Is 1920s roof underlay breathable?

[tickletoe]

Hello,
I am looking into insulating a pitched roof in a house built around 1915 - 1920. The roof is in good conditon and water tight with tiles, batons, underlay, board then rafters. No signs of leaks and no damp or rotten boards.

The underlay is about 5mm thick and made of a fine straw or hessian matting, dark brown/black in colour. My question is whether the underlay used in those days would be considered breathable / diffusion open in todays world?

many thanks....

 

[freddiemercurystwin]

no

 

[Possel]

Hello,
I am looking into insulating a pitched roof in a house built around 1915 - 1920. The roof is in good conditon and water tight with tiles, batons, underlay, board then rafters. No signs of leaks and no damp or rotten boards.

The underlay is about 5mm thick and made of a fine straw or hessian matting, dark brown/black in colour. My question is whether the underlay used in those days would be considered breathable / diffusion open in todays world?

many thanks....

I'm surprised that you have underlay from then. I once owned a house built in 1935 and that had no underlay, neither does my parents' house built in 1925. Anyone know when it was generally introduced?

If it's that old and made of those materials, I'm surprised it is still fit to be called "underlay". It has probably deteriorated so much that it IS now breathable!!

 

[datarebal]

It was used way before that time. The OPs is still intact (ish) because its supported by close boarding.
Try taking it off and it'll fall apart getting **** everywhere..

Later on underlay/felt was often an optional extra..

 

[tickletoe]

Thanks for the replies. Yes it is brittle and would probably fall apart if the boards weren't there. But seems to me it's done well over the last 95 years.

What I was looking into is whether I would need the required 50mm air gap between the insulation and the underlay. The insulation manufacturer says it's dependant on the underay's diffusion properties.

But I won't risk it and will leave the gap. The boards are 15mm thick, do you know if the 50mm is between the insultaion and the boards or the insulation and the underlay? Might gain an extra 15mm insulation.

thanks again..

 

[freddiemercurystwin]

It should be a clear air gap of 50mm.