Insulation buildup of small pitched roof

[Super_Paulie]

Hi guys. The pitched roof of my extension at the front of the property needs insulation. What's the preferred buildup, see attached. The ceiling/plasterboard is to go flat straight across so that leaves the void above. Would it be best to use 50mm PIR between the rafters (leaving room for ventilation), wool in the void, then PIR between the joists to finish or is this overkill?

 

Or PIR between the joists with 200mm wool layed over the top for sound/heat retention? Any thoughts would be most welcome.

[Redbeard]

I was about to say forget the PIR and go for quilt alone, but that would limit the insulation value at the 'point of the triangle'. You want either a U value of 0.16W/m2K at all points or an average U value of 0.16 for the whole area.

 

You are talking basically 150 PIR and 300 quilt even not allowing for the 'timber fraction' (the proportion of the timber-and-insulation total which is not insulation).

[Super_Paulie]

38 minutes ago, Redbeard said:

I was about to say forget the PIR and go for quilt alone, but that would limit the insulation value at the 'point of the triangle'. You want either a U value of 0.16W/m2K at all points or an average U value of 0.16 for the whole area.

 

You are talking basically 150 PIR and 300 quilt even not allowing for the 'timber fraction' (the proportion of the timber-and-insulation total which is not insulation).

Yeah it's tricky. I was thinking, 50mm between the rafters (red), made airtight. 75mm between the joists, airtight again (green). Then just fill the void "triangle" in-between with as much wool as I can (blue). Any issues with that?

[Redbeard]

Checking I have got my facts right:  This tiny roof void is open all the way from one side to the other (past the porch in the middle, yes? We have already noted that it has ventilation 'in' (at the soffits) but it has no 'out' above that, as it stops on the wall. The only way, then, for that 'in air' to get out is via a 'plenum chamber' at the top of the roof slope against the wall which is open all the way across the whole roof and then exits via soffit vents on the small hips at each end. That way you might actually get adequate cross-ventilation (although I suspect that 'top chamber' might be a little 'under-fed' by the small soffit vents on the hips). You can check cross-sectional areas and I cannot. IIRC it's a C/S area equal to a (some say 15, some say 25) mm slot along the whole length of the eaves.

 

Usual caveats re really tight fitting of the PIR, and v close attention to sealing at jts and perimeters. Air-tight tape and ait-tight foam. (I use Illbruck FM330 as many have mentioned. Expensive but good. (Come to think of it I could maybe apply that to the majority of the materials I use!)

 

Have fun!

[Super_Paulie]

1 hour ago, Redbeard said:

Checking I have got my facts right:  This tiny roof void is open all the way from one side to the other (past the porch in the middle, yes? We have already noted that it has ventilation 'in' (at the soffits) but it has no 'out' above that, as it stops on the wall. The only way, then, for that 'in air' to get out is via a 'plenum chamber' at the top of the roof slope against the wall which is open all the way across the whole roof and then exits via soffit vents on the small hips at each end. That way you might actually get adequate cross-ventilation (although I suspect that 'top chamber' might be a little 'under-fed' by the small soffit vents on the hips). You can check cross-sectional areas and I cannot. IIRC it's a C/S area equal to a (some say 15, some say 25) mm slot along the whole length of the eaves.

 

Usual caveats re really tight fitting of the PIR, and v close attention to sealing at jts and perimeters. Air-tight tape and ait-tight foam. (I use Illbruck FM330 as many have mentioned. Expensive but good. (Come to think of it I could maybe apply that to the majority of the materials I use!)

 

Have fun!

morning fella. 
No, this roof is totally separate to the porch and canopy section. It has the original brick wall at the house side and is block on the new gable side. Im well versed with FM330 and its a hell of a product, i used that when foaming the joins on my rear extension before cutting back and taping.

So this roof is separate to all the others. It has no vents but i have used wedges so i can see daylight between the membrane and the eaves tray every foot along its length, so it has "air in".

Would insulating this in this way be acceptable, im not so sure. PIR foamed and taped in the rafters, fibreglass in the void the PIR foamed and taped into the joists? I worry that any moisture could get trapped in this "inbetween section" or am i worrying about nothing? As the ceiling level will be flush with the rest of the house i need this section to be well insulated as its within the envelope. 

Cheers fella.

[Super_Paulie]

i just got my crayons out.
So basically, can i go with option B is I suppose what im asking, i just dont know if 2 airtight layers with a void inbetween is such a good idea or not. Obviously the more insulation the better, think BC wanted 300mm if going the wool only route.

[Mr Punter]

I think A looks better.  A very well fitted VCL under the joists will prevent warm moist air getting into the roofspace.  Don't ruin it with a load of downlights lashed in.

[Super_Paulie]

Looking at it now I'm at home, I could potentially put 120mm in the 70mm joists and just have the PIR heigher then the joist top. I'd just angle the pir on the end to cover the cavity. Then I can lie as much wool as I can over the top of that?

 

Can also add 50mm directly to the block wall above the joist.

Edited September 22, 2023 by Super_Paulie

[Dave Jones]

i know this is existing but a heads up for newbuild, this would be a fail as the cavity insulation must meet the loft/roof insulation and be continuous.

[Super_Paulie]

53 minutes ago, Dave Jones said:

i know this is existing but a heads up for newbuild, this would be a fail as the cavity insulation must meet the loft/roof insulation and be continuous.

It's about as continuous as I can get it! I can get the pir in the small roof space over the cavity of the main wall. I guess the only place I can't is where the header joist is up against the steel beam.

 

I'm open to any suggestions with this, although I'm hoping to do the job today so any great ideas would be welcome.

[TonyT]

15 hours ago, Super_Paulie said:

Looking at it now I'm at home, I could potentially put 120mm in the 70mm joists and just have the PIR heigher then the joist top. I'd just angle the pir on the end to cover the cavity. Then I can lie as much wool as I can over the top of that?

 

Can also add 50mm directly to the block wall above the joist.

In your situation I would do the same.

ensuring that the pir, is sealed to the timber, and foil taped.

many chance of double sheeting plasterboard?

[Super_Paulie]

5 minutes ago, TonyT said:

In your situation I would do the same.

ensuring that the pir, is sealed to the timber, and foil taped.

many chance of double sheeting plasterboard?

Not unless I do the entire ground floor... it might be a possibility depending on my ceiling height, could very well end up lowering it to meet the main steel which we are leaving exposed. Could happen, but not sure.

[TonyT]

I’m that could be a future option to add insulated plasterboard over the existing to let it ride over the steel

[Super_Paulie]

Yeah, potentially. I was planning on not doing that so that the heat from downstairs will transfer upwards, however I'm not at that stage yet and it could well be the way to go.

 

I guess I'll add in 120mm PIR between the extension roof joists, put in 200mm wool over the top of that, foam and tape and move on. Seems about all I can do here.

[Super_Paulie]

To close this, I went 120mm PIR between the joists, 120mm PIR on top of that, both angled to provide the ventilation void and span the cavity. Then I threw 100mm of wool over the top just because I had it. So 240mm PIR and 100mm wool, which is surely excessive. Got a cold bridge on the underside of the joists and the beam but I'll look at that when it comes to ceiling time.