape is being applied to along the base of the window, forming a seal between the window frame and the previously parged internal window reveal. We probably have not approached this very well, as the process took longer than expected and was very fiddly. Because the parge layer was rougher than is ideal, we felt we needed to use an adhesive primer in addition to the Tescon Profil tape. It was hard to control what then becomes a very sticky combination of materials! We will need to check back with suppliers, Ecological Building Systems, to try to do it better next time.

We have two large service penetrations into the roof: for the soil vent pipe (SVP) and for the flue. We are using a small gas boiler, which modulates down to about 4kW, to provide our winter hot water - solar thermal will deliver the rest - and any residual heating we may need in the coldest weather. As well as an airtightness issue, SVPs and flues create a potential thermal bridges. The flue has a pair of concentric pipes, the inner one to vent exhaust gases and an outer ring to take in air to the boiler for the combustion process. The flue runs through a grommit/Intello, past 350mm of Warmcel, 22mm of Steico wood fibre board and through the Solitex roof underlay. It could therefore, unless insulated around the pipe within the house, be a significant thermal bridge. We are planning to enclose it, and the SVP, which presents similar issues, with sheep's wool insulation within the boxing. The lack of a need for a flue in an electricity based heating system, i.e. a heat pump, makes gas a less attractive option in a Passivhaus. However, I must admit that the extra cost of a heat pump based solution put me off a bit when we were at the design stage. The Passivhaus Institute are keen to encourage manufacturers to develop and sell "compact units". These have about the same footprint as a fridge freezer - so are super space efficient - and combine the MVHR, hot water (DHW) and space heat product functions in a single unit. One, the Compact P made by Danish manufacturer Nilan, has been Certified by the Passivhaus Institute, and is being promoted in the UK. It looks quite promising, however, most combined devices don't deliver the same performance as the "separates" units would. Our MVHR unit is almost twice as electrically efficient as the Nilan Compact P. And I think compact units need to be completely modular in design, so that parts that fail can be replaced independently of the rest of the machine. The other argument in favour of using heat pumps for DHW and space heating is that they will be better in climate/CO2 terms than gas, as the renewables portion of the electricity grid mix grows. I think that in another five or ten years, the balance - and the economics - may well have shifted in favour of heat pumps and I hope compact units.

 
We also have two large penetrations for the heat recovery ventilation (MVHR) unit's intake and exhaust ducts. The ducts themselves are 160mm but are wrapped in 125mm of insulation, making the penetrations 410mm each! This photo is from the inside, where the ducts meet the airtightness barrier, which in this part of the building is the internal plasterwork (parge coat). The walls are still to have their finishing layers of plaster, which will encase the grommit, intello and tescon tape. The continuation of the ducts will also have 125mm of insulation right up to the insulated surround of the MVHR unit.

Budget and payback

It is about time I talked about how much all this is costing. So far we have managed to keep close to our estimated budget and schedule. At this point, it is looking like the build costs, excluding VAT and professional fees, will come out at £1500 per m², which, given that we are having a high standard of internal finish (hardwood floors, granite worktop, decent taps and appliances etc) is not unreasonable. If you strip out the Passivhaus elements, it would have costed £1200 per m².

I think we have learned a lot about how we will approach new Passivhaus builds to bring the cost differential as close to £0 as possible. This is something we are looking at now. For this project, the extra cost will pay for itself easily over the time I hope to be living in the building, especially now that fuel prices are shooting up - very small increases in energy prices have a disproportionate effect in shortening the payback period. There will be many other non-energy benefits for us - better air quality, much more comfort (no cold drafts or 'cold radiant') and less noise from outside, no radiators on the wall taking up valuable space, loads of natural light, cooler summer temperatures - I think all those must be worth something.

Other Passivhaus Refurbs

Passivhaus refurbs are like London buses, you wait for ages for one, then suddenly three in a row come along. The other two projects are both part of the Retrofit for the Future scheme.

First, is the Princedale Road refurbishment of a Victorian Terrace in Holland Park, West London. The building is owned by a social landlord, Octavia Housing. It gained its Passivhaus certification on 1st March from Peter Warm, making it officially the UK's first refurbishment to reach the full (15kWh/m2.a; 0.6 ach @+-50pa) Passivhaus standard. It is a fantastic achievement, particularly as the building is a mid terrace in a conservation area, so there were many complex problems to overcome.

The second project is here in the south west. The client is North Devon Homes and the project, known as Barbrook, is currently on site. The site is much more challenging than our one so that, even though they are 'only' going for the EnerPHit standard, their project is every bit as challenging. I am looking forward to visiting Barbrook soon.

Forming an airtight wall-ceiling junction

We have started to create the airtightness layer at various points in the building. In this post, I will describe the sequencing for one of our ceiling-wall junctions. The airtightness layer in this part of the build is formed by plaster on the wall and by Intello vapour barrier on the underside of the roof I-beams.

Stage 1) The Intello was stapled to the underside of the I-beams. Tescon tape was applied to join up each row of Intello. We also taped over the staples, in case the Intello is stretched and a small gap appears around the staples, although apparently this is not strictly necessary to achieve the Passivhaus 0.6 air changes per hour (ach) target.

Stage 2) In order to join the plaster to the Intello, we used Contega PV tape - half adhesive backed, half mesh. We had to use Tescon tape to stick the Contega to the Intello, as the Contega adhesive would not stick properly to the Intello. I had a very helpful conversation with Niall Crosson from Ecological Building Systems where he told me that if the Intello is exposed to humidity, it can affect its ability to stick to the Contega. I guess this must be because of the property of the Intello to vary its vapour permeability according to the ambient humidity.

Stage 3) After the first photo below was taken, an initial parge layer was applied under the Contega. We took care to put a kink along the length of the Tescon tape by pushing it up slightly, to allow for any possible future movement between wall and ceiling.

 

 Stage 4) Once it had dried, a second layer was applied to encase the mesh and the Contega tape above it. Then battens were screwed through the Intello to the I-beams. The Intello grips the screws and forms an airtight seal around them, which is only compromised if the screw is removed: it is vital therefore not to remove a screw if it has been incorrectly positioned - better to leave it in place. The space above the Intello (i.e. between the I-beams) is to be filled with Warmcel cellulose, so the battens will help the Intello support the weight of the Warmcel and will provide a 25mm service void to the ceiling plasterboard. 25mm is normally too narrow but I didn't want headroom to be reduced any more than is necessary in this attic space.

The remainder of the wall will be plastered later, along with the ceiling.

Airtightness around existing floor joists (and along the top of internal walls)

The last few weeks has seen tremendous progress on the build and one issue that I wanted to focus on in this post is how we managed airtightness around the ends of the existing floor joists.

The picture below shows how the floor joists looked before. Normally, the block work would only be plastered above and below the joists, where the finished wall would be visible. In a Passivhaus refurbishment, the plastering needs to run continuously around the joists.

We had originally intended to keep the plasterwork below, if it was sound enough, but mostly it just fell off with minimal if any encouragement. As discussed in a previous post, water penetration in the cavity, due to poor detailing at the original roof-wall junction, together with some poor choices with the original render and paintwork seemed to have been the cause.

The issue of how to make airtight joist ends has been covered in this AECB YouTube video. One of the key points they mention is that the approach needs to take account of whether the wall is going to be insulated internally or externally. If a wall is to be internally insulated, there will be a significant temperature drop at the end of the joist, which brings a risk of condensation because the joist end sits within the cold wall. In the film, to avoid this, they have mounted the joists on joist hangers on an internally insulated wall. Steps then need to be taken to achieve airtightness around the joist hangers.

In our project, the existing house is being insulated on the outside, so the inner leaf of the existing cavity wall will be the same temperature as the inside - i.e. no condensation risk. This has been our approach to achieving airtightness around the joist ends:

Stage One

Carlite Bonding parging around the joists onto the original concrete block wall or sometimes new built sections of Thermalite block wall:

The same principle has been applied to a steel girder fabricated with an additional L-shaped piece, welded with the bottom of the L welded to the web of the girder. This means that the profile of the end of the girder is a solid rectangle, like a joist end, rather than an "I" profile, simplifying making it airtight.

Stage Two

Priming around the joist ends with Pro Clima's Tescon Primer adhesive. This helps to make sure the Tescon tape sticks to the plaster.

Stage Three

Taping around the joist and girder ends using Pro Clima's Tescon Profil tape.

 

Stage Four

Dotting the corners with Pro Clima's Orcon F adhesive. This is a "belt-and-braces" step. We were not absolutely sure that our tape corner junctions would stay completely airtight. We applied dots of Orcon F to be sure! 

Stage Five

Apply Unibond to the Tescon tape to provide a key for the second parge layer (next stage).

Stage Six

Apply a second parge layer to seal in the Tescon tape. In the picture below, the joist on the left was originally a double joist. We cut short the thinner of the two rather than try to treat the double joist along its length. Obviously, if the double joint were needed for structural reasons, we would not have been able to cut one joist back.

 

Dealing with the edge joists
The joists run north-south in our build. There was no edge joist on the east side due to the run of the original roof rafters, however the edge joist on the west wall was mounted close up against the wall and there wasn't enough space around the joist ends to parge and tape around them. One option was to wrap the entire length of the joist in Intello Plus membrane, then tape it down above and below the joist. We decided it was more practical to cut out the existing joist, reduce its length a few cm and re-attach it with coach bolts along its length of the west wall. We used resin anchor gel to secure the bolts in the wall but also applied a generous ring of Orcon F around each a penetration to provide a flexible airtight seal (picture below).

YouTube video of project

I've uploaded an interview about the project given in December about the project.

View it here: http://www.youtube.com/watch?v=EjNbNJ3YgfQ

Photos from weeks 9 and 10

I've uploaded photos of the build - weeks nine and ten - on flickr. Our progress was slowed down by the snow, but the team has been trying to catch up since the new year.