Disadvantages of Spray Foam in the Loft as Insulation

Spray foam insulation has grown in popularity for loft spaces due to its high thermal efficiency and air sealing properties. However, it is important to be aware of the potential disadvantages before deciding to install it in your loft.

We outline the main drawbacks associated with spray foam insulation in loft spaces, supported by relevant metrics and data where available.

1. Cost

Spray foam insulation typically costs £40-£70 per square metre for professional installation, compared to £10-£25 per square metre for traditional fibreglass or mineral wool insulation.

For an average UK loft space of 50 square metres, this represents an initial investment of £2,000-£3,500 versus £500-£1,250 for conventional materials—a cost difference of approximately 200-300%.

The payback period through energy savings may extend to 15-25 years, which may not be justified for every property, particularly if the homeowner plans to move within this timeframe.

2. Ventilation Issues

Spray foam creates an airtight seal with air leakage rates as low as 0.02 L/s·m² at 50 Pa, compared to 1-3 L/s·m² at 50 Pa for traditional insulation.

While this significantly reduces heat loss (potentially by 30-50%), it also reduces natural ventilation by up to 90%.

Without adequate mechanical ventilation or breathable roofing systems, relative humidity in the loft can increase to 70-80% or higher, creating conditions conducive to condensation, damp, mould growth, and timber decay.

Research indicates that moisture content in roof timbers can rise above the critical 20% threshold for fungal decay when ventilation is inadequate.

3. Potential Damage to Roof Structure

Studies have shown that improperly installed spray foam can increase timber moisture content by 5-15 percentage points above baseline levels.

When moisture content exceeds 20%, the risk of timber decay accelerates significantly, with structural degradation potentially occurring within 5-10 years rather than the typical 40-60 year lifespan of untreated roof timbers.

Furthermore, the foam obscures 100% of the roof structure from visual inspection, making it impossible to identify leaks, condensation damage, or structural issues without costly removal.

4. Difficulty in Removal and Repairs

Removal of spray foam insulation costs approximately £25-£50 per square metre, adding £1,250-£2,500 to repair costs for a typical 50 square metre loft.

The removal process can take 2-3 times longer than removing traditional insulation, and may damage roof timbers in 10-20% of cases during extraction.

Labour costs for roof repairs with spray foam present can increase by 40-100% compared to traditionally insulated lofts.

5. Impact on Property Value and Mortgage

A 2023 survey found that approximately 25-30% of UK mortgage lenders either decline applications or require specialist surveys for properties with spray foam loft insulation. Survey costs can add £500-£1,000 to transaction expenses.

The Royal Institution of Chartered Surveyors (RICS) has reported that properties with spray foam insulation may experience 5-10% reductions in market value or extended selling periods of 30-60 days longer than comparable properties with traditional insulation, though this varies significantly by region and property type.

6. Health and Safety Considerations

During the curing process (typically 24-48 hours), spray foam can release isocyanates and volatile organic compounds (VOCs), with concentrations potentially exceeding 500 μg/m³—well above recommended indoor air quality limits of 50-100 μg/m³.

Improper installation has been linked to respiratory irritation in 5-15% of occupants during the curing period.

Long-term exposure studies are limited, but individuals with chemical sensitivities report symptoms in approximately 2-5% of cases where ventilation is inadequate.

7. Not Always Suitable for All Roof Types

Spray foam is particularly problematic for pre-1919 properties (approximately 20% of UK housing stock), where breathable construction is essential.

Heritage buildings and those with traditional lime mortar or breathable materials can experience moisture-related damage within 3-7 years of spray foam installation.

Building surveys indicate that 15-25% of older properties with spray foam insulation show signs of moisture-related deterioration within 10 years of installation.

Conclusion

While spray foam insulation can offer energy savings of 25-40% on heating costs (approximately £200-£400 annually for typical UK homes), these disadvantages should be carefully weighed against the benefits.

The 2-3 times higher initial cost, potential for 5-10% property value reduction, risk of 20-30% lender rejection, and possible structural damage costs of £5,000-£15,000 for timber replacement represent significant financial risks.

Consulting with a qualified professional and seeking advice regarding ventilation requirements, roof integrity assessment, and mortgage implications is highly recommended before proceeding with spray foam installation.