CIAT Student Competition Technical Report Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 1 - Daniel Bates Architectural Venue Design BA, Stage 3. Faculty of Art, Design and Technology, University of Derby. 2012. CIAT Student Competition. Technical Report. Membership ID : 025344
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CIAT Student Competition Technical Report
Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 1
-
Daniel Bates
Architectural Venue Design BA, Stage 3.
Faculty of Art, Design and Technology, University of Derby. 2012.
CIAT Student Competition. Technical Report.
Membership ID : 025344
CIAT Student Competition Technical Report
Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 2
Figure 4 - Boxing with penetrations from the blowing nozzle of Warmcel insulation. Source: barbrookpassivhaus.wordpress.com, 2012.
CIAT Student Competition Technical Report
Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 7
External wall insulation may involve knock on costs of alterations to roofs typically the extension of
eaves so as to cover the new thickness of walls. This however is an opportunity to replace an old
poorly insulated roof. Two distinct methods of EWI involve either the use of foam boards or the
construction of a timber frame to hold batts. There are pluses and minuses to both. Namely, that the
speed with which foam boards can be placed means a great cost saving. Primary buildability
considerations here are scaffolding, weather, and excavation works for insulating the foundations.
Internal Insulation
Internal insulation is more challenging technically and more costly due to the difficulties in working
around existing structure. It has often been utilised in the case studies because of the aesthetic
value of the façade, which is often of Victorian or Georgian origin in hard to heat homes.
In the Lena Gardens case study the walls were internally insulated with 130mm of phenolic foam
board insulation hung off aluminium spacers, 12mm OSB (airtightness layer), up to 50mm additional
thickness phenolic insulation depending on the presence of electrical or other services, finished with
12mm gypsum plasterboard and skim.
Figure 5 - Aluminium hangers holding the phenolic foam to the wall. Note part of the void has been filled with polystyrene beads, not carried out during the entire project due to moisture related risk. Source: ecohome.tumblr.com, 2012.
The air gap between the masonry wall and first layer of insulation was of critical importance to
minimise interstitial condensation risk. An analysis of this type of risk is held in Appendix A.
CIAT Student Competition Technical Report
Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 8
This method of construction was made simple due to the internal joists being cut back and hung off
steel beams, set into insulated wall pockets. These steels and their pockets will be discussed further
on. Not having to make individual joists airtight on a project of this size resulted in cost-effectiveness
in terms of time and ultimately in airtightness and heating costs.
An alternative method of internal insulation utilises
natural materials. This simultaneously addresses
the interstitial condensation issues and embodied
energy in materials. In the St. Luke Street retrofit
project the method involved creating an internal
timber frame to hold batts of sheep’s wool. This
hygroscopic material reduces the risk of interstitial
condensation.
The sheep’s wool was easy to handle, making the
solution buildable on the whole, however in the
locations where the batts had to be cut there were
minor delays due to the difficulty in cutting the
batts (Davenport, 2012).
Figure 6 - Insulation details. Airtightness layer set back from plasterboard to avoid puncturing. Hygroscopic materials used. Source: Anne Thorn Architects LLP, 2010.
CIAT Student Competition Technical Report
Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 9
Airtightness
Window Strategies
Referring to the previous section, the construction of the window sub-frames was crucial before the
placement of insulation. 2x6 lengths of treated softwood timber were fixed through the parget coat,
pebbledash and into the masonry wall using Rawlplugs. The intelligent vapour control airtightness
membrane that lined the interior of the building was brought around the front of the sub-frame as
shown in the image below:
Figure 7 - Preparing the window sub-frame for airtightness. Source: towerhamletspassivhausretrofit.retrofitdiaries.org
The sequence of work was to apply the parget coat first, fix the window sub-frames, perform the
required airtightness works, fix windows and make airtight, the 3rd party installers adhere foam
boards to the wall working around window openings where appropriate.
The airtightness interface of the windows to primary layer is often a tricky one to navigate:
CIAT Student Competition Technical Report
Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 10
Figures 8 & 9 - The application of 'Rabbit ears' in taped window installation. Allows for the relative expansion of tape and frame, as well as making the installation easier. Source: towerhamletspassivhausretrofit.retrofitdiaries.org
Figure 10 - Expanding foam between the window tape and the external timber frame. Source: towerhamletspassivhausretrofit.retrofitdiaries.org
CIAT Student Competition Technical Report
Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 11
Figure 11 - Airtightness tape to the outside. Source: towerhamletspassivhausretrofit.retrofitdiaries.org
Figure 12 - Airtightness tape to the inside. Source: towerhamletspassivhausretrofit.retrofitdiaries.org
An alternative strategy for the construction of windows involves the use of window boxes (shown in
the Barbrook Passivhaus case study above). The comparison of creating a sub-frame versus a
window box has to be looked at in two main ways. Material cost and material function. Functionally
a window box may cause a reduction in the window size, for example a 900mm opening with 18mm
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Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 12
plywood surrounds will be reduced to 864mm, a 4% reduction. This could be beneficial for north
facing windows to minimise heat loss or may negatively impact heat gains from south facing
windows, often in short supply in refurbishment projects.
The economics of the materials is highlighted in the box below, indicating a more cost effective
boxing method.
Internal Airtightness Strategies
Investigation into the case studies found a myriad of different products and materials used to
achieve airtightness. However, key methods did emerge:
Plastic membrane. Either tough building plastic or intelligent vapour control layer.
OSB taped at the joints. Usually good quality OSB and specialist tape.
Parget coat, using either cement or natural lime based products.
The parget coat method is typically done when exterior insulation methods are employed. For the
sake of depth, a focus on the other two methods will be done here. An intelligent plastic membrane,
first stapled and then taped, costs approximately £4.37/m2. Compared to 11mm OSB at £5.36/m2.
Both are functionally effective in different ways. The intelligent vapour control membrane restricts
the passage of moisture from the interior to exterior during winter and allows it during summer,
theoretically reducing the mean annual moisture content of subsequent layers. OSB taped at the
joints is cost effective and reasonably quick to install, because this method is relatively new time will
tell as to its effectiveness. The Lena Gardens and Balham Passivhaus case studies contained OSB as
their airtightness layer. Lena Gardens achieved the lowest air test result (0.49 ach) of all the 7 case
studies looked at; pointing to its initial effectiveness, especially in a project of its size and complexity
(Open Day Visit, 2011).
900x900 window 0.225 m2 ply per side
0.9 m2 ply box £17.02 per window excl. wastage
8 m treated timber 47x150mm
£25.52 per window excl. wastage
both exclude fixings
prices from buildingmaterials.co.uk
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Investigation into the buildability and cost-effectiveness of Passivhaus refurbishment - - - Daniel Bates - - - Page 13
Post-construction of the internal airtightness layer, be it OSB or membrane, an inspection can be
carried out for the entire airtightness layer. The A blower door test can be run at this stage to check
for any leaks.
Figure 13 - Continuous OSB airtightness layer. Easily visible at this stage of construction. Source: Prewett Bizley Architects.
Joist treatment is one of the most important considerations when choosing
an internal treatment strategy. The main issue is one of interstitial
condensation and the increased risk associated with the application of
internal insulation. One can imagine the theoretical dew point within a wall
moving closer to the interior upon the application of internal insulation
(Little, 2009). Potentially exposing timber joists to increased risk.
There exists a wide variation in the risk factor depending on the wall type,