Mixed roof, pitched & flat what insulation levels apply?

[imsparticus]

Hi, we have an ongoing saga with regards to trying to fit a large (2100x1400mm) skylight into an area of flat roof over piched sides above an existing double story extension. We originally wanted an open shaft through the roof void (approx 1000mm) to the room below but as the construction is trusses this has proved to be too expensive to achieve for the benefits it would yield.

Therefore we are now considering the option of having the same size skylight but leaving the trusses intact and building the shaft (splayed on the sides) around them so they would be visible from the room below.

We have spoken to our local BCO who has said that the insulations levels required would be at current standards rather than the over ceiling rockwool we currently have.
So the current list of questions are:

1) As the roof is of a mixed design i.e. pitched with a flat top which U value should we be aiming for as it seems there is a different requirement for flat roofs and for pitched?

2) I assume the new shaft through the roof void would need to be insulated but again what U value are we trying to achieve for this aspect?

3) The skylight has a U value of 0.6 W/m2k which appears to be well within the requirements for skylights but does the roof taken as a whole have to meet a minimum insulation level or is each separate component assessed individually?

One last thing what types/methods of insulation would be appropriate for each aspect? I know it's a big ask but if you don’t ask..................

Many thanks.

 

[freddiemercurystwin]

The U value required for the flat roof will need to be that of a Flat Roof.
The U value for the pitched roof will need to be that of a Pitched Roof.

The U values for the roof will need to meet the required standards for roofs and the window must meet the required standards for a window, they will not measure the two elements as one.

Question for you though, you will need to insulation the walls of the light shaft as you know. So why the questions about insulating the either the flat or pitched roofs?

I would have thought that insulating the shaft walls with some rigid insulation such as Kingspan or Celotex between the vertical studs + maybe a thin continuous layer on the back of the studs and adding a few extra layers of Rockwool above the ceiling would satisfy Building Control.

BTW you're aware that the new Part L regs start 1st October?

I'll look up the actual U values later if you like, I'm just nipping out for a bit right now.

 

[imsparticus]

Many thanks,
To answer your question, I knew that the roof (both pitched and flat) would need to be insulated but I wasn't sure if they would need to meet differing criteria or if they would be treated as one.

Yes the BCO did mention about the new regs but I don't know what it will mean to our project. Is there some where that you can point me to that lists the differences? Should I try and get my application in quick?

 

[^woody^]

You could average out the heatloss of "the roof" so that the total heatloss of the two roof types together is no worse than treating them individually

The lightwell will have to be insulated to the same standard as a wall next to a cold space - 100mm celotex will do. And if you don't have a room in the roof, then you would just insulate the ceiling and up the lightwell walls, and not insulate the roof

 

[imsparticus]

Many thanks, there is a gap of around 1000mm between the ceiling and the underside of the flat roof so in theory I could have up to 1000mm of rockwool?

 

[^woody^]

Yes ..... if the joists are strong enough

 

[mikric]

As of Oct 1st
Pitched roof /insulation at ceiling level 0.16m2k
Flat roof requires a min 0.18m2k
Sloped roof 0.18m2k
Wall 0.28m2k
The shaft as you call it would be classed the same as a wall
If you have a 1m void 300mm mineral wool will do to achieve 0.16m2k
And the shaft walls will require 120mm Cellotex or similar between the studs or 75mm between and 25mm across the face will do it.

 

[imsparticus]

Many thanks for that, not too bad then. Any idea what the skylight value would need to be after the 1st?

 

[mikric]

Many thanks for that, not too bad then. Any idea what the skylight value would need to be after the 1st?

1.6Wm2k so your well in.

 

[DevilDamo]

I've had a "similar" situation on a recent job I've dealt with. The clients wanted to change the roof type from a crown flat (i.e. pitch roof with flat top, like you have), with a traditional pitched roof as to continue the line of the existing roof. Obviously insulation would be an issue if we were to comply with current standards but the BCO on the job has agreed to match that of the existing house as it's a small area and the works are minor enough to not warrant current/additional insulation and ventilation.