Warm roof insulation

[Kev]

Hi All 

 

Is it possible to use 75mm kingspan above the joist and 50mm below between the joist's i. e will this still comply with building regs? 

[PeterW]

Any reason not to put some between the joists ..??

[Dee J]

If you have a void between two layers of insulation, do you ventilate it? If so do you ventilate to the cold side, with heat loss issues, or the warm side, with condensation issues? Or try to keep the void airtight? Isn't insulation simpler in one thickness?

[ToughButterCup]

You have us all puzzled.

What motivates you to separate 125 mm of insulation in such a way that it would be likely to both cause condensation problems and reduce the effectiveness of the insulation ?

[nod]

If you choose to install a warm room there is no need to ventilate

I chose to you the multi layer foil on top of the joist and 200 mil kingsman between the joists 

Covered with a 15 mil foil back PB

I was restricted to how much build up I could have on top of the joists

But there is nothing to stop you fixing kingsman in top of the joists   

[Mr Punter]

For warm flat roofs, 100% of the insulation should be located above the roof structure on top of the deck.  In practice this does not always happen but you are best to follow the BBA certificate of the insulation manufacturer.

[Temp]

15 hours ago, Kev said:

Hi All 

 

Is it possible to use 75mm kingspan above the joist and 50mm below between the joist's i. e will this still comply with building regs? 

 

So a total of 175mm? 

 

The outside surface of the insulation is cold, the inside warm. In between these two places the temperature transitions between warm and cold. The definition of a "warm roof" construction is that the joists (or rafters) are located near enough to the warm side that condensation doesn't occur on them. If all of the insulation is above the joists then clearly the joists are on the warm side.

 

Your proposal has part of the joist 75mm from the cold outside and 100mm from the warm side. So its actually nearer the cold side than the warm side which isn't good.

 

Ideally you should get a proper condensation risk analysis done. Some insulation suppliers may do one for you.