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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Roof insulation https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1506573187 Message started by Drdtkk on Sep 28^th, 2017, 5:33am

Title: Roof insulation
Post by Drdtkk on Sep 28^th, 2017, 5:33am

I have a 1925s terracotta tile roofed house that does not have any sarking under the tiles- Is it possible/safe to put sarking (insulation fabric) inside the rafters and add a layer of foil insulation under it by adding some extraspacing timber?

Title: Re: Roof insulation
Post by woodsmith on Sep 28^th, 2017, 3:45pm

A couple of questions.

Are you trying to insulate the loft space to make a warm room in the loft or inslulate the roof in order to warm the house?

What sort of insulation and sarking are you planning to use?

Title: Re: Roof insulation
Post by Drdtkk on Oct 1^st, 2017, 12:38pm

Hi, I only want to seal the attic space and add insulation so I am open to any suggestions, I'm don't want to take off tiles to add sarking so alternatives from the internal side is what I need really
Thanks
Dominic

Title: Re: Roof insulation
Post by woodsmith on Oct 2^nd, 2017, 11:46am

Sealing the attic is very difficult to do effectively as you would need to be absolutely thorough sealing every gap but the big problem with adding insulation like this is condensation. Unless you have a vapour barrier fitted in the ceilings of the rooms below, warm moist air will permeate into the loft. With a ventilated loft space this is not a problem, but once it is sealed in it wants to condense on the coldest part of the loft, which is often the rafters. You can even get condensation forming inside the timber (interstitial condensation) which rots the timber from the inside out.

It is possible to add insulation to the roof but I think you would be better off using a rigid foil sided insulation board like Kingspan. You would need to ensure that you provided plenty of ventilation on the cold side of the board though, so you may need add ventilation tiles to the ridge and create ventilation at the eaves. Altogether with the cost of the insulation this would be a very expensive project, which is why insulation is normally added to the ceiling and the void is left cold

As for rolls of foil insulation, they come in many forms and I've used a couple but they appear to be next to useless. There are several houses in our village with this type of insulation and they rarely have any frost or snow on the roof in the winter.

Title: Re: Roof insulation
Post by CWatters on Oct 7^th, 2017, 10:51am

If you fit a membrane from the inside there is a risk that any water that gets through the tiles ( eg wind blown snow) will become trapped against the rafters by the membrane causing the rafter to rot.

Attics/lofts of that age are generally meant to be well ventilated to remove water vapour that gets in so best put any additional insulation on the floor of the loft.

If you are doing a proper loft conversion then foil type insulation might not be enough to meet the building regs. You would probably need to fit rigid insulation between the rafters leaving a 50mm ventilated void between the insulation and the tiles. Then fit more insulation below the rafters, then a vapour barrier and plasterboard.

If the tiles aren't in great condition then consider re-roofing when doing the loft conversion.