Barn Conversion

[simonr]

I'm new to this forum so apologies in advance for any errors!

 

i'm planning to convert existing barns to our family home and now have planning permission. i'm a structural engineer and have been round block a few times but really don't know enough about latest technology/thinking so will be reading earlier contributions in the weeks to come.

Initially i'm trying to understand best way of insulating warm sloping roofs....i'll be stripping pantiles off 75% of structure (retaining ex rafters) whilst retaining a recent reed thatch on remainder. 

Where stripping pantile roofs i'm thinking insulation over and between rafters (PIR/fibre board combination ?) 

Where retaining thatch (250mm thick) , i'm hoping i can simply add extra insulation between rafters (75mm deep) perhaps with thin insulated plasterboard as i want to retain exposed purlins/principal rafters  

 

i havent read previous threads so apologies if advice repeated elsewhere

 

 

[Temp]

Welcome to the forum.

 

Before you do anything on site have the council said anything about the CIL in your planning permission?

 

https://www.chichester.gov.uk/article/29769/Would-CIL-be-chargeable-on-a-barn-conversion

 

Would CIL be chargeable on a barn conversion?

Potentially, but a change of use for a barn to residential use should not result in CIL liability as long as the barns are permanent usable buildings in lawful use. To be usable, such buildings should be weather tight with four walls, floor and a roof. To be exempt from CIL the barn should have been in lawful use for a continuous 6 months out of the three years prior to planning permission first permitting development. However, any additional new build floor space would be liable for CIL.

 

If you are liable then it is essential you apply for the self build exemption before doing any work. Failure to follow the multi step process to the letter and you become liable, possibly with no way back. One letter or form lost in the post has cost people £lots.

 

1 hour ago, simonr said:

Where stripping pantile roofs i'm thinking insulation over and between rafters (PIR/fibre board combination ?) 

 

If you're an SE you will probably know a lot of this (so sorry in advance)...

 

That's called a "warm roof construction" because the rafters are far enough on the warm side of the insulation to prevent interstitial condensation. Ideally you should aim for as much insulation above the rafters as possible but usually a compromise has to be made. That will also give you nice exposed rafters on the inside if you want. 

 

Typical construction might be:

 

Tiles

Tile Battens

Counter Battens

Vapour Permeable Membrane

Insulation

Rafters with insulation between (possibly insulated plasterboard).

 

There can also be an OSB layer somewhere in the mix to prevent racking. Not always necessary. Seems more common in Scotland?

 

The counter battens lift the tile battens off the membrane allowing any water that is blown under the tiles to run down and not pool above the battens causing them to rot. Typically you need special long fixings to fix the battens to the rafters through the insulation. I gather these can be quite expensive but other can advise better than me. It can be tricky getting them to go in straight and hit the rafters. 

 

Ideally a condensation risk analysis should be done to confirm the ratio of insulation above/between is ok. Some insulation suppliers may do one for free.

 

Many insulation suppliers have apps notes..

https://www.celotex.co.uk/assets/between-and-over-rafters_application-datasheet_nov18.pdf

 

 

 

 

 

 

 

 

[simonr]

appreciate the cil reminder 

I am thinking along similar lines but perhaps using pir board between and  pavatherm board above rafters to increase Decrement delay...a new concept to me!