Comment by building inspector for warm deck roof

[Mr_T25]

We're building a GF bedroom extension (single storey). We have created a warm deck roof with 120mm celotex and then GRP over the top of the structural deck.

As the room is for an elderly parent, I was thinking of adding further insulation (rockwool) in between the 400mm centre ceiling joists. I asked my building inspector if he would have any issues with this and he replied "Additional insulation (Rockwool or similar) can be installed between the joists but this should not leave a void between the two insulation layers. It would therefore be better to fully fill in between the joists and the firings without squashing the insulation too much. To do this you will need more than 100mm of insulation though. As the roof is a ‘warm’ roof, the top of the external walls should be built up or sealed with insulation to the underside of the warm deck roof to minimise any heat loss through the fascia."

Does he say this because of concern that there might be a temperature differential in the void between the top of the rockwool and first layer OSB3 which the 120mm celotex is sitting on? If so, surely the temperature differential wont be that much since the warmth inside will not really be hitting a cold ceiling?

Also, am I wasting money by adding further insulation between the ceiling joists as 120mm celotex on the roof is sufficient? The walls are insulated with 50mm celotex and the concrete floor will have 75-100mm celotex under the screed.

 

[^woody^]

He says it because of the are any voids, there will be potential for air to condense in them.

You won't be wasting money.

 

[chappers]

Does he say this because of concern that there might be a temperature differential in the void between the top of the rockwool and first layer OSB3 which the 120mm celotex is sitting on? If so, surely the temperature differential wont be that much since the warmth inside will not really be hitting a cold ceiling?

Yes your secondary insulation will prevent the bottom of the OSB equalising in temp with the air in the room. He mentions heat loss through the fascias. that should already have been taken care of. The open joist ends should have been insulated to the same degree as the roof deck.
Hybrid roofs are to be avoided for those reasons really and mixing insulation medias might compound the problem.
If you want to do anything I would full fill the voids with celotex and then a vapour barrier under the joists. Whatever you do don't partial fill to the bottom of the joists)
In all honesty though I think you will find the present set up efficient enough. You should really have addressed this by putting more insulation on the deck.
You probably won't be wasting money, but there will be a point at which it won't be cost effective. Also no point mega insulating the roof if the rest of the house is uninsulated

 

[Mr_T25]

If the extension room has a door way and windows which are still open to the elements, will any mineral wool type insulation absorb moisture from the atmosphere or is it still ok to insulate it even though the plasterboard and room wont be air tight for some weeks?

 

[Mr_T25]

Chappers - the rest of the house is already well insulated, I'm just trying to over insulate the extension as it is a bedroom for an elderly parent so the £80 odd that I will spend on minteral wool for a 3m by 7m extension wont be much. The architect stipulated 120mm celotex for a warm deck, I think this good enough for a minimum standard warm deck (worth more than 200mm of mineral wool equivalent I think) and besides, since we have a sloping garden we cant risk having the extension too high.

When you say dont partial fill, There will have to be a bit of a gap between the top of the plasterboard and where the rockwool starts, I presume that is ok? What about with electric cables for lights etc and the LED GU10 housing, is it ok for that to touch or be pushed against the rockwool?

 

[Robbox]

We just had our flat roof dormer re-surfaced in GRP. The guys put Celotex in between the joists as per regs. But could only fit so much to allow for air movement between GRP and Celotex. If I wanted to increase the insulation by fitting insulated PB instead the old stuff, would I need to fit a membrane to the joists before fitting PB? Our neighbour has a new extension, they used a combination of Celotex in between the joists and that silver blanket stuff below that, then reg. PB. Is that an alternative?

 

[geraldthehamster]

Yes your secondary insulation will prevent the bottom of the OSB equalising in temp with the air in the room.

Why does this matter, if the bottom of the OSB is still above the dew point?

 

[tony1851]

Received opinion is that if the insulation has to be split into two layers, and the vcl can't be below both layers, it should be immediately below the insulation layer with the higher reistance.

 

[chappers]

When you say dont partial fill,

Ok to partial fill but fill at the top not the bottom

 

[Mr_T25]

Can the mineral wool insulation be installed in the ceiling joists if the room is still exposed to the elements i.e. no doors or windows to stop cold air coming in? Will it absorb moisture and create more problems?

 

[chappers]

will be fine, its the constant supply of fresh new air from day to day living rather than trapped air in the structure when built that causes the problems.