Insulating between rafters and joists in barn conversion bathroom

[vagrantly3893]

We are renovation our bathroom and this room was originally in the barn. The rafters are spaced 90cm apart and are 120mm deep. The ceiling beams are 80cm apart and 15cm deep. There is a breathable membrane under the tiles, and above the ceiling is the cold barn area. There is a 50mm airgap between the felt and the tiles.

 

Previously there was glass wool insulation between rafters and ceiling beams.

 

My plan is to add new 120mm insulation wool between the beams and rafters. Because the roofing felt is breathable, I understand the insulation can be placed directly against the felt. Under the insulation, rafters and beams I will lay a vapor control barrier. Underneath that plasterboard. We'll also be installing an extractor fan in the bathroom.

 

I've attached some pictures and would like some feedback regarding my plan. Is it going to cause any issues with condensation in the future?

[Radian]

2 minutes ago, vagrantly3893 said:

Because the roofing felt is breathable, I understand the insulation can be placed directly against the felt.

You definitely need a second opinion on this. It's certainly always been the case that with rigid insulation a 50mm air gap must be left to transport any water vapour away from the structure - I can't see how mineral wool would be any different. While you may have a vapor barrier above the plasterboard, vapor from the outside can make its way into that area, and if it takes a long time to clear (due to inadequate ventilation) the temperature may drop in the meantime causing interstitial condensation. I think that the objective with ventilated (cold) roofs is to make sure the RH & temperature of the interior voids equalise with outside conditions as rapidly as possible.

[vagrantly3893]

3 minutes ago, Radian said:

While you may have a vapor barrier above the plasterboard, vapor from the outside can make its way into that area,

 

I thought perhaps the breathable felt prevented this by allowing moisture in the air from the heated area through to the cold area, but prevented any water from the cold area coming through?

[Jilly]

I you do need the 50mm ventilation gap. We’ve just done a cold roof like this and the builder was incredibly fussy to get the detailing right as you need to have a vent at the eaves and ridge to get air flow through each bay. You need a draft whistling through to remove any moisture laden air to stop it condensing. You could improve the insulation by putting a layer under the plasterboard, if you have enough head height. 

Edited May 6, 2023 by Jilly

[vagrantly3893]

1 minute ago, Jilly said:

That’s my understanding of it too, we’ve just done a cold roof like this and the builder was incredibly fussy to get the detailing right as you need to have a vent at the eaves and ridge to get air flow through each bay. 

I checked today and it doesn't seem that this part of the roof has any venting. There are no eaves so unless it is accomplished some other way, I don't think the roof is ventilated.

[Jilly]

Is there not a strip vent behind the guttering somewhere?

Is this bit an extension?

Edited May 6, 2023 by Jilly

[Radian]

7 minutes ago, vagrantly3893 said:

 

I thought perhaps the breathable felt prevented this by allowing moisture in the air from the heated area through to the cold area, but prevented any water from the cold area coming through?

It's a breathable membrane so vapor will be breathed in/out along with the air from the outside (uncontrolled) and the inside (controlled but practically impossible to control 100%). The insulation wool is also full of air pockets so the vapor will hang around in there. If it hangs around when the outside temperature drops, it will condense on the timber. The air gap is there to make sure the air doesn't hang around.

[Radian]

1 minute ago, Jilly said:

Is there not a strip vent behind the guttering somewhere?

That could be the case, but I think having a breathable membrane makes this optional. The membrane should be laid with a droop and as such ventilates the space at each overlap (the droop is primarily to prevent water being trapped on the battens).

[vagrantly3893]

3 minutes ago, Radian said:

 

9 minutes ago, Jilly said:

Is there not a strip vent behind the guttering somewhere?

Is this bit an extension?

The guttering is against the wall, so I don't think so. Yes I think this part may have been built after the original building. It is part of the barn.

[Jilly]

Ok…we’ll as I understand things, it should be ventilated, as Radian says I think you need advice. We’ve just done just this and the bays from the main roof (with their 50mm ventilation gap) were made to communicate with the eaves in the flat roof so there is continuous ventilation from ridge to eaves. 
We had bitumous F1 membrane though, (bats). 
We couldn’t put a warm roof in (where you put the insulation over the rafters and so don’t need the ventilation gap) because we didn’t have planning permission to raise the roof height.

 

 

Edited May 6, 2023 by Jilly

[vagrantly3893]

Just now, Jilly said:

Ok…we’ll as I understand things, it should be ventilated, as Radian says I think you need advice. We’ve just done just this and the bays from the main roof (with their 50mm ventilation gap) were made to communicate with the eaves in the flat roof so there is continuous ventilation from ridge to eaves. 
We couldn’t put a warm roof in (where you put the insulation over the rafters and so don’t need the ventilation gap) because we didn’t have planning permission to raise the roof height.

 

 

I know there are also building regulations regarding these things. Are they mandatory or recommended for DIYers? What are the consequences of not doing something like insulation according to regs?

[Jilly]

The Building Regs should have been adhered to by the original builder, whatever they were at the time, as they change over the years.
The risk to your roof is interstitial condensation, irrespective of whether you involve them now. Hopefully @Nickfromwales will come along and tell you whether bathroom refits need Building Control 

[Nickfromwales]

As long as it’s like for like I don’t think it’s notifiable. It’s not a change of purpose. 
Ventilation, and air admittance on the SVP will be the hot topics, and insulation on the external walls. 
If lighting / electrics etc is all new that’s going to be notifiable.

[Dave Jones]

maintain 50mm air gap then fill with PIR and overboard with insulated plasterboard. You are going to lose a bit of head height but at least you wont freeze taking a bath!