It’s official: Straw bale social housing is energy-efficient
Two years after they were designed by members of the Straw Works design team, the strawbale social housing in Martin and Waddington has proven to be an energy-saving success.
Research has been undertaken by North Kesteven Officers to provide evidence of the benefits of straw bale construction.
By measuring the overall energy efficiency of the buildings, heating loss and taking into consideration electricity costs and tenants’ feedback, Officers have been able to prove the high insulation factors of building with straw bales.
Councillor Stewart Ogden, executive member with responsibility for housing strategy said; “North Kesteven’s straw houses are designed to last around 200 years and have so far proven to be low costing and energy-efficient for the tenants. We are dedicated to finding new initiatives in our council homes and working with contractors to always exceed requirements for energy efficiency.”
Overall, straw bale houses have insulation levels which are twice as effective as traditional construction, with the walls on their own providing three times the insulation value of the minimum requirements of the Building Regulations.
On average the properties are using 238 KWh/m2 per annum less energy than that of a traditional, solid fuel heated property.
Council Leader, Councillor Mrs Marion Brighton OBE said; “The district council is proud to have been the first in the country to build houses of this kind; with huge energy saving benefits for the tenants they have proven to be a great success.”
U-VALUES
The U-Value is a measure of the thermal conductivity (heat loss) of a building element measured in watts per square meter. The U-Values achieved on the straw bale houses are shown below compared with the minimum requirements of traditional construction.
| Straw Bale construction | Traditional Construction |
| Straw Bale walls with a U-value of 0.1 | Building Regulations require 0.3 max |
| Highly insulated roof with a U-Value of 0.11 | Building Regulations require 0.25 max |
| Highly Insulated floor with a U-Value of 0.12 | Building Regulations require 0.25 max |
| Triple glazed windows with a U-Value of 1.32 | Building Regulations require 2.2 max |
SAP SCORE
The SAP (Standard Assessment Procedure) score is a measure of the overall energy efficiency of the buildings. It uses a number of criteria to provide a score, such as:
- The space heating system, fuel type, controls etc.
- The building fabric (U-Values, above)
- The water heating system, fuel type, solar heating, cylinder insulation etc.
- The number of low energy light fittings installed.
The actual SAP scores achieved for various types of comparable sized properties are:
| Property Type | SAP Score |
| Straw Bale House at Martin with wood burning stove | 88 |
| Code 4 New build at Branston with Air Source Heat Pump | 82 |
| Well insulated traditional property with solid fuel heating | 57 |
| Building regulations minimum property with solid fuel heating | 73 |
HEATING COSTS
It is notoriously difficult to assess the true cost of heating any property due to the many variances of lifestyle, for example, how long the heating is used each day and the temperature that is suited to the occupants.
The Energy Performance Certificate produced for each property gives an indicative cost based on the SAP rating. There are also national tables published that give average heating costs for various types of property and fuel. From these sources the Council has been able to estimate the likely running costs based on average use.
| Property Type | Energy Use | Lighting Cost | Heating Cost | Hot Water Cost |
| Straw Bale (wood) | 83 KWh/m2per annum | £44 per annum | £700 per annum | £64 per annum |
| Code 4 Sustainable Housing (Air Source Heat Pump) | 111 KWh/m2per annum | £44 per annum | £1160 per annum | £254 per annum |
| Traditional Solid Fuel | 321 kWh/m2per year | £62 per year | £1100 per annum | £209 per year |
| Traditional Gas | 156 kWh/m2per year | £78 per year | £1006 per annum | £105 per year |
The published figures assume where the main fuel is wood, logs are being brought at commercial rates. The council cannot know what tenants are paying for fuel, but some of the tenants have been able to obtain wood to burn at little or no cost.
The Energy Use column gives the best indicator of the amount of heat that should be needed based on the insulation values of the fabric.
Designed by Kuba Wihan and Barbara Jones and built in partnership with Taylor Pearson Construction for North Kesteven Council in 2009/10. Built for £103,000 per house above foundation, excluding balcony and porch.
In this National Building Specification video, Barbara explains the various preparations made before the first bale is secured to the ladder plate and the process of laying the wall begins in earnest.
The role of render as both an aesthetic waterproofing and a strength-giving material is discussed; including experimental evidence from the BRE.
As well as explaining other key considerations, the programme compares the cost of building a typical three-bedroom straw bale house, to that of building using traditional materials. Full details can be found here: https://www.thenbs.com/knowledge/straw-bale-construction
This article is, in part, adapted from the North Kesteven District Council website.