On July 1, 2019, all rental properties must have adequate insulation.

Tens of thousands of rental properties will not be insulated by the government"s deadline of July 1 next year, insulation companies are warning.

And that could result in a $4000 pay-day for tenants who dob them slack landlords in.

All rentals must have ceiling and under floor insulation by mid-2019 following a law change introduced by former Housing Minister Nick Smith under the last National government.

But Richard Arkinstall from the Insulation Association of New Zealand, which includes Pink Batts' maker Tasman Insulation, said a survey of its members indicated tens of thousands of rentals would not be insulated on time.

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Landlords who fail to make the deadline could end up out of pocket.

"There's a $4000 penalty for landlords when the provisions kick in on July 1," Arkinstall said.

"The part that is not understood is that if the landlord is fined, that $4000 goes to the tenant."

"There's an impetus for the tenant to check," Arkinstall said.

The association's survey of members, which is yet to be released, indicates tens of thousands of rentals won't not be insulated by the deadline.

"In the next 12 months there are more rental homes that need to be insulated than our members can get done by July 2019," Arkinstall said.

It's member companies have insulated between 40,000 and 45,000 houses in the last 2-3 years.

Before July next year, they had capacity to do another 50,000 or so.

But that would leave a shortfall of tens of thousands of rentals, assuming the 2015 estimate of around 180,000 uninsulated rentals was correct.

"The concern for us is that potentially, that leaves about 100,000, give or take, that need to be insulated by July next year," Arkinstall said.

STUFF Jacinda Ardern visits Pink Batts and talks about healthy homes. The Labour government looks like it will push even further than National did on forcing landlords to make their rentals healthy, warm and damp-free.

Some landlords would have done the job themselves, said Andrew King from the Property Investors Federation.

Tenancy Services estimated the cost of getting an installer in to do both ceiling and flood was around $3400 excluding GST for a 96 sqm property.

King said good, professional landlords would already be insulated, he said.

"The clock has been ticking for two years," he said.

King supported compulsory insulation of rentals.

"It should have been done years ago," King said. "A lot of tenants over the past ten years have been asking more and more for insulation."

DAVID WHITE/FAIRFAX NZ Andrew King of the NZ Property Investors Federation is a big supporter of insulation in rental properties.

​Arkinstall warned badly-laid DIY insulation jobs may fail to stand up to scrutiny, if a tenant dobbed their landlord in.

But Arkinstall warned landlords to document their insulation with photos, just in case dishonest tenants played games in a bid to pocket $4000.

There will be no hiding a lack of insulation.

All new tenancy agreements must include an "insulation" statement, Tenancy Services said.

The insulation standards rentals are expected to meet are set out by Tenancy Services in a brochure for landlords.

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