Advice RE installing insulation

[joelg]

Hi,

First a quick intro. I'm not a builder. Far from it. I'm a bit handy and have been doing all work so far with the advice of a couple of friends. I've never taken on anything even a tenth as big as this project.

I am now at the stage of installing insulation. My rafters are only 75mm, so to leave the 50mm air gap I've gone for 25mm kingspan (well Extratherm actually - Travis perkins' rip off version!) Which is foil backed both sides. I'm also going to put insulated plasterboard over the top to provide more insulation and avoid thermal bridging. Now the dilemma. I've got half way through putting the insulation up and friend 1 says - I need to rip it all down and start again as I've put no plastic membrane up, so if (he reckons all roofs leak even new ones so not if but WHEN) I get a leak, it will come through the tile and bitumen, make the insulation soggy and useless and ruin my nice new boarded ceilings and plaster and decoration etc etc. So he's suggesting staple visqueen between all rafters up against the bitumen just in case of a leak. My issue with this is A. Its a nightmare and B. Surely if there's a leak and there's plastic up, doesn't the water just run down the plastic and ruin somewhere lower down?

Friend 2 says that's a load of b*llocks. He says you only need visqueen in certain circumstances and as I have a double layer of bitumen under tile (rear of house) and slate (front of house), not to mention foil backed insulation, that I have nothing to worry about and to carry on putting the insulation up.

So I don't know of friend one is being overly cautious and taking the job to the extreme, or friend 2 isn't being cautious enough! I don't fancy ripping it all down again as most of it is wedged in tight and it'll be a nightmare to rip down, but then I also don't want in 4 years time to be dealing with rotting rafters, soggy insulation and ruined plasterboard.

Both are doing me a favour offering their advice so I feel bad questioning them.

Any advice would be much appreciated!!

As a quick side note, the roof wasn't ventilated either, so in between each rafter I've bored a 16mm hole at the front and back of the house. Would you say that's big enough to allow sufficient ventilation?

Thank you massively and look forward (ish) to hearing your thoughts.

Joel

 

[geraldthehamster]

You don't need visqueen in that application.

I would consider adding 50mm foiled Kingspan beneath the rafters as well. The 25mm you're putting between won't do much. When I did something similar, I battened the 3x2 rafters out so they were 6x2, so I could put 100mm insulation between and 25mm underneath, which is (or was in 2007) the building regulations standard (we had Building Control involved because of some structural work, but strictly speaking what you're doing is notifiable to them).

I'm not clear where you are drilling these 16mm holes. If you have old-style impervious roofing felt, then you should be providing ventilation at the eaves and at the ridge, if it isn't present, so air can flow.

Cheers
Richard

 

[joelg]

Hi Richard,

You're a star, thanks so much for the help!

I'm not too worried about building regs as Building Control are already involved because of some work I had done putting 2 x rsjs in (done very badly and left dangerous by builders - I won't name and shame them on here as tempting as it is!). BC have said because there was no insulation to begin with, anything we add is considered an improvement so even if it's only the 25mm in between and 50mm beneath the rafters, they're happy with that.

The holes I've drilled are where the external brick wall meets the roof. I was originally hoping to put holes in the wall plate but it's external and not accessible, so this was the next best thing. I'm just hoping 16mm will allow enough airflow. I read somewhere that 50mm gap is advised, so wasnt sure whether to redo the hole to 25mm and drill another one. I've attached some pics (apologies for the poor quality).

Thanks again!

Joel