How long do insulation batts really last?

Fletcher Insulation, the Pink Batts manufacturer, answers the question “Can insulation be compressed to fit into a smaller frame?” on its own FAQ page with one line: “any compression will result in a reduction in performance”.

A batt is a blanket of fine glass fibres with air trapped between them, and the trapped air is what slows the heat. Squash a 165 mm R3.5 batt to half its height and you have roughly half the insulation because you have pushed out half the trapped air. From below, the ceiling looks the same, and the house runs hotter in summer and colder in winter until someone climbs up and looks.

So does a batt wear out with age? A 2017 laboratory test on old batts says thickness is the whole story. Eight wall batts taken out of American houses after 31 to 54 years measured an average of 95.5% of the R-value on their label, and they had kept their thickness. The lab wrote that the effect of ageing “cannot be inferred” because nobody knew what the batts measured on the day they went in. A batt that holds its thickness holds its R-value, and the batt in your ceiling has had decades of possums, tradies and leaks to lose it. Our R-value depth tables give the thickness each R-value needs.

What the documents say about storage and recovery, with the page and clause beside each line:

• BRANZ appraisals for Pink Batts, Knauf glasswool and Bradford Gold all say glasswool batts are “sensitive to the length of time they are stored under compression packaging” and that a batt which does not recover its thickness “may not achieve the stated thermal resistance (R-value)” (Appraisals 238, 648 and 301, page 3, clause 5.2 in each; 301 reads “stored in compression packaging” and “thermal performance (R-value)”). Packs “must be released from the packaging and allowed to re-loft prior to installation”, and the time to loft “will depend upon the length of time the product has been packaged and stored” (238 clause 15.3; 648 and 301 clause 16.3).
• Fletcher (Pink Batts): “Product must be allowed to fully recover before use (cannot be used crushed)” (TDS Pink Batts Ceiling, Rev 7, “Product limitations of use”). “Product must remain dry during transportation, storage, installation and usage”, and the warranty excludes damage “as a result of handling, storage or installation” (warranty, page 1 “Installation” and page 2 clause 6(b)).
• Knauf (Earthwool): batts must be “transported and stored in dry conditions at all times between purchase and installation and without bearing the weight of other materials”, and the warranty excludes damage from “storage, shipping, handling or installation of the Product in an improper manner” (Earthwool Glasswool Warranty, sections 3.B and 4(a)). The instructions say “Don’t take the insulation out of the packaging until you’re in a position and ready to install” (Installation Instructions, STEP 2).
• Bradford (CSR): “Store in the original packaging in a cool, dry area” and “Avoid packaging being stored under UV light (direct sunlight) for long periods” (Bradford Gold Ceiling Batts Product Technical Statement, Issue D, page 1).
• What the Fletcher, Knauf and Bradford documents we read leave out: a maximum time in the pack, a recovery time in hours or days, and a stacking height.

A batt that stays flat is paid for at the full R-value on the pack and delivers less, which is why the next two questions, how long and how old, matter to your power bill.

Both sheets tie the six months to the product’s “initial performance”, which is the thickness and R-value it recovers to when the pack is opened. Neither sheet mentions the warranty, and neither gives a reason or a test behind the number. It is two manufacturers’ recommendation for their own products, not an industry rule.

So are the Australian manufacturers doing something different, and does anyone track it? The Fletcher, Knauf and Bradford documents we read set no time limit in the pack and describe no packaging technology, no stock rotation, no batch tracking and no storage rule for the shops that sell their batts. The New Zealand sheets say six months. The Australian sheets say nothing about months, so the question is yours to ask before you buy.

• Ask the supplier: when was this batch made, and does the brand set a maximum time in the pack?

One fibreglass manufacturer has measured what humidity does to a packed product. Owens Corning’s own patent application reports fresh duct insulation held in wrapped bundles at 23°C and 92% humidity for 21 days, which recovered 14.6% to 20% less thickness than the same product stored dry, while the dry control bundles lost only 1.2% to 3.3%. That is duct insulation and recovered thickness, not ceiling batts and not R-value, and it was humidity in the pack, not time alone, that cost the thickness. In a Queensland shed in February, humidity is the condition the pack sits in.

The Australian Standard’s label rule asks for a batch code, a declared R-value and a thickness, and a manufacture date is not on the list. AS/NZS 4859.1:2018, Table 3.1, requires “batch identification or other traceability information” on every pack. None of the Fletcher, Knauf or Bradford documents we read carries a key to its batch codes, so the code on the wrapper tells you the batch and nothing else. Fletcher’s consumer warranty asks for “written proof of the date of purchase” when you claim (Rev 2, 29 Apr 2024, page 2, clause 7(c)), which is the day you bought the batts, not the day they were made.

Why does that matter to you? Because the appraisals of these same brands say time in the pack can cost the R-value, and a batt that stays flat is paid for at full R-value and delivers less. Ask the supplier when that batch was made. If they can’t tell you, there’s no way of knowing how long it has sat in the pack.

The same article modelled smaller gaps and found that “4 mm gaps result in 12-15% lower system R-values” (page 69). Four millimetres is the thickness of two coins. BRANZ wrote that with 16 mm gaps the R2.8 batt behaved as if it were “half its nominal value”. Those were wall panels, where the batt is held between the linings and fits tightest, so a ceiling is a different test and the figures above are the wall’s, not the roof’s.

In one New Zealand ceiling, BRANZ measured R1.5 with 5 mm gaps open at some joins between sections, and R3.5 once the gaps were closed. One house, so an example, not a rule.

The report itself is not online and Fletcher’s page gives no method, product or conditions, so the 20% stands as Fletcher’s own figure from a report it paid for. It is the gentlest of the gap figures on this page.

A Canadian government laboratory measured 25 to 33% less thermal performance from full-size frame walls with a batt installation defect covering 6% of the batt area, at the largest gaps and temperature differences tested. Walls, not ceilings.

Gaps were the most common roof-insulation defect the BRANZ inspectors found (survey below), and our guide on whether insulation gaps matter shows where they hide.

How common is all this? BRANZ inspectors found more than half of New Zealand houses surveyed had at least one defect in their roof-space insulation, and gaps were the most common, in around 25% of houses (BRANZ, Warmer, drier homes, 2019, page 4; all insulation types). BRANZ inspected; it did not define a defect, and it measured no gap size and no R-value lost.

Every lifespan figure on this page assumes nothing has been living in the batts. In the roofs I open, that is a big assumption.

Our rodent damage guide shows what that looks like from inside a roof. A batt that rodents have been through is a batt to replace, whatever its age, because the thickness has gone and the smell comes down through the ceiling in summer.

Our premium cellulose insulation goes in at about 35 kg/m³, two to four times as dense as most glasswool batts (CSIRO lists glass fibre batts at 8 to 16 kg/m³). At that density our premium cellulose insulation resists air moving through it, so it doesn’t lose performance to air currents in the roof the way loose fibreglass can, and your home holds its temperature through extreme summers and freezing winters. A glasswool batt holds its shape with far less fibre, so the batt squashes the moment anything sits on it, while our premium cellulose insulation is pumped in as a dense mat of fibres that grip each other, so it holds its depth and keeps working.

The other difference is joins. Our premium cellulose insulation is pumped in as one continuous layer around every joist, pipe and wire, so there are no joins to open up and no edges to leave short, and the gap losses measured above have nowhere to start. Because the fibres grip each other, the layer stays where we put it when the wind comes through the eaves, which is what the video beside this paragraph shows on a finished job.

We allow for a little settling when we pump it in. Our settling guide has the longer answer.

R-values for our premium cellulose insulation are worked out from CSIRO’s published figure for loose-fill cellulose (about R2.5 per 100 mm at 35 kg/m³), applied to the settled depth, so the R-value on your certificate is one you can check yourself with a ruler. That figure has a long history at our factory.