The scheme, which has been running since 2009, has insulated 230,000 houses in its first four years, more than the target of 188,500. But just 26,000 of those homes were rental properties.

The Green Party is now lobbying for funding to be extended when it runs out in 2014.

The scheme covers 33% of the cost of insulation, or up to $1300. If a homeowner or tenant is a community services cardholder, the subsidy increases to 60%.

While the scheme has been popular right across the country, landlords have been noticeably reticent, those involved say.

Mike Underhill of EECA, the Government partner in the scheme, said it was a challenge to appeal to property investors. "We're doing our best but it is an issue."

The Green Party has a bill in the ballot that would introduce minimum energy efficiency standards for rental properties.

Auckland Property Investors Association president David Whitburn acknowledged that landlords had been slow to take up the subsidy offer.

He said some landlords were concerned that installers were upping their prices in line with the subsidy, so there was only a discount if a tenant had a community services card.

"Landlords are savvy and wait until they have a tenant with a CSC in to get a better discount. Some have their own financial issues with properties in small towns or rural areas being vacant, so they don't have the cashflow to afford it. Others I know believe their rental properties are warmer than their own homes, so they don't do it."