A bureaucrat has disputed an email claiming she and former environment minister Peter Garrett believed installing batts was "not that hard".
Then-environment department secretary Robyn Kruk says the conversation in question was about the do-it-yourself nature of the industry, rather than the ease of installing insulation batts.
It was a do-it-yourself industry.
The pair also discussed how hopeless they were at renovating, she said.
A royal commission into the troubled scheme was on Wednesday shown an email from another bureaucrat that said: "The Secretary and the Minister compared notes on their personal experience in installing batts! 'not that hard'."
Ms Kruk said she disagreed with the comment.
"The sentiment of that discussion was in effect the installation of batts could be done by anyone," Ms Kruk told the inquiry in Brisbane on Friday.
"It was a do-it-yourself industry."
Ms Kruk also told the inquiry that environment department staffers devising the home insulation scheme worked their "butts off".
But they were up against it because the home insulation program far exceeded the scale of any project the environment department had ever delivered.
The department also lacked project management experience and the appropriate systems to deliver the program, Ms Kruk said.
The inquiry has already heard how environment department staffers were working at 110 per cent capacity before being told to devise the home insulation program.
Ms Kruk said she was worried about the capacity of her department to deliver the scheme.
"It caused me considerable concern," she said.
"I want to stress it's not a criticism of the staff."
The royal commission is investigating what advice the government received about the home insulation program and whether the deaths of four installers could have been avoided.
Queenslanders Matthew Fuller, Rueben Barnes and Mitchell Sweeney, and Marcus Wilson from NSW, died working under the scheme.
The inquiry before commissioner Ian Hanger QC continues.