Traditional Scottish slate roof and insulating sarking boards?

[YodhrinForge]

Ahoy. So, I'm about(*touch wood*, the offer's accepted but you never know) to be moving into a late 1800's stone built semi-detached cottage with a traditional Scottish slate roof, ie, slates nailed through felt directly to sarking boards in turn nailed to the rafters, no battens/counter-battens. I'm planning to retrofit the property to radically increase its efficiency and comfort, but that obviously has to be done sensitively to avoid future issues. There's an existing velux conversion of the roof space that appears to have been done inside-out without disturbing the existing roof excessively, but it was done pre-2000 and the insulation appears pretty minimal. I'd like to increase the insulation & thermal mass, but ideally without having to pay for big honking dormers on the rear elevation or stealing too much of the internal space.

 

Given the roof needs a good once over and potentially a complete strip-and-reslate anyway, modern insulating wood-fibre sarking panels such as from Steico are appealing since they would allow the creation of a full warm roof rather than the present room-in-roof construction, but as I've never done this sort of thing before I'm wondering whether that kind of panel is suitable for nailing into? Every example of its use I can find online is in a battened/counter-battened construction. The current exterior dimensions of the roof cannot be significantly altered, both because the house is in a conservation area and because of the somewhat unusual construction(rather than a typical abutting-rectangles approach, it's more like two interlocking L-shapes with one cottage biased to the front and the other the rear - this includes the roof space). 

 

Any advice or experience would be welcome.

[ToughButterCup]

35 minutes ago, YodhrinForge said:

...I'm wondering whether that kind of panel is suitable for nailing into? ...

 

There's so much behind your question .....  Help us with some images, or maybe a sketch drawing please.....

[Russell griffiths]

You don’t nail into the wood fibre insulation, you fix through it, so either your sarking boards or counter batten and batten get fixed through the insulation into the rafters, the wood fibre boards just get trapped in position. 

[YodhrinForge]

8 hours ago, ToughButterCup said:

 

There's so much behind your question .....  Help us with some images, or maybe a sketch drawing please.....

 

Obviously the rafter section is different since the space has already been converted, but that's how traditional roofs are done up here; basically no eaves, slate over felt nailed through directly to the sarking. But it seems Russel has answered my question definitively - if they can't take a nail securely they're not an adequate substitute for the boards in this case. A shame but it is what it is, I'll just have to shrink the internal space a little to fit adequate insulation and put vents in

[Russell griffiths]

You can use the wood fibre insulation, but you put your sarking boards over the top of it. 
look up the term warm roof. 
as long as the little parapet wall is high enough, or you might need to raise it a couple of hundred mm. 

[YodhrinForge]

2 hours ago, Russell griffiths said:

You can use the wood fibre insulation, but you put your sarking boards over the top of it. 
look up the term warm roof. 
as long as the little parapet wall is high enough, or you might need to raise it a couple of hundred mm. 

Aye but as I said in the OP, I can't alter the dimensions of the existing roofing as it's a conservation area, and it would be awkward anyway given the property's shape. I'll just have to stay with room-in-roof and try to boost the thickness internally without losing too much space or introducing moisture issues. Something to chat to the architect about anyway.

 

Thanks folks.

[ProDave]

This was a relevant recent thread worth a look.

 

For the roof they have insulated with wood fibre between the rafters

 

The interesting bit is they have done half the roof with a ventilation gap and half without to see if it is necessary with this type of roof

[sgt_woulds]

Don't discuss this with your architect - it's very rare that they will have the technical knowledge of woodfibre insulation to be able to advise you correctly.

 

Speak directly to the technical department at STEICO - you can send a message via their website.  The STEICO technical team is incredibly busy at the moment and it might take a while for a reply, but they will get back to you as soon as possible.

 

You can also speak to one of the STEICO distribution partners who have their own technical departments; they will be able to help with specification advice, U-values, and compatible components: 

 

Mike Wye                                            01409 281 644                               sales@mikewye.co.uk

 

Back to Earth                                    01392 861 763                               Chris@backtoearth.co.uk

 

Ultimate Insulations                     01786 447 997                               Technical@ultimate-insulation.co.uk

 

Ecomerchant                                   01793 847 445                               info@steicoinsulations.co.uk

 

As stated by others, you cannot fix directly into the boards - you will need battens and counter battens.  You need to maintain a minimum of 40mm ventilation over the insulation to allow it to 'breathe' moisture away - ridge and eaves ventilation is required.

 

This is still the case if you fit the sarking boards directly over the sarking insulation as Russell suggests.

 

It 'might' be possible to specify this as a moisture-restricted construction with the correct build-up of membranes inside and out.  This has been used a couple of times in difficult cases such as mansard roofs where ventilation is hard to arrange, but it does require a WUFI assessment to confirm that the specification won't cause any long-term issues.

 

STEICOuniversal sarking insulation comes in 22mm thickness - so with 50mm of battens the overall roof raise would be 72mm.  This has been approved in some properties in conservation areas by arguing that the raise is only truly visible at the gable ends;  this can sometimes be further disguised with architectural detailing. 

 

Worth discussing with your conservation officer as external insulation, even a tiny amount with produce much better thermal performance, and, in combination with insulation between the rafters creates a warm roof construction that lowers the risk of moisture damage to the rafters.  It helps to create a building that will be more comfortable for the occupants, will be better protected from the elements, (excess summer heat as well as winter cold and rain) and is likely to last long into the future as a result. 

 

N.B STEICO external insulation is treated with a hydrophobic surface coating which provides an extra element of waterproofing, (in most circumstances an external breather membrane is not required).

 

 

[YodhrinForge]

21 hours ago, ProDave said:

This was a relevant recent thread worth a look.

 

For the roof they have insulated with wood fibre between the rafters

 

The interesting bit is they have done half the roof with a ventilation gap and half without to see if it is necessary with this type of roof

Oh nice one thanks man, that does look interesting, I'll give it a watch over lunch.