The new external brick skin of the walls is now built up, and the cavity created between that and the original solid concrete walls has been fully-filled with insulation.
Wall insulation showing in new cavity at front door
The insulation is between 250-300mm thick; the variance is due to the fact that the original walls had shifted slightly over time, so in some places the cavity was slightly wider than others, and we took every opportunity to get in as much insulation as possible! You can also see in the photo above that the cavity extends down below ground level, to the bottom of the original wall foundations, and insulation was packed down here as well to minimise thermal bridging from the internal floor to outside.
Wall insulation being packed into the cavity at roof level
The photo above shows the insulation being packed into the cavity just below the existing eaves. The insulation is then continued up over the top of the original wall, between the rafters and into the roof space to again minimse thermal bridging at the top of the original walls. This is the weak point of the overall insulated envelope however, as the void over the orignal wall between the rafters only allowed for around 150mm of insulation. The roof was then extended by two rows of tiles to meet the corbelled external skin.
Wall cavity at an opening showing wall tie
The photo above shows the newly created cavity at one of the windows, and if you click on the photo to expand it you can just see the wall tie spanning the cavity four brick courses down. These ties were secured into the original wall by drilling a hole and inserting the tie with a mortar fill and they then span the cavity and are secured in the mortar layer of the external skin. The ties used are Teplo Ties from Ancon, which can be cut to length; traditional ties do not come in sufficient length for this sized cavity.
The new floors have now been laid. The floor slab is 150mm of dense concrete, which sits on top of a new layer of insulation which is 250mm thick. Around the side of the floor slab is 50mm of edge insulation.
Floor insulation being installed, which will be below the new concrete floor
The new floor slab will act as a heat store to help keep the house warm through winter, with heat loss from the slab to outside of the house minimised by the insulation below and around it. The heat then only has one way to go; back into the house as the air temperature drops below that of the floor.
The new concrete floor slab being poured over the insulation layer
In summer the floor slab will act in reverse as a cooling mechanism, because as the air temperature rises above that of the floor, the excess heat will be absorbed into the dense concrete.
The mass of the original solid walls will act as a heat sink in a similar way, helping to cool the house in summer and heat it in winter, once it is fully encased in insulation, which is the next major phase of the retrofit.
Although we believe we’ve come up with a very energy-efficient retrofit design, and combined with the energy offset provided by the investment in the community wind turbine will deliver a zero carbon solution, there is still one critical element over which we have limited control – the tenants.
In highly energy-efficient houses that combine high thermal mass and significant levels of insulation to remove the need for a space heating system, it is vital that the occupier appreciates the design concepts and how to live in the house; if you leave a window open in winter the house is going to get cold!
Therefore we will be working with the tenants moving into the retrofitted houses, to help them understand how the houses are designed, and more importantly how they need to occupy the house to ensure this design is effective.
This subject was covered during a seminar at the recent National Energy Management Exhibition (NEMEX), and included a name check for Hockerton Housing Project.
In order the take advantage of the thermal mass of the existing solid concrete walls, and turn them into a heat store for the houses, we are building a new external wall and incorporating a large (250mm) cavity fully-filled with insulation between the two walls. This significant layer of insulation will stop the heat absorbed by the existing solid walls transferring to the outside of the houses and being lost, as it does at present.
Before we build these new external walls, a couple of steps are required first:
- Removal of the existing bays on the houses to make it easier to build a single wall right across the front
- Addition of new foundations on which to build up the new walls
The video below shows the houses with the bays removed and the new foundations being laid. The new foundations were combined with underpinning for the existing walls to make the overall structure more secure.
[youtube=http://www.youtube.com/watch?v=UjthbR13YcI]
The houses currently have solid concrete floors with no insulation underneath them. As a part of the retrofit we are excavating these original solid floors, and replacing them with a combination of insulation laid under a new concrete floor. Putting the insulation underneath allows the mass of the concrete to store heat and then give it back up as the air temperature in the house cools, thereby helping to keep the house warm.
Effectively the floor is then acting as a storage heater. The heat absorbed into the floor is from a combination of passive solar gain and incidental gains from occupants, cooking and appliance use. Basically whenever the air temperature in the house exceeds the temperature of the floor slab, the floor will absorb that excess heat. The insulation under the floor slab stops the heat escaping to the ground below, storing it to be returned to the house as the air temperature subsequently cools.
The existing solid walls will act as a heat store in a similar way once the new external wall is built up with the insulated cavity outside the existing wall.
The video below shows one of the houses with the old floor excavated and Nick summarises our plans for the new floor.
[youtube=http://www.youtube.com/watch?v=yjePXnOUgKM]
