Minimising Thermal Bridging

[Pappa]

Hi guys

 

I'm trying to design a single story extension and was thinking about roofing structures with a view to maximise insulation.

I've created 5 models:

https://app.sketchup.com/share/tc/europe/RFf1yIra8Sw?stoken=g_uLQpD-6PkI1IdWUdy2VuAq5YQBVi8u8sePl-mi4LEE1gDgtJWcjI6y8nd3o1Lu&source=web

There will be external PIR insulation along the entire wall which I have not included in the model.

 

Model 1 is a traditional pitched roof with insulation at ceiling level. The problem is its almost impossible to remedy the thermal bridging at the wall plate.

 

Model 2 is a traditional warm roof. The problem I have is that there is too much thermal bridging at the parapet walls where the ladder frame is.

 

Model 3 is a variation of the above. Its a hybrid that allows me to enjoy the benefits of a pitched roof and a flat roof.

 

Is Model 4 possible? In theory it is the most thermally efficient as it creates a perfect thermal envelope all around the building. However it relies on the horizontal ceiling joists being hung directly off the vertical wall studs. Is this possible? I'm wondering if I could notch the vertical wall studs and rest the horizontal wall stud in the notch with the usual mechanical fixings and adhesives.

 

Model 5 is a variation of the above. Its a hybrid that allows me to enjoy the benefits of a pitched roof and a flat roof.

 

Thanks in advance

 

Pappa

 

 

[JohnMo]

Model 6

 

Insulation at roof rafter level, make the loft a warm space. No need to ventilate the loft space or insulation to floor except for noise. Floor whole area. Have a ledger board on two walls, span with suitably sized timber on joist hangers.  Ridge beam and cut roof rafters. Mineral wool Insulation between joists and under draw in PIR with joints taped, and or, vapour control and airtight layer taped at wall. Breather membrane above and batons and tiles.

[Pappa]

23 hours ago, JohnMo said:

Model 6

 

Insulation at roof rafter level, make the loft a warm space. No need to ventilate the loft space or insulation to floor except for noise. Floor whole area. Have a ledger board on two walls, span with suitably sized timber on joist hangers.  Ridge beam and cut roof rafters. Mineral wool Insulation between joists and under draw in PIR with joints taped, and or, vapour control and airtight layer taped at wall. Breather membrane above and batons and tiles.

 

Haha - I love your answer. 
To the extent I went  and modelled it on Sketchup, please check the original link.

However, it still suffers from the same problem as Model 1 (traditional pitched roof with insulation at ceiling level)... There is no way to mitigate the thermal bridging at the wall plate.

 

Also, more generally, if you don't need the loft space insulated (which in this case I don't) I just feel like insulation at rafter level achieves the same thing but at a greater cost as it uses more PIR to go along diagonal rafters as opposed to horizontal ceiling joists. The end conclusion is the same either way, the room below is perfectly sealed thermally and hydrostatically. Happy to be corrected.

 

Thanks again

Edited July 27 by Pappa

[Dave Jones]

are you external walls really just stud ?

 

option 1 is perfectly fine as long as the cavity insulation and loft insulation overlap.

[Pappa]

1 hour ago, Dave Jones said:

are you external walls really just stud ?

 

option 1 is perfectly fine as long as the cavity insulation and loft insulation overlap.

 

Thanks for the reply, thats right, its a single stud wall.

There is no cavity for the wall insulation to meet the loft insulation.

As the model shows, there is a massive thermal bridge at the wall plate with no way to insulate over it on the inside or outside.

 

Thanks again

 

P

Edited July 28 by Pappa

[Dave Jones]

if all you got is stud wall the wall plate is least of your issues.

[Pappa]

On 28/07/2024 at 15:37, Dave Jones said:

if all you got is stud wall the wall plate is least of your issues.

 

How so?

The stud thickness and spacing is spec'd by structural engineer so strength won't be an issue.

There will be a rain screen on the external wall. ie. battens and counter buttons to provide a ventilation gap with render board mounted for rain proofing.

[Dave Jones]

1 hour ago, Pappa said:

 

How so?

The stud thickness and spacing is spec'd by structural engineer so strength won't be an issue.

There will be a rain screen on the external wall. ie. battens and counter buttons to provide a ventilation gap with render board mounted for rain proofing.

 

U value ?

[Pappa]

3 hours ago, Dave Jones said:

 

U value ?

 

70mm PIR between studs and 50mm over the studs on the external side. All fastened with tube washers.

Should give me a u value of 0.17 W/m2K

 

Thanks again

 

P