This is the second in a series of case studies that use a new methodology to assess the deconstruction potential of new build properties. The case studies and methodology have been funded by the BRE Trust to raise awareness amongst architects, designers and contractors of the potential of Design for Deconstruction (DfD) to create more sustainable buildings and lead to better deconstruction outcomes.
A Design for Deconstruction (DfD) assessment has been undertaken on a PassivHaus system used in Scotland for a 2-3 bed house. PassivHaus is a low energy design standard, developed in Germany, providing year-round comfort without active space heating or cooling systems. Design criteria includes a simple building form and orientation to maximise solar gain and reduce heat loss, super insulation, no thermal bridging, air tight fabric and mechanical vent and heat recovery. There is also a maximum primary energy demand: 120kWh/m2/yr. Offsite construction was used in the form of structured insulated panels (SIPs) which were mechanically fixed together onsite.
The buildings is composed of Structurally Insulated external wall panels (SIPs) use mechanical fixings and screws to form fully insulated (258mm) timber stud walls. OSB boards form the sandwich and the joints are sealed using air tight barrier tapes. The internal vapour control barrier itself is stapled to the OSB boards. The wall panels are nailed to timber load bearing sole plates, which in turn are nailed into masonry sub-structure. The roof is made of cassette panels glued to timber rafters and the glulam ridge beams. The ground floor reinforced concrete slab carries 200mm rigid PIR insulation and an air tight barrier on top. The flooring is tongue and groove solid timber boards; this is assumed to be glued to the base structure. The type of elements within this PassivHaus case study are summarised in Table 1.
|Frame||Timber studs, lined both sides with plasterboard, 90mm miner wool insulation between the studs|
|Foundation||Concrete trench fill foundations. External brick block sub-structure, PIR insulation to underbuilding cavity, wooden sole plate (140x30mm) nailed to base layer|
|External walls||SIPs – breather membrane on OSB, fully insulated 258mm Space stud timber framed walls, OSB, 70mm rigid insulation, 38mm service zone, plasterboard|
|Ground floor, upper floor and Ceiling||Ground floor – tongue and groove boards, air tight barrier on 200mm rigid PIR insulation on ground bearing reinforced concrete slab
Upper floor – tongue and groove boards on Timber floor truss, mineral wool insulation between the floor truss
|Roof||Roof cassettes – with slate, roof membrane on t sheathing board, fully insulated 300mm joists, OSB, 55mm rigid insulation, 38mm service zone, plasterboard|
|Cladding||Timber or render cladding|
|Floor finishes||Tongue and grove solid timber (22mm) flooring|
|Windows and doors||Timber framed externally aluminium cladded and triple glazed|
|Sanitary ware||No information available|
|Services||Mechanical Heat Ventilation System, Wood burner and Solar Thermal panels|
|Fixtures and Fittings||No information available|
Table 1: Elements of the PassivHaus case study
There was no consideration from the client’s side apart from the target for achieving Passivhaus standard. The design team were aware of DfD principles and incorporated it from the start. Along with Passivhaus design requirements, the installation of wall and roof panels considered optimum fixing details for effective deconstruction. No specific drawings were made for deconstruction. There is a visual record of documentation provided for Passivhaus certification which shows all the junctions and the details of how the components are fixed together.
Overall Score 70%
- The insulated substructure (Figure 1) can be dismantled with relative ease after removing the external wall panels and timber sole plates (Figure 2) (which are nailed into the masonry). It is likely that portland cement has been used between the bricks and as such would make it difficult to remove the bricks intact.
- The timber truss for upper floor has metal connections and can be dismantled easily. The tongue and groove timber floors are glued, they would have to be cut out in sections. The plasterboard on the underside of the ceiling, screwed to the timber truss can be demounted easily.
- For the roof, the roof cassette is screwed to the ridge, making it easier to recover the whole section intact. The vapour barrier is stapled to the panelvent board, and it is possible to remove it. The glulam ridge beam can be removed, once the triangular section of the gable end is exposed.
- The cladding is attached using an external rack board and is either sprayed on render, making it difficult to remove or timber cladding hung by nails, making it easier to remove (Figure 4).
Overall Score: 64%
- The roof cassettes (Figure 5) and wall panels are accessible, however they are likely to require specialist handling equipment to remove, due to their size and to re-instate airtightness if they are to be reused elsewhere.
- For the internal walls, the plasterboard is accessible as are the non-load bearing stud walls.
- In terms of the foundations and ground floor, it would be difficult to access these without damage to surrounding materials.
- A Mechanical Ventilation and Heat Recovery (MVHR) system located is located in the utility room at the ground floor level. This allows good level of access to service the system.
- It is unclear how the duct work is linked between the two floors. If the duct work runs through the floor, between the floor trusses, the floor boards would have to be removed to access it.
Reuse and recycling potential
Overall score: 58%
- The SIPs are suitable for reuse, if dismounted with minimal damage and have an estimated service life of 80 years.
- The brick and block sub structure can be recycled as aggregate
- The concrete slab can be recycled as long at the steel can be segregated from the concrete; alternatively it could be reused in-situ
- The PIR insulation used in the ground floor and the air tight vapour barrier, cannot currently be recycled, energy from waste is currently the best option.
- Where plasterboard is used, this is recyclable, though there are limited end markets
- The Internal and external timber battens for installing plaster board (internal) and wall board (for cladding/rendering) – recyclable and partly re-usable
- The OSB used in the panels is not recyclable and would have to be sent to a waste to energy plant along with the PIR insulation used
- The external racking boards used for the timber cladding can be recycled; and the timber cladding could be reused, the rendered racking boards are not recyclable
- For the internal stud walls, around 20-30% could be reusable, the rest could be recycled
- Mineral wool has been laid within the panels (wall and roof), and this is currently difficult to recycle
- The timber truss used for the upper floor is recyclable
- The tongue and groove timber flooring could be recycled if not glued.
- For the roof, the fibres cement slates are recyclable, though there isn’t currently an end market. The timber battens and timber joists are also recyclable. The remainder of the materials i.e. the vapour barrier, OSB and mineral wool are not currently recycled. The panel vent maybe recyclable (this is made from wood waste). The glulam ridge beam is reusable
- The windows could be reused.
Optimisation of deconstruction
Overall score 77%
The upper floor, internal walls, windows and doors and floor finishes can all be dismantled with traditional equipment. For the ground floor and foundations, these can also be dismantled using non-specialised equipment once the external and internal walls have been removed. Specialised equipment would be required for the external walls (SIP panels) and the roof cassettes, due to their size and the items glued together within the panel.
Key considerations for deconstruction of the SIPs are:
- The SIPs will need special tools for disassembly for handling large panels. Other equipment will be needed for the removal of insulation, nails and screws. Similarly, it could be manually harder to separate the metal bars from the OSB and remove screws and nails out of the panel.
- Manual deconstruction of the pods and cassettes on site may not be cost-effective, due to the labour cost, value of the recovered material and time involved in deconstruction. The scrap value for the metal may be too small to make it worthwhile to accommodate specialist shredding costs of SIP panels and transport cost to specialist facilities to recycle.
The PassivHaus case study scored 64% overall for its DfD potential. Figure 6 shows the overall score by element. Elements which had greater potential for DfD are the windows and doors, internal walls, sanitary ware and floors. The deconstruction process (77%) and connections (70%) scored the highest and reuse and recycling potential the lowest. Where the connections are screwed based i.e. for the SIPs this scored highly; where nails are used (i.e. fixing the panels to the sole plate), the score is slightly less, as nails are more difficult to remove. Most of the elements of the house could be dismantled using non-specialised equipment apart from the SIPs panels. The following issues are summarised:
- Though the SIPs panels may present issues for recycling due to separating their constituent parts, their speed of erection can be up to 4-5 times faster than other construction types and these needs to be considered within the context of a project and its whole life value.
- A number of materials used are currently hard to recycle such as the mineral wool, OSB, fibre cement slates and PUR insulation; alternative products which may be easier to recycle, could be considered.
- Attention should be given to the type of cement used between the bricks and to ensure that the bricks could be removed intact for future reuse.
- Nailing tongue and groove flooring is preferable than using glue as it will be able to removed for reuse.