Using multifoil under steico insulated roof

[Gus Potter]

2 minutes ago, JamesP said:

Perfect timing from Gus

My timing, spelling,  grammer and choice of language etc is often well below the mark, but thanks for that.

 

12 minutes ago, JamesP said:

We can cope during the day but night was a challenge. Would consider Solar glass and external blinds next time. The AC is great especially  if you have PV.

On 06/09/2025 at 21:33, lookseehear said:

We’re in the final stages of planning our re-roof.

But have a look again at your roof make up, just in case you have missed something fundamental. The last thing you want is you SE turning round and saying, we never signed off on that. 

 

I do like how you are taking the time to get your U values and material build up sorted and it's clear to me that you have put a lot of time and effort into this, I admire you. 

 

If it turns out you need to resolve an SE issue then you will  be able to adjust as you have already put in the hard work.

 

Onwards and upwards, you are on the right track!

 

 

 

 

12 minutes ago, JamesP said:

We can cope during the day but night was a challenge. Would consider Solar glass and external blinds next time. The AC is great especially  if you have PV.

I love your idea of external blinds, easy to maintain, passive over heat protection. 

 

You could retro fit when your glass units start to fail? 

 

I was on holiday recently in Lake Garda and stayed in an "ECO hotel" They made much use of external blinds.. which were not that Architecturally unappealing. Yes it's a different mind set but cf the cost of special coated glass and the inevitable replacement cost then it seems like not a bad deal. 

 

For all.. see when you get fancy glazed units.. the warranty is often 7 years.. so ask if they fail how much is that cf some external blinds, brise soleil for example.

 

21 minutes ago, JamesP said:

Perfect timing from Gus, On top of the 200mm rafters I counter battened 70mm C16 to then fix the Fibre board and add another layer of insulation.

Every external wall and roof had 11mm OSB fixed on the inside before batten and plasterboard. That was our racking layer.

Bet your house is roasty toasty,  a joy to live in and won't be falling down any time soon. A house like yours could still be standing in two hundred years or more if maintained and kept dry. 

 

[lookseehear]

On 07/09/2025 at 21:08, Gus Potter said:

START HERE!

 

You might be as your SE may be relying on the sarking boards to stiffen the roof.

 

Just ask your SE if the sarking you propose is compatible with the building stability design intent. 

 

If your Architect has not picked up on this then you have options, but ask the question now and direct these to the SE,  rather than leaving it to later. If a loft conversion say this could blow the gaff in terms of clearance height when you go up the stair and so on. 

 

If your SE has relied on the boards on top of the rafters to stiffen the roof then it's probable that your Steico boards are not providing enough stiffeness, and then you need to rearrange the roof make up. 

 

I've checked with the SE and no sarking board or 'racking' layer required.

 

I'm thinking about taking the simple route here and going with PIR between and under the rafters. I've been on a few websites (ecotherm, recticel etc) and they seem to indicate that a 50mm ventilation gap isn't required providing a breather membrane is used, but that maybe you need to leave at least 25mm of rafter 'unfilled' and potentially counterbatten as well. Ecotherm calc below showing 'fully filled' with 150mm PIR, with 40mm underneath to get to 0.15, and Recticel only showing 120mm in 150mm rafter.

 

Is going 'unvented' a bit of a risk or are modern breather membranes combined with a good VCL underneath sufficient to ensure no risk of condensation?

 

 

Edited Monday at 17:28 by lookseehear

[lookseehear]

Any thoughts here?

[ab12]

The answer below is based on non direct experience. BTW i'm in a similar position in that I need to get roof insulation done but just haven't decided how its going to be done.

I think the risk with non breathable materials is that if any moisture seeps through any compromise in the air tightness then there will be risk of condensation on the rafters when the moisture reaches the cold side of the insulation. Although breathable materials won't provide a comparable U-value to PIR for the same level of thickness it is considered possibly safer route especially for retro fitting old properties.

Incidently what is the depth of your rafters?

 

[lookseehear]

It’s a brand new roof, so it’s actually less of a retrofit. Rafters are 150mm.

 

i think you’re right that the risk is higher, I just can’t work out how much higher and if it’s allowable by Recticel etc then maybe the risk is mitigated by decent breather membranes and a small amount of ventilation from counter battening and not having the insulation jammed against the breather.

[Nick Laslett]

This is a very good video by Robin Clevett on 2024 BR Warm Roof install. Might be a bit OTT for you requirements.