Improving the Thermal Performance of UK 1930s Semi Detached Houses
When my family and I moved into a 1930s semi-detached property a couple of years ago we made a few improvements to the house. One aspect was addressing the thermal performance as the building appeared to be poorly insulated and felt cold. In this story I would like to share what we have learnt and done, as well as what would be possible beyond that. Given that there are quite a few of those houses in the UK, I hope that our experience will be useful to others, with some measures being even transferable to other property types. As such all may contribute towards combating climate change by reducing the energy expenditure for domestic space heating, mostly through insulation.
1930s Semi Assessment
According to the 1930s House Manual, “The 1930s witnessed nothing less than the biggest building boom ever seen in the UK (before and since), with around 300,000 new houses being built every year by the middle of the decade, …”. These 1930s houses come in various shapes and styles, most in the well recognisable mock-Tudor style, some in the Continental Modern Movement, and a few with features of the latter in the former; one can find some detached houses but more often a whole road of semi detached pairs either side. I reckon there may be up to 3 million dwellings of that kind in the UK, all with a very similar layout, built from the late 1920s and going well into the 1940ies.
The 1930s Semi Blue Print
The term blue print deserves its name with British housing. There seem to be only a couple of designs in the 1930s semis that were rolled out across the whole country, with only a few variants such as whether the entrance halls are in the middle, next to the party wall, or towards the lateral walls. In addition, one can find variations of features such as the shapes of bay windows with either flat or pitched roofs. Overall, the layout is very practical for a small family, with entrance hall, front room, dining room, kitchen, two large bedrooms, a small spare bedroom and a bathroom as shown below.
Thermal Performance of Surfaces
Any property will lose energy by transferring heat through its external walls and surfaces as well as through uncontrolled air exchange, e.g. draft through doors and windows. The thermal performance of the house is determined by the “weakest link”, i.e. the area with the highest conductivity and largest surface. The first stage in working out a baseline thermal performance is some thermal modelling and analysis of the entire house with an understanding of the types of surfaces, their area and conductivity. Therefore, in addition to the floor plan I am using two different vertical cross sections:
Ideally I would like to make a Finite Element model using a proper 3D CAD drawing, but these tools are not available to me. Instead, and in order to calculate an estimate for the overall thermal performance, I have categorised the most important surfaces that convey heat to the outside as follows.
SW — Solid brick walls
Solid brick walls with apparent bricks externally are used at the bottom of the window bay, but most importantly for the side and rear elevation. Together with the internal plaster, these walls are about 225mm+20mm thick and exhibit the following thermal performance (U-value):
SP — Solid walls with pebble dash
The majority of our front elevation is a pebble-dashed solid brick wall. Together with the internal plaster, these walls are about 20mm + 225mm+20mm thick and show the following thermal performance:
SR — Single brick with render
At one side of the entrance porch as well as on either side of the entrance door only a single brick, or possibly a breeze block, is used with some external render. Together with the internal plaster, these walls are about 20mm + 110mm+20mm thick and show the following thermal performance:
ST — Toppled brick with pebble dash
The window bay above the ground floor window to the first floor window of the front elevation is not very thick, it uses possibly a wooden framework and solid bricks toppled onto their side; the outside is pebble-dashed. Together with the internal plaster, these walls are about 20mm + 65mm+20mm thick and show the following thermal performance:
LP — Ceiling with lath & plaster
As shown in the cross section above, the ceiling has both slanted and flat parts plus a small part in the entrance porch. They are in essence lath & plaster and about 20mm thick with very poor thermal performance:
WD — Wooden door and frame
The entrance door is 40mm solid wood with a wooden frame of various thickness, ranging from 30mm to 60mm, all around, and single glazed windows set on either side and three small windows on top. Assuming an average thickness of 40mm the thermal performance is:
GX — Glazed Windows and doors
By this time, most houses would have double glazed windows and doors fitted. Therefore it is assumed that their performance is at least:
WF —Wooden floor
All ground floor is suspended in a traditional way, 20mm floorboards on joists on top of wall plates being laid in mortar on sleeper walls (with some slate as damp proof material), in turn resting on a cement screed on top of some hardcore and the clay soil. The thermal performance is about:
Baseline Thermal Performance
After measuring all wall surface areas for all types and using the previous values on their thermal resistance, it is possible to calculate the total heating power [Watt] that would be needed to keep the inside of the house at a comfy 21 degrees Celsius for a worst case outside temperature of -10 degrees Celsius (to accommodate some quite cold days I remember). The temperature of the ground is assumed to be +10 degrees Celsius.
As the table shows nearly 50% of the heating power of 14,297W (and the money spent on heating) is lost through the roof. The latter is something that can easily be addressed, together with some small but easy wins that mitigate cold spots and corners where mould can build.
Current Thermal Improvements
Our aspiration was to keep the look of the building as much as possible, e.g. brick walls and pebble dash remain so far as they are.
Loft Insulation
The building regulations nowadays stipulate 250mm rock wool for loft insulation. However, most people like some kind of storage area in the middle part of the loft where it is possible to stand up. Also, quite some heat loss occurs through the slanted part of the ceiling, going into the eaves, where there is only little vertical space. Therefore, I went for the following solution:
After having removed everything from the loft (including the thin layer of old and dusty insulation between the joists), I cleaned and hovered between the joists to create a clean space. Then the following steps were carried out:
- Use 50mm Knauf rock wool slabs, cut into appropriate pieces and slid between rafters in order to add insulation on the sloped ceiling part of the upstairs bedrooms. This leaves some space above the rock wool to breath.
- Install 100mm rock wool between the 4” (100mm) high joists.
- Build a platform in the middle of the loft using 40mm x 150mm timber on top of and perpendicular to the existing joists, because I had some timber left at the time; an alternative is to use a set of plastic stilts.
- Fill the gaps between the new 150mm joists with 150mm rock wool.
- Install loft boards on the 150mm joists to create a new platform.
- On either side of the platform, use 200mm rock wool on top of the 100mm layer between the original joists, perpendicular to the joist.
The new U-values can be estimated for the three different areas as follows, quite an improvement on the original 5.189W/m2K:
Entrance porch ceiling
The bathroom, notably our bath tub is right above the entrance porch. It is very easy to glue some Kingspan/Celotex solid insulation onto the ceiling of the entrance porch and put some marine plywood on top which can be painted on. The thermal performance is an improvement on the original 5.189W/m2K:
Upgrade entrance porch glazing
Having removed all single glazing window panes from the entrance porch, double glazing replacement units were ordered from Go-Glass in Cambridge and fitted. This brought the U-value from 4.8 down to 2.8W/m2K.
Entrance porch bottom wall
On either side of the front door in the entrance porch is a very thin brick wall. It is very easy to glue some Kingspan/Celotex solid insulation on these small surfaces in the entrance porch (i.e. externally) and put some marine plywood on top which can be painted. The thermal performance is an improvement on the original 2.802W/m2K:
Bay Window Insulation
While carrying out some work at the property before moving in, a neighbour drew my attention to the fact that the up-stairs bay is very cold. I then realised that indeed bricks are used on their sides, making the wall as thick as a brick is high (65mm). Therefore I immediately installed 100m rock wool insulation on the inside, before installing the radiator there. The resulting performance is down from the original 3.351 to 0.307W/m2K, a factor of about 10:
Double wooden floor
In theory, it is possible to lift the floor boards and install some insulation between the joists (possibly blocks of 100mm Celotex/Kingspan). I did lift some boards in order to clear out all the rubble that was left there and created moisture bridges but refrained from installing insulation. Instead we simply put down some floor boards on top of the old floor; the new board were made from reclaimed Victorian timber sawn and planed into floorboards by a company in Yorkshire, Ecopine. This improved the insulation by a factor of 2:
A-rated double gazing
Even though the property already had double glazing on most windows and the side entrance, we had all windows and doors (except the front door) replaced with new A-rated uPVC equivalent at about 1.4W/m2K.
Improved Thermal Performance
Putting all the improvements together, accounting again for all the different surfaces, their area and conductivity, we obtained:
As it can be seen by comparing this table with the previous one, the heat loss has dropped from 14,297W to 5,934W, i.e. by about 60% to 40% only. Now the ceilings make little contribution to the heat loss.
Possible Further Improvements
After insulating the loft space, the external walls contribute now 70% with 50% of the losses due to the solid brick walls, in particular the one on the side of the property. What can be done?
Entrance porch side insulation
It is very easy to glue some Kingspan/Celotex solid insulation on the small surface of the entrance porch side wall (i.e. externally) and put some marine plywood on top which can be painted. Performance as above. This knocks off another 100Ws of the total energy expenditure and avoids a cold corner:
Replacing the pebble dash
A reasonable gain in thermal performance could be achieved, if the pebble dash is replaced by insulating render. Removing the current pebble dash gives an allowance of approximately 20mm; it should be possible to make the render a bit thicker, perhaps 25mm, and over the window bay, say 30mm.
A commercially available render is called ThermoPor and its thermal conductivity is comparable to that of Rockwool, around 0.053 W/mK. This type of render does not change the appearance of the property too much, in particular if the pebble dash was already painted. The solid wall and window bay, both pebbled dashed, would obtain the following performance:
The overall improvement is not too much, about 200W less heating power:
External wall insulation
In order to address to heat loss of nearly 70% on external walls significantly, external wall insulation would be needed to get substantial improvement on the thermal performance of the solid walls, e.g. adding 100mm Celotex or Kingspan or equivalent and an thin render on top:
If this external insulation was applied to all solid walls (including the pebble-dashed ones with the pebble dash removed, but no insulating render) the overall thermal performance would be much better (now exposing the glazed surfaces as the worst offenders):
Now the heat loss is down to 2,335W for a very cold day, perhaps down to 1kW for clement winter days. Consequently, the heat generated by appliances and people would nearly be sufficient, producing inadvertently a passive house. However, it would not look the same any more. Is it worth the effort?
References
- Insulation materials and their thermal properties
- U-Value for dummies — an introduction
- Thermal conductivity, R-values and U-values simplified
- Building Regulations and Standards — Energy
- Approved Document L — England
Post-scriptum
Following a discussion with Simon Chung from Sow Space, an Architecture & Planning outfit in Oxford, UK, there is professional software to carry out thermal building analysis such as but not limited to:
- IES (Integrated Environmental Solutions), a software and consultancy company that specializes in building performance analysis,
- PHPP (Passivhaus Planning Package) by the Passive Haus Institute, an independent institute for outstanding energy efficiency in buildings,
- Ecotect Analysis by AutoDesk is an environmental analysis tool that allows designers to simulate building performance from the earliest stages of conceptual design,
- ArchiCAD is an architectural BIM CAD software for handling all common aspects of aesthetics and engineering during the whole design process of the built environment and includes their own in-built EcoDesigner,
- WUFI performs dynamic simulations of coupled heat and moisture transfer and can be used for thermal bridge calculations,
- Sefaira is a software company specializing in cloud-based computing solutions for high-performance building design.
