Garage Converaion Flat roof - Height issues

Hey all,

First time poster so take it easy on me!

Converting a detached garage to a playroom/ office and am having issues with making it building regs compliant (will be notifying council etc) whilst trying to avoid massively compromising ceiling heights.

In terms of U values, the roof needs to achieve .18.

Basically, if I install a traditional “warm”’ flat roof with Kingspan 130mm insulation or similar, the roof buildup from the bottom of the plasterboard to the external surface will be circa 300mm.

Ideally, I would need this to be as close to 225mm as possible so I would end up with a floor to ceiling height of c 2.15m.

What I need is some sort of high density, high performance insulation that does the same as the “off the shelf” products in terms of u value but is only approx half the thickness.

I appreciate if any such product exists it will be more expensive but willing to take the hit.

I’ve tried to source online myself but not having much luck. My roofer hasn’t been much held either if I’m honest...

I think aerogel products give very high performance per thickness, not sure about the price though:
https://www.proctorgroup.com/products/spacetherm

Can you use smaller joists?

Sometimes its possible to have a couple of universal beams 152mm high -fit to divide the roof into thirds.

Then use 95mm x 45mm joists with 120mm insulation cover. You will have to cut the insulation around the steels, but they will still have enough to provide a thermal break and prevent condensation issues.

With conversions, you make best efforts to acheive the u-values, but it's not critical that you meet them exactly if the structure you have to work with makes it unfeasible.

Fill the void with insulation (foil faced PIR is best), leave no gaps, and if possible have a thin 10-20mm layer across the underside but may not be so important if the existing joists are over 200mm.

Thanks all for your replies!

Woody,

is there anywhere in the regs that state it’s down to “best efforts” on conversions of unfeasible?

The "regs" you refer to are the approved documents which are just guides, but there is common mention of "reasonable provision" so things aren't prescriptive. But I think it's table A1 that specificaly mentions the adverse effects of lower ceilings which allows for relaxing the requirements for flat roofs.