Part of the BRE Trust

Energy Performance of Solid Walls Funded by BEIS

Project overview

John Riley

Director, Housing Stock performance,

BRE

What is the SWI research project?

What is a solid wall home?

– Found in older homes in the UK (pre-1930s, depending

on region)

– Mix of stretcher and header bricks (compared to

stretcher only cavity walls)

– Approximately one-third of English Homes are solid

walled.

Understanding why solid wall insulation in housing may

not save as much as anticipated

What is the potential for SWI?

Why doesn’t SWI achieve the predicted savings?

Why is there a gap between calculated and measured consumption?

Are the measured & calculated U-

values correct?

Do we understand what

the occupants are doing?

How can we better predict savings?

Can we give energy

surveyors the right tools and

info?

Does SWI perform as expected?

Are there risks with SWI?

Can we identify them?

Can we mitigate them?

Six work packages

1 Literature review

Desk study

2 Existing wall performance

Fieldwork & lab tests

3 Pre-, post-insulation study: Understanding the gap

Fieldwork & occupant survey

4 Measurement and calculation method

Hot-box & field tests, desk study of calculation

5 Unintended consequences

Review of evidence, observations, route-map

6 A system for more accurate U-value estimation

Classification database, product review & competition

What have we found so far?

Existing results (2012 work by BRE)

Wall type

Number of cases

Measured U-values:

mean

W/m²K*

Calculated (theoretical)

U-values:

Mean

W/m²K*

Typical RdSAP U-valuesa

W/m²K

Ratio of

(Mean measured U-value)

(Typical RdSAP

U-value)

Solid wall, standardb 85 1.57

1.90 2.1 0.75

Why there is a large difference between the RdSAP

and calculated values.

Is there a problem with the measurements?

Hot box tests and modelling

+5% to

measured

values

More fieldwork:

In-situ U-values

– Measurement of U-values

• 120 properties

• Two weeks measurement period

• Heat flux plates (transducers)

• High specification data logger

• Floor to ceiling poles to pressure

fix to wall

Solid wall U-values measure lower than currently

assumed (2.1 W/m2K)

On site investigations provide clues to why

this might be the case

Mortar fill

– In standard calculation methods

assumed that all joints are filled.

But full fill is not the usual situation.

– On average:

Space between stretchers are

67% full.

Perpends were 70%.

Frogs

– Frogs are not accounted for in standard

U-value calculations.

– Fieldwork:

When laid frog down it is very

seldom filled.

When laid frog up it is usually

filled.

Split headers and broken bricks

– Standard calculations take no account of broken or

split bricks forming part of the wall.

• Fieldwork

» Around 10% of locations showed some evidence

of broken or split bricks being used.

Moisture content

Standard

assumption

Fieldwork data

Moisture content by

volume

5% ~1% (Brick)

~2% (Mortar)

• Thermal conductivity of the materials in the wall

are strongly dependent upon moisture content

• Brick and mortar samples from the field were

tested

• The walls are drier than have usually been

assumed

Producing a new U-value

– Taken together, these findings can help explain why U-

values are lower than originally thought

• Lower moisture content

• Air gaps between bricks

• Presence of air filled frogs

• Broken bricks and snapped headers

– Taking all of these into account produces calculated U-

values of ~1.7 W/m2K.

Lab work allows us to study these effects in a controlled

environment – NPL hot box

The construction process for the test walls (Test wall 3)

Also investigating the people - Those studied do not

achieve the assumed heating regimes

10% lower

20% lower weekdays

50% lower weekends

Site visits identifying poor practice: Unintended

consequences are not well understood by the industry or

controlled for

Cost minimisation seems to be the main driver in most

cases observed

Next steps

Next steps

– Integration of laboratory and field test results

– Final analysis

– Outputs

– Reporting and conclusions

– Solid wall U-value default database.

– Updating SAP and BREDEM.

– Further work

– On-going monitoring

– Monitoring historical cases

– Further fieldwork

Part of the BRE Trust

Energy Performance of Solid Walls Funded by BEIS

Project overview

John Riley

Director, Housing Stock performance,

BRE