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Detached House, Perth

Monitoring of fabric performance and indoor air quality following foam insulation install


The team at the Scottish Energy Centre, part of Edinburgh Napier University, were asked to conduct thermal performance and indoor air quality monitoring work for the refurbishment of a detached two-storey dwelling in Perth. The work consisted of increasing the thermal performance of the walls.

These were built with:

  • an external roughcast, block outer layer,
  • a 50mm void,
  • and a timber frame structure consisting of
    • building paper layer,
    • OSB sheathing board,
    • 95mm timber studs and
    • a plasterboard finish.


The timber frame is originally un-insulated and therefore belongs to 2% of pre-1982 timber frame housing stock in Scotland. Properties of this nature have been labelled as ‘hard to treat’ by the Scottish Government as they are dwellings that pose difficulties in improving their thermal performance. There are benefits to insulating walls of this type, both to alleviate thermal conductance and ventilation heat losses. Cavities between the masonry block wall and the timber frame should remain un-insulated in order to mitigate moisture transfusion.


The insulation product used is a kiln-dried urea-formaldehyde (UF) resin, with a lighter and less dense consistency both in its liquid form and its completed curing process. These qualities provide the ability for the foam to flow and expand easily into a cavity and cover the totality of a contained volume. In appearance the insulation in its liquid form is comparable to shaving foam while in its cured form it retains a light pumice porous consistency.  The foam is injected with a hose in between the timber frame supports (stud and noggins).  Two inch holes are drilled into the plasterboard to manoeuvre the hose into the cavity slowly releasing the foam until each timber frame cavity is fully filled. 

Detached House - Perth (ENU)

Perforation of 2" holes © Edinburgh Napier University

One of the objectives of the study was to both qualitatively and quantitatively evaluate changes in the wall thermal response. The use of in-situ measurements and surveys facilitated this work. These include the measurement of heat flux and air permeability tests which gave performance figures before and after the interventions. Equally the use of infrared thermography was key to the study as thermal deficiencies were able to be observed before and after the work was done.

As a precautionary study, the use of modelling software was used to observe if the interventions would impose a level of condensation build up in the layers. This was latterly measured in-situ with a six month hygrothermal study using temperature and humidity probes embedded into the walls. It was also important to measure the indoor air quality of the property. Particular attention was made on formaldehyde concentration as exposures through inhalation and indoor concentrations may be high enough to cause adverse health effects. This pollutant commonly originates from fabrics, carpets, aerosols, or other chemically produced materials.   Many urea-formaldehyde (UF) insulation products contain isocyanates to synthesize polymethylene which are known to raise severe toxicity issues.


Measurement of the acoustic performance of the insulation also took place. The property was tested for in-situ façade sound insulation and latterly tested in a laboratory for the effects of reverberation.

Additional to the study was the monitoring of indoor air quality consisting of temperature, relative humidity and formaldehyde concentration. Equally, it was vital to record baseline in addition to actual figures during installation and the curing process. According to distributors and manufacturers of the product, kiln-drying the UF removes free formaldehyde and stabilizes the resin into a dry powder, hence reducing any environmental and health implications


Pre- and post-intervention results were compared with each other providing an insight into thermal benefits and air quality reassurances.

The air permeability tests showed improvements from a base-line figure of 6.0m3/hm2@50Pa to just above 5.0m3/hm2@50Pa.

In-situ U-values gave an improvement of >60% from 1.06W/m2K to 0.38W/m2K.

Indoor air quality remained consistent and below hazardous levels of ≥0.36 mg/m3 except at high occupancy occurrences, for example during gatherings where concentrations appeared to rise for its duration.

On a more qualitative measure, infra-red thermography showed insulation consistency and improvements on minimal heat loss areas.


Post-intervention highlights the benefit of insulation in the cavity. The insulation pumped into a 95mm timber frame allows the building to achieve a 60% reduction in thermal transmission which benefits the internal thermal comfort of the property and helps to reduce the energy demand. It is hoped that this saving in energy demand will help to aid as a pay-back to the investment of capital and labour costs through the decrease in energy bills.

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