In his latest newsletter, Auckland property investor and mentor Lee Whiley predicts a new wave of leaky "old" homes.

As people rush to insulate their old homes, filling up the ceilings and walls with fibreglass batts, polyester and so on, they may be eliminating ventilation gaps that have kept buildings dry for years.

"Once water gets into bats etc, it's pretty much forever. And the kauri of North Island homes and rimu of the South Island was never treated. It lasts forever but not long when wet."

Architect David Wingate of Wingate + Farquhar agrees it could be a real problem, if people don't allow air movement in homes when installing insulation.

"If the insulation stops the house from breathing, naturally it will retain water. If that affects the insulation, it can make the whole house damp and cold. It's critical to get good air gaps and the correct ratio of insulation in your building."

Wingate adds that, from what he's seen, EECA providers are doing a good job installing insulation correctly. The only concern is when people do it themselves, he says.

Builder Stefan Hulme says home handymen haven't got a clue. "That's why they should use professionals who know what they're doing."

He adds there shouldn't be a problem with old villas because they have high pitched roofs and insulation shouldn't interfere with the airflow.

Another builder, Dave Brown of Taylor & Brown Builders agrees there could be a problem when home handymen install insulation in walls and forget to put building paper against the weatherboards first.

"The same thing that makes it ventilate will bring moisture into the cavity. It will hold water and the framing can rot. The gib will also go mouldy and fall off."

BRANZ had not returned calls for comment.