Milton Keynes’ First Passivhaus on Site
One of the important requirements of PassivHaus is to ensure that the fabric of the building is cold bridge free and the insulation layer that wraps the building is as continuous as possible. The foundation details are an area where this becomes difficult as insulation is inherently light and does not have great strength, so how do you support the weight of the house just on some polystyrene? Well, if you spread the load then the polystyrene is strong enough. The next problem is to ensure that the insulation stays intact for the life of the building – not eaten by mice or dissolved by petro chemicals; it will probably be OK as it is well wrapped in DPM’s etc. With our Engineer Andy Allan of Allan Consult, we have done things slightly different. We have introduced a series of lightweight thermal blocks as piers within the insulation that would support the house should the insulation fail structurally. As these are reduced to just piers of blocks, heat loss is less than if they were a continuous structure. It does produce a slight cold bridge but this is counterbalanced using slightly thicker insulation and as good a thermally efficient block as we can find.
The other different aspect to our foundation detail is that we wished to provide a brick plinth to the outside of the house to protect the insulation and form a dry strong base for the timber cladding, but we wanted as much insulation as we could get in the upstand at the edge of the slab to join with the insulation in the wall. Our solution was to use cant bricks and bridge the gap with the outside of the I joist sitting on the bricks and the inside of the joist on the slab. The frame engineers and the builder took some persuading but we all now feel we have a robust and fairly cold bridge free detail.
Why Timber Frame?
We started the project with the idea of using rendered block work, but as the site was located at the edge of a wood, timber cladding seemed the most appropriate, plus the clients preferred option. Due to the high U values required using timber cladding with block work was difficult due to the number of metal fixings needed to support timber battens, causing excessive cold bridging and reducing the overall U value excessively. Therefore using timber frame became a more viable option. Of the possible solutions the offerings of Touchwood homes were selected using engineered I beams in the walls to reduce the cold bridging through the web. The frame is stick built with pieces pre-cut to length and soleplates notched to receive studs.
Airtightness layer inside or out?
When using timber frame for passivhaus’s the airtightness is normally provided using OSB3 on the inside of the frame, with all joints and junctions taped up. However Pat and his team at Touchwood Homes who won the contract to provide the timber frame had other ideas, they use an external tongued and grooved wood fibre board (Agipan DWG) on the outside and then seal up all the joints with a non setting mastic – Butyrub (no jokes please). At the time of detailed design Touchwood had not had any airtightness tests below 0.6 and no data on the longevity of the non setting mastic could be found. So the decision was taken to do airtightness layer inside and out.
3/5/12 – Explanation of some of the features of the Passivhaus, enjoy.
First Airtightness Test
Great results today, just had our first air tightness test, as the timber frame is up and the windows installed. We had a fantastic result of 0.16 ! and we only needed to get under 0.6. I spoke to Nial at Ecological Building Products, who supply the Proclima tapes and is an expert on airtightness. The best result Nial has seen is 0.15 in Ireland, and that was the second test, after they had taped up all the staples holding the airtightness membrane in place. So this could be the best result for a house in the UK! will need to double check the certificate once we see it. So many thanks to the guys on site particularly Adam from Touchwood homes who was making sure everything was good. I can’t wait for the final results could be better still!
Final Airtightness 0.065! Best in the UK!
The final blow door, airtightness test,has just been done, which we believe is the best airtightness for any building in the country with an air change rate N50 of just 0.065 h-1 @50pa. The only lower result we have found is a 0.05 for a large building in Germany, if you know any lower result please let us knows. But don’t worry you won’t die of suffocation, it would take many days of the MVHR unit not working and you not going out or opening a window, even when you feel muggy! We have also installed a CO2 Alarm just in case.
The house will be open to view on Saturday 23rd of June, as part of The Parks Trust Little Green Fair, and part of the RIBA’s Love Architecture week. The event will include tours of the house and talk by ourselves as Architects and PassivHaus Designers. If you can’t make the open day give us a call and we may be able to show you around before the tenants move in in early July.
Great News the house is now a certified PassivHaus, and the official plaque is fixed. The new tenants have now moved in and are willing for the house to be monitored and have promised to write a blog of their experience. We are also hoping that Oxford Brookes University will be monitoring the house and providing post occupancy evaluation. So watch this space for a link to the blog, and the findings of how the house works in real life.