Our rafters are 225 mm by 47mm. 

The insulation specified is 200 mm  of Kingspan K7 between the rafters (underlayed by 82.5 mm of K18 )

Hence the 25 mm issue.

And the 82.5 mm issue I suspect. Architects are like that aren't they?

 

I have 225mm of rafter to fill: 

• should I leave an air gap? (I bet the answer is No)
• should I fill the gap : 100, 100, 25 and then underlay it all with 60 mm K18 (not 82.5 mm as above)

 

Or do something more intelligent?

 

Which is where you come in.

 

In older designs there'd be eaves and ridge vents, and that 25mm between insulation and sarking felt would be ventilated. But if that 25mm is within the sealed envelope then by all means fill it with insulation. But don't cut down too much on the overlaying insulation. The rafters need a good cover to reduce cold bridging...

Is there a cost saving opportunity in not using K18 insulated pb? Can it be treated as 2 elements?

 

Would perhaps be cheaper but more interesting to do.

Edited April 9, 20188 yr by Ferdinand

Questions ...

 

Do you have a sarking board ..?

Is this flat or sloping ..??

In my 45 degree pitched roof i have

slate

breather membrane 

sarking boards

50mm breather gap with 50mm x 25mm battens atached on either side of the rafters with the 25mm up against the sarking 

50mm calotex foamed and taped on top of the battens 

120mm calotex up to the inside face of the rafters 

25mm calotex full sheets over the rafter ends, fully foamed and taped. 

 

Its a PITA of a job, took for ever and was just awful. 

 

I have 4  roof vents  spread out along the ridge and x2 soffit vents per rafter gap  

 

i am happy that its done, i know its been done  with 100% attention to detail and will work to its maximum potential but would not do it again.  

 

I went with the initial 50mm calotex  because i wanted to be sure that at that layer i had a good seal between the vent gap and all other layers of insulation, i did not want any cold air getting between the layers of insulation. Much easier sealing 50mm with foam and tape than a big thick slab of insulation. 

 

if there was  ever to be a water leak through the slates and somehow past the breather membrane then i wanted a good ventilated gap to keep things dried out as it would be very unlikely to be spotted from the inside. 

 

 

 

1 hour ago, PeterW said:

[...]

Do you have a sarking board ..?

Is this flat or sloping ..??

 

Sloping sarking. 

We have a flat roof too, but one roof at a time will ding dang do for me....

Right so it should be ventilated above the insulation. Batten on edge will stop you pushing the insulation up to the boards. 

7 hours ago, recoveringacademic said:

Our rafters are 225 mm by 47mm. 

• should I leave an air gap? (I bet the answer is No)
• should I fill the gap : 100, 100, 25 and then underlay it all with 60 mm K18 (not 82.5 mm as above)

 

Given the sarking board there should be an air gap.

 

Assuming rafters at 400mm centres 200 + 82.5 ha U approx. 0.093, 200 + 60 has U approx. 0.106. A difference of 0.01 in U value has an energy implication of about 0.5kWh/m².yr

We have sarking but 200mm rafters and no gap. It is ventilated from above the sarking.

 

Putting in three lots of insulation will be unnecesarily expensive. There are three lots of cutting etc. As A_L says 25mm of insulation would make negligible difference.

 

But it may be that yours has been specced for a 25mm gap and 200mm of insulation. The sarking cannot be air tight, it has to breathe, so that gap will not make a difference to air tightness. 

 

BTW your design has specified unnecessarily expensive insulation. K7 and K18 can be changed for Celotex or cheaper Kingspan. This will reduce the insulation level by 5% but the cost considerably.

 

 

 

 

Edited April 10, 20188 yr by AliG

Thanks for your help folks. I feel a conversation with the architect coming on.

 

And it doesn't help that sarking means different things to different people

Would you consider swapping the K7 for mineral wool? It would be much easier to fit and a huge amount cheaper. You could then use a rigid board on the underside to get to the necessary U values and reduce the cold bridging.

I think that sarking is normally 6" x 1" boards with a small gap between for ventilation.

 

It also seems to be sheets of OSB used as an airtight layer over the rafters but I don't know how the ventilation bit works there as I don't think it is very vapour permeable.