Building Air Pressure Testing and MVHR Fitting

Case Study: The Gatehouse

The Gatehouse, built in the early 19th century, was in various previous existences: a toll house, a farm labourer’s tithe cottage  and a sweet shop. The “bricked up window”, which is not bricked up but was built that way (and contrary to common belief this was nothing to do with the Front wall original“Window Tax”). The recess would have had the “bill of fayre” or toll rates displayed there. The two strips of land either side of the house parallel to the road are an eighth of an acre – all toll houses had and eight of an acre for the toll keeper to grow his own vegetables. I’ve learnt this from people calling in to see what was going on as I worked on the house. Other than this I have no knowledge of the history of the house. When I bought the house the old lady who lived here had moved out only 5 years earlier P1030605and the house was boarded up. There were 4 light bulbs, one plug socket (but the electricity was turned off as it had been condemned as unsafe), a single cold water tap feeding a Belfast sink and two fireplaces (complete with rooks nests) in the house. The toilet was a “long drop” outside in an out-house over by the “internal” hedge. There was a stone lean-to to the left of the rear door which had an oven built into it. I was reliably informed (by an old guy in the pub) that it was built with the last stone quarried at Nesscliffe quarry. 


I built the extension and refurbished – more like re-built – the old house. The house had a corrugated asbestos roof when I bought it. English heritage’s stipulated that I should replace this with “Welsh Slates as this is what it would have had originally”. (Taking off the asbestos revealed the nail pattern for the previous roof which indicated that it was tiled – slates were still required). P1030625The roof timbers were riddled with woodworm and a good quarter of the wall plate was infected with dry rot so it all had to be removed. In fact apart from the stonework there is nothing original left of the old house such was the state of the timbers. English Heritage on their first visit, were impressed with my covering the first floor and stairs with 12mm OSB board to protect the floor boards until I demonstrated that this was to protect me from falling through the floor! On a later visit, when English Heritage finally allowed me to remove all of the roof timbers, they remarked “I begin to wonder why we listed this building”.


The old house.


The old house has been insulated internally; the ceiling has 450 mm fibreglass insulation on the horizontal and 150mm Celotex on the sloping parts. P1030555Under the insulation is an air-tight layer of 12mm OSB board, taped at the joints. The walls have 70mm Knauf “eco-studs” fastened to the external wall and the volume between them filled with isotherm spray insulation which is air-tight and moisture resistant. OSB board, taped again at the joints was fixed to the eco-stud to form an air-tight layer and thenP1030607 battened to allow the wires and pipes to run in the gap. This was then plasterboarded with insulated plasterboard with 40 mm insulation. The original floor (60mm concrete in places and tiled dirt in the rest with briar roots growing through it in places) was dug out and replaced with a new concrete slab covered with 150 mm insulation and then a screed floor with underfloor heating pipes fitted. The glazing is all Krypton-filled slimline double glazed units and the front door is screwed shut to help with the airtightness levels. The timber panels in the doors are filled with 25mm polystyrene and the joints siliconed to seal them.


The new extension.


The new extension is built on an insulated raft using Isoquick insulation blocks and the walls are built using Logix ICF blocks (ICF – Insulated Concrete Formwork). A polystyrene “Lego” build.Logix 4 The ground floor blocks are 70 mm of insulation either side of 150 mm concrete giving a u-value 0.23w/m2k. Damp protection 3 The concrete is waterproof concrete. The first floor blocks have an internal 70 mm layer of insulation and an external insulation of 102 mm. giving a u-value 0.17w/m2k. Before back filling the outside of the ground floor ICF blocks, they were painted with a waterproof coating and then lined with a drainage membrane.


The ceiling has 150mm of celotex between the 200 mm rafters leaving a 50mm ventilation gap (to guard against condensation).P1030552 Again a taped OSB air-tight layer and then 20 mm insulated plasterboard. On top of the rafters is 18mm plywood and then breathable membrane with Eco-Slates nailed directly on top of this. P1030580(Eco-slates are made, in part, from recycled car tyres) The Velux windows, the French Doors, the bedroom window, the bathroom window, and the big window by the kitchen are either Green Building Store Ecocontract windows or by Velux, all are triple glazed giving a U value as low as 0.81w/m2k. The glass wall is double glazed with 10mm TXD Sunbelt A60 HT, 16 Spacer Argon filled, 8mm TXD giving a u-value of 1.8w/m2k. The Logix ICF blocks are clad with feather-edge green oak boards. All in all the extension is super insulated and because of its construction method very airtight.


The amphitheatre was constructed using blocks from the old out-house, the lean-to oven and cutting the door openings by the kitchen and the walkway to the first floor in the old house.

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Energy efficiency.

To make the house as energy efficient as possible great attention to detail was paid to both the air tightness and the insulation during the construction – probably more important than chasing those design increases of 2nd decimal places by using more expensive insulation materials. Thermal bridges and holes in the insulation were designed out as much as possible, when the P1030589solid Celotex was fitted – carefully – the joints were sealed with expanding foam. (Also for instance the 1st floor joists in the old house are mounted on a steel frame in the dining room as the joists ends mounted in the wall were rotten and were cut off. This allowed a clear run of continuous insulation and air tightness OSB to pass vertically between this steel and the stone wall thus designing out thermal bridges and holes in the airtightness line). P1030494

The air leakage result is 1.3 m3/hr per m2 @50 Pascals differential. To give an idea of how good this is; Building regulations require 7 m3/hr per m2 @50 Pascals for a new build and Passivhaus demand 0.6 m3/hr per m3 @50 Pascals for certification.

Underfloor heating is throughout the ground floor and a MVHR fitted extracting air from the two bathrooms and the kitchen areas and supplying air to the other rooms.

A large evacuated tube solar thermal array heats the 200 litre thermal store with two electric immersion heaters as an alternative to heat the thermal store (the underfloor heating is part of the thermal store water). It was felt that the costs of fitting either a propane or fuel oil tank and boiler would be unjustified given the levels of insulation, excellent airtightness and the MVHR unit.

LED lighting is fitted almost throughout. The hob is an electric induction hob and separate oven is fan assisted.

Future proofing.

A 2.5 mm2 twin and earth cable is in place from the fuse board to under the soffit by the French doors should Solar PV panels be fitted at a later date.

A corrugated flexible duct (with a draw cord) has been buried from near the electric supply cupboard to a point where a garage may wish to be built.

So after all this work to make the house energy efficient it gets a EPC of C! This is because the way EPC are measured does not take into account the levels of insulation and airtightness installed. EPC assessment has a is a box for ceiling insulation – there are no extra points for the amount of insulation, the same with wall insulation, air-tightness has no influence on the rating and fitting an MVHR unit has no box to tick! If I had fitted a condensing propane boiler it would have improved my score but made little difference to the energy consumed by the house.

  Gallery of the finished house.

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