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ISO 29471:2008 — Thermal Insulating Products — Dimensional Stability Under Constant Normal Laboratory Conditions
Measuring dimensional change of insulation materials at 23 °C / 50 % RH
Purpose of Dimensional Stability Testing
ISO 29471:2008 specifies a method for determining the dimensional stability of thermal insulating products when exposed to constant normal laboratory conditions of 23 °C and 50 % relative humidity. Dimensional stability — the ability of a material to maintain its original dimensions over time under specified environmental conditions — is a key indicator of product quality and long-term performance reliability.
Insulation boards that exhibit significant dimensional change after installation can cause cracking of render systems, buckling of cladding, or gaps in the thermal envelope — all of which compromise building performance and durability.
The test involves measuring the length, width, and thickness of conditioned specimens, exposing them to the standard laboratory environment for a specified period (typically 24 h, 48 h, or 7 d depending on the product standard), and re-measuring to determine the percentage change in each dimension.
Test Procedure
| Step | Detail |
|---|---|
| Initial measurement | Length, width (per ISO 29465), thickness (per ISO 29466) after initial conditioning |
| Exposure conditions | 23 ± 2 °C, 50 ± 5 % RH for specified duration |
| Re-measurement | Same measurement points as initial; measure immediately after exposure period |
| Calculation | Dimensional change (%) = [(L₂ − L₁) / L₁] × 100 |
Specimens are the full-size product or representative portions, conditioned initially at 23 ± 5 °C for at least 6 hours, then placed in the controlled environment. The number of specimens and exposure duration are specified in the relevant product standard or agreed between parties.
The standard requires measurement at the same marked points before and after exposure — this is essential because dimensional changes can vary across a specimen, particularly in products with non-uniform density or moisture gradients.
Engineering Relevance
Dimensional stability under normal conditions (23 °C / 50 % RH) is the baseline against which more severe exposure conditions are compared (such as those in ISO 29472 for elevated temperature and humidity). Products that fail to maintain dimensions even under mild conditions will inevitably perform worse in real building environments where temperature and humidity fluctuate.
For manufacturers, dimensional stability testing identifies formulation issues such as incomplete polymerisation in foam insulations, residual stress from manufacturing processes, or moisture sensitivity in fibrous products. Early detection enables process adjustment before large quantities of non-stable product are produced.
In multi-layer wall systems where insulation is sandwiched between structural layers, even small dimensional changes (0.5–1 %) can induce stresses that lead to long-term creep, fastener loosening, or serviceability issues. Specifying dimensional stability limits in procurement documents is a prudent engineering practice.
Originally published in 2008, the standard remains current. Users should reference the latest edition of the relevant product standard for specific stability requirements and acceptance criteria.
Understanding the mechanisms behind dimensional stability is important for interpreting test results. In foamed plastic insulations, dimensional changes under constant conditions are primarily driven by two phenomena: diffusion of blowing agents out of the cells (causing gradual shrinkage over weeks or months) and moisture adsorption by the polymer matrix (causing slight expansion). The balance between these competing mechanisms determines whether a product shows net expansion or contraction. For fibrous products, dimensional changes are almost entirely moisture-driven — fibres swell as relative humidity increases, and the binding matrix may relax over time. The 6-hour conditioning period specified in the standard is designed to bring the specimen to moisture equilibrium before the baseline measurement is taken.
Frequently Asked Questions
Q: How long should specimens be exposed to the test conditions?
A: The exposure duration is specified in the product standard — typically 24 h, 48 h, or 7 days. In the absence of a product standard, the duration should be agreed between the parties and stated in the test report.
A: The exposure duration is specified in the product standard — typically 24 h, 48 h, or 7 days. In the absence of a product standard, the duration should be agreed between the parties and stated in the test report.
Q: What is the difference between ISO 29471 and ISO 29472?
A: ISO 29471 tests dimensional stability at constant normal conditions (23 °C / 50 % RH). ISO 29472 tests under specified temperature and humidity conditions (e.g., 70 °C, or 70 °C / 90 % RH), simulating more severe environmental exposures.
A: ISO 29471 tests dimensional stability at constant normal conditions (23 °C / 50 % RH). ISO 29472 tests under specified temperature and humidity conditions (e.g., 70 °C, or 70 °C / 90 % RH), simulating more severe environmental exposures.
Q: Can dimensional change be negative (shrinkage)?
A: Yes — both expansion and contraction are possible depending on the material. Some foams may shrink due to cell gas diffusion, while fibrous products may expand slightly due to moisture adsorption. Both positive and negative changes should be reported.
A: Yes — both expansion and contraction are possible depending on the material. Some foams may shrink due to cell gas diffusion, while fibrous products may expand slightly due to moisture adsorption. Both positive and negative changes should be reported.
Q: Is this test relevant for all insulation types?
A: Yes — but the practical significance varies. Rigid foams are generally stable under normal conditions, so changes > 0.5 % may indicate a quality issue. Fibrous products may show slightly higher variability but this is often within acceptable limits.
A: Yes — but the practical significance varies. Rigid foams are generally stable under normal conditions, so changes > 0.5 % may indicate a quality issue. Fibrous products may show slightly higher variability but this is often within acceptable limits.
