DIY Doctor

Vapour Barrier or no Vapour Barrier

Postby teddy_amok » Sat Aug 25, 2018 2:46 pm

Hello, I hope you are all well <3

I live in a very old sold wall granite building (approx. 100 year old). The building has 2 floors and a small loft. The bottom floor is owned by a property leasing company and me and the wife are on the 2nd floor.

The building has a solid granite wall exterior, a gap (which runs from sides of walls into loft), and then plaster boarded interior walls. We have no central heating. As you can imagine, the flat is cold.

The loft had badly laid fibreglass insulation. This had been compromised and ruined over the years as holes in the roof had allowed water to penetrate. There was also numerous birds that had at one point made the loft their home.
The roof has now been patched up to fix the leaks. As condensation was an issue in the flat a kitchen and bathroom extractor fan had been fitted. The ducting runs to 2 newly installed pipes int he roof. The loft roof also has roofing nails protruding through the ceiling every couple of inches.

Theres not much in the way of ventilation in the loft. I presume where in the past heat had been escaping into the badly insulted loft in abundance this is what has caused numerous condensation problems in the loft as well as the flat. There are numerous salt build up areas on the roof of the loft.

I have spent a few weeks cleaning up the main area of the loft as well as the awkward skeilings. I have safely disposed of the old ruined fibreglass and am now ready to insulate although I need help ironing out a few reservations first.

Apologies to all as I have a few questions. Hope you don't mind I number them. Im just trying not to forget key questions.

1) - As I am going to insulate the loft, and I dont want the heat to continue to escape into the loft, after I draughtproof the loft (i have also remedied external repointing and etc of walls) should I lay down a vapour barrier between the joists in the loft before laying down insulation on top of it?

2) - Also, as I cannot reach down into the sloped areas of the loft sides (skeilings) to fit normal insulation, should I lay down foil backed, rigid insulation boards as I cannot reach down that far to install vapour barrio sheet.

3) - Is it definitly a Vapour barrior I would need and not a Damp Proof Membrain?

4) - Should I cut individual sections of VP for in between joists or just one big continuous sheet accross all the joists and neatly stapled to fit?

5)- I have noticed that where the roofer insatalled 2 ventillation pipes through the roof for ducting, one of the pipes protrudes into the loft to such a degree where it is almost touching the floor of the loft. Obviously this would make it difficult for me to fit insulation under this ducting pipe. Is it possible for me to trim the pipe a couple of inches to allow insulation underneath or should I just insulate around it?

Thank you for your patience and help.

Kind regards,

Old Insulation

Cleaned Skeiling

Ducting Insulation Pipe

Side View Illustration
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Loft Light attic lighting products

Postby collectors » Sat Sep 01, 2018 11:43 am

Remember a loft needs good air ventilation to avoid condensation. That's why they use breathable membrane under roof tiles & a good vent all the way around the soffit boards, so there is a flow of fresh air.
When a house has wood walls with plastic sheeting behind the plasterboard, its to stop the damp air in the house permuting through the plasterboard into the cavities & creating mould. I have not heard of plastic sheet being laid in between the joists.
If your planning to put insulation in the Skeiling areas, leave a good inch above for some sort of airflow.
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