GRP vs Steel vs Concrete Tank Insulation: Which System Works Best?

Why Substrate Matters for Spray Foam Insulation

The long-term performance of any externally insulated tank system depends on two things: the insulation itself, and the bond between the insulation and the tank wall. Nexseal LE closed-cell spray foam has a well-documented adhesion profile on the three substrates used for commercial water storage in Ireland — GRP (glass-reinforced plastic), steel (welded, galvanised and riveted) and concrete. But each substrate has distinct preparation requirements, movement characteristics and long-term failure modes that must be taken into account at specification stage.

Getting this wrong is the single most common cause of tank insulation failures in the field. A foam layer that is correctly specified thermally but poorly bonded will debond within a few seasonal cycles. A foam layer that is well bonded but coated with the wrong topcoat will degrade under UV or chemical exposure and lose its weather protection, which then compromises the foam beneath. This article goes through each substrate in turn and explains what Duratite specifies and why.

GRP Sectional Tanks

GRP sectional tanks — built up from bolted 1m × 1m panels — are the dominant cold water storage format in commercial buildings across Ireland. They are light, corrosion-resistant, easy to retrofit and widely available. From an insulation point of view, they present a smooth, sometimes waxy gel-coat outer surface that must be properly prepared before spray foam application.

The key preparation steps for GRP are: pressure wash or hand wipe to remove dust, algae and loose material; solvent degrease to remove mould release agents and surface wax; and a light mechanical abrasion (scuffing) of the gel-coat to give the foam a physical key. Done correctly, Nexseal LE forms an excellent bond to GRP and remains bonded through normal thermal cycling for the service life of the tank.

The other factor specific to GRP tanks is panel flexure. Sectional GRP tanks expand and contract slightly with water fill level and temperature. Nexseal LE has sufficient tensile elongation (typically 5–10%) to accommodate this movement without cracking. Rigid board insulation cannot — which is why we see historic jacket-and-board tank insulations fail at panel joints after only a few years.

Welded and Galvanised Steel Tanks

Steel tanks — especially older welded or riveted break tanks in industrial facilities and historic hospital estates — are often the ones that most urgently need insulating, because they are also the ones most prone to condensation, corrosion and winter freeze damage. Steel is an excellent thermal conductor, so an uninsulated steel tank reaches ambient air temperature almost instantly.

Nexseal LE bonds very well to prepared steel. The preparation protocol is more demanding than for GRP: all loose rust, scale and failed paint must be removed, typically by wire brushing or mechanical abrasion to a visual standard equivalent to SSPC-SP2/SP3 (hand or power tool cleaning). Heavy pitting and active corrosion require pre-treatment with a rust-inhibiting primer before foam application. Once bonded, the Nexseal LE layer encapsulates the steel surface and arrests further atmospheric corrosion by excluding oxygen and moisture.

Galvanised steel tanks present a specific challenge: the zinc coating is intentionally passivated and somewhat non-reactive, which can affect adhesion. Duratite specifies a solvent wipe and light abrasion on galvanised surfaces, and in marginal cases a specialist adhesion primer before the foam pass. We've never had a bond failure on galvanised steel when this protocol is followed.

Concrete and Precast Tanks

Reinforced concrete tanks — typically semi-sunken or fully underground — are the simplest substrates for Nexseal LE application. The bond to clean, sound concrete is essentially a mechanical and chemical lock into the surface pores of the concrete. Preparation requires removal of efflorescence (salts leached from the concrete), removal of any free surface moisture (foam will not bond to wet concrete), and cleaning of release oils on new precast tanks.

Concrete tanks benefit from the concrete's own thermal mass, which buffers temperature swings. For that reason, lower Nexseal LE thicknesses (50mm rather than 75mm) can be sufficient on concrete tanks where the installation is sheltered. For exposed above-ground concrete tanks or rooftop systems, 75mm remains the standard specification.

Thermal Expansion and Differential Movement

Every material has a coefficient of thermal expansion. Steel expands at approximately 12 × 10⁻⁶ per °C, concrete at around 10 × 10⁻⁶ per °C, and GRP somewhere between 20 and 30 × 10⁻⁶ per °C depending on the resin system. Nexseal LE itself expands at approximately 70 × 10⁻⁶ per °C. This means the foam expands and contracts more than the substrate as temperatures change — but because Nexseal LE has significant elastic recovery, this differential movement is accommodated within the foam layer without debonding, provided the bond at the substrate interface is sound.

The practical implication is that substrate preparation is non-negotiable. A foam layer bonded to a contaminated surface will tear loose within one or two seasonal cycles as differential expansion stresses the weakest bond. A foam layer bonded to a properly prepared surface will remain attached for the service life of the coating — typically 15+ years with the correct Gaco topcoat.

Topcoat Selection by Substrate

The Gaco topcoat range gives Duratite three distinct options depending on tank substrate, environment and longevity targets:

Comparison Table

Duratite's technical team will match Nexseal LE thickness and Gaco topcoat grade to your tank substrate and environment at survey stage. Email info@duratite.ie.