PHOTO: SUPPLIED Forty Tenancy Tribunal complaints over insulation were taken in the first four days of a new law.

Forty complaints have been taken to the Tenancy Tribunal since a new law forcing landlords to insulate their properties kicked in on Monday.

The July 1 deadline has been on its way for three years since the last National Government passed a law change mandating that all rental properties must have underfloor and overhead insulation, unless installing it was not practical.

The law lets tenants claim up to $4000 from their landlords if they do not comply.

But despite the long lead-in time, the insulation industry warned in recent months that tens of thousands of rentals remained uninsulated.