Insulating raised tie trusses

[willraymond]

Evening everyone,

 

My architect has specified 450mm rockwool insulation in my raised tie trusses, the sloping section of the ceiling has 120mm PIR between (147mm thickness timbers) and 45mm PIR over the top. I did look at the comparison of PIR between the flat timbers and below but just couldnt get the same U value out of PIR vs the rockwool. 

 

However I cannot understand how to insulate quite a small area that certainly cant be crawled through. I havent got the exact measurements to hand but from the bottom of the rafter to the ridge is no more than 600mm so its almost a full fill with 450mm in there. Going to have to be very careful to not let the rockwool touch the underside of the membrane.

[ProDave]

How far into the build are you?  or just at design stage?

 

I may start sounding like a stuck record but the difficulty in insulating, ventilating and making air tight this exact situation is why I am completely sold on the idea of make ANY room in roof situation with a WARM roof, that means insulation above rafters and possibly in between as well.  It is SO much easier.  But it has to be planned for at design stage.

[willraymond]

Unfortunately we were already beyond that point. We were restricted to a 6m ridge height so a warm roof would have reduced our already fairly restricted ceiling heights although I was pushing for it in the design stages. The higher ceiling heights won the argument in exchange for this headache now!

 

also just checked the truss drawings, underside of tie to ridge is 915mm so slightly more than I estimated

Edited March 6, 2023 by willraymond
Added sizes

[ProDave]

19 minutes ago, willraymond said:

Unfortunately we were already beyond that point. We were restricted to a 6m ridge height so a warm roof would have reduced our already fairly restricted ceiling heights although I was pushing for it in the design stages. The higher ceiling heights won the argument in exchange for this headache now!

No.  It would have changed the roof construction but some of the insulation would have been above the rafters and some in between so no reason to be any thicker overall, just different.

 

there is almost a case (again if not too late) to detail the insulation above the raised ties from above, before the tiles go on.  At least take the PIR between the rafters ALL the way to the ridge, (leaving the ventilation gap) and then fit the rest of the insulation from below.

[Iceverge]

Groan... pity you didn't post earlier. 

 

An airtight membrane underneath the rafters and ventilation above the breather membrane with pumped insulation in between would have been simple in comparison. 

 

None the less we are where we are and a good solution can still be had I'm sure. 

 

Apply the breather membrane to the lower  portion of the roof (over the PIR) and batten for tiles. 

 

Put one or two flat timbers longitudinally along the top of the rafter ties to hold up the insulation. 

 

On a dry day with roll out 350mm of rockwool over the top of the battens. 

 

 

Put some of these over the rafters to maintain ventilation .

 

Working both sides concurrently apply membrane to the rafter bays as you go to protect the insulation from rain. You can roll it out one rafter at a time. 

 

Finally push up the remaining 100mm (or whatever is needed to fill the rafters) from below. 

 

Apply a vapour barrier /Airtightness layer below the 45mm pir and rockwool. Then 20mm batten to provide a service cavity and finally plasterboard.

 

Edited March 7, 2023 by Iceverge

[Dave Jones]

11 hours ago, ProDave said:

How far into the build are you?  or just at design stage?

 

I may start sounding like a stuck record but the difficulty in insulating, ventilating and making air tight this exact situation is why I am completely sold on the idea of make ANY room in roof situation with a WARM roof, that means insulation above rafters and possibly in between as well.  It is SO much easier.  But it has to be planned for at design stage.

 

and looks crap on a pitched roof. Not acceptable for a lot of builds in conservation areas. 

[Dave Jones]

10 hours ago, willraymond said:

Unfortunately we were already beyond that point. We were restricted to a 6m ridge height so a warm roof would have reduced our already fairly restricted ceiling heights although I was pushing for it in the design stages. The higher ceiling heights won the argument in exchange for this headache now!

 

also just checked the truss drawings, underside of tie to ridge is 915mm so slightly more than I estimated

 

the way that is detailed will fail building regs, the roof insulation needs to be brought down over the internal wall insulation to make a continues barrier. your wall plate is now a massive cold bridge.

 

I wouldn't have bothered with PIR and instead stuffed as much rockwool into the eaves (after fitting the eaves ventilation trays). You will get a better U value for much less money and its easier to do.

 

Warm roofs have their place where the room has to be habitable but are expensive and labour intensive to get right with correct vapour barriers etc.

[ProDave]

55 minutes ago, Dave Jones said:

 

and looks crap on a pitched roof. Not acceptable for a lot of builds in conservation areas. 

Making it a warm roof makes no difference to the look of the finished roof, so completely irellevant to conservation areas.

[ProDave]

49 minutes ago, Dave Jones said:

 

Warm roofs have their place where the room has to be habitable but are expensive and labour intensive to get right with correct vapour barriers etc.

We must agree to disagree.  I found detailing a warm roof to be air tight and well insulated without cold bridging far easier than the previous house I built with a cold roof.  Both room in roof, that is the situation we are discussing.

[Sparrowhawk]

12 hours ago, willraymond said:

@willraymond Do you have a photo of the inside, showing where a rafter (with PIR either side) rests on the wall plate? I find that to be a weak spot re airtightness and currently thinking about how to improve it in our house. In your picture I see foam going up the sides of each rafter but not underneath.

[Dave Jones]

2 hours ago, ProDave said:

Making it a warm roof makes no difference to the look of the finished roof, so completely irellevant to conservation areas.

 

massive soffets, look at the size of those trusses. 75mm would have been plenty.

[Dave Jones]

2 hours ago, ProDave said:

  Both room in roof, that is the situation we are discussing.

 

not in the pictures, unless its a  hobbit.

[willraymond]

Unfortunately that photo was from a while ago and has since been fully slated, guttering on and the scaffold has been dropped.

 

there is an open eaves detail that I need to sort out so no soffits but was fully aware I needed to pack rockwool into the space above the cavity while maintaining the air gap. Annoyingly the roofers didn’t use the eaves vent tray as shown above but did fit over fascia vents, which do nothing as they only vent the space up to the first batten and not into the roof space. So the only ventilation the roof will have is under the eaves. It will (eventually) be clad in timber weatherboarding so planning on putting a 25-50mm slot at the top and putting some mesh over to stop insects/birds.

 

My first new build project so this is all new to me and assumed that all the trades would do things the proper way. But that’s definitely not turned out to be the case already