Can you add insulation to an existing sandwich panel roof?

Consider the condition of the covering alongside its intended remaining service life. Adding insulation to a sound roof is a different undertaking from refurbishing one with widespread defects. In the latter case, replacing the panels may give a more predictable result. Factor in access for repairs and which existing components can be retained.

For a preliminary estimate, use R = d/λ. Suppose the existing roof has a U-value of 0.45 W/(m²·K). Adding 100 mm of insulation with a thermal conductivity of 0.022 W/(m·K) gives an extra resistance of about 4.55 m²·K/W. Adding this to the existing resistance of 1/0.45 and taking the reciprocal gives an indicative U-value of about 0.15 W/(m²·K).

This simplified example leaves out new thermal bridges, fastener effects, roof geometry and the condition of the existing assembly. For the actual design, determine each layer’s properties and apply the relevant corrections. Assess airtightness and the risk of condensation separately, even where the calculated U-value meets the target.

A layer above the existing panels needs suitable bearing surfaces, fixings and a new means of keeping water out. The profiled metal facing may offer only partial support for insulation boards. The design should show how the chosen system carries loads and how the insulation is supported across the troughs.

Check whether fixings can reach the structure and, where bonding is proposed, whether the coating is sound. Metal facings, membranes and waterproofing layers all influence how the assembly dries. Inspect the existing panel joints before relying on the panels as a vapour control layer. Assess the completed roof as one assembly.

Insulation fitted on the inside lowers the temperature of layers on its outer side. Warm, moist indoor air can leak through gaps and condense when it reaches a colder surface. Detail a continuous airtight layer around service routes and at the perimeter. Filling the available space with insulation leaves these junctions unresolved.

Specify the layers, their fixings and the required internal finish. If a ceiling void is proposed, include it explicitly in the design. Check the implications for the fire classification of the roof. In buildings with high humidity or intermittent operation, base the moisture assessment on the actual operating cycle.

The new insulation, secondary framing and roof covering all add weight. Have a structural engineer check the entire load path, including supports and connections. Assess wind uplift and the restraint of the new layers. Even lightweight insulation boards form part of an assembly whose total load must be checked.

Increasing the roof thickness changes levels at the eaves, ridge, walls and service penetrations. Adapt the flashings and drainage to the new section. Check the effective height of waterproofing upstands above the finished surface. Confirm that equipment and designated maintenance routes will remain accessible.

Compare the complete insulation retrofit with the cost of replacing the covering. Include defect repairs, alterations to services, downtime and future maintenance. For each option, specify the target U-value, expected moisture conditions and which existing elements will stay. This puts the costs of the two finished roofs on the same basis.

Protect the work against rain and plan the sequence around the continued use of the building. Check that layers are dry before closing them in. At handover, inspect junctions, the continuity of seals and the fixings against the design. Keep as-built records for subsequent inspections, when much of the original roof will be out of sight.