zoe61 Posted October 12, 2018 Share Posted October 12, 2018 So I'm in the deep end here frantically trying to learn as much about roofs as possible in a short space of time. I have a joiner and roofer lined up (both close family friends), very good and reasonably priced hence using them, but old fashioned in there methods and knowldege and not that helpful at communicating the finer details so I'm trying to ensure I have a good solution and that they wont miss anything out. I'm swayed towards a cold roof. Reason being height build up. At the moment I'm proposing the following: Slate tiles Battens (do I need counter battens as well, also how do you determine thickness of these?) TLX Gold or Actic Hybrid R (thoughts welcome?) 150mm rafters at 400 centers with a 50mm air gap followed by 100mm insulation (ideally PIR- but dreading the cutting so maybe go for earthwool- thoughts again please?) 50mm PIR Internally 25mm battens Plasterboard. Does this sound like a reasonable solution? Any better alternatives please? Am I correct in saying I don't need any extra venting? When it comes to the wall plate is there any special detail required here? And at the ridge do I just need to lay the following https://www.roofingsuperstore.co.uk/product/cembrit-ridge-vent-roll-5m.html Thanks Link to comment Share on other sites More sharing options...
PeterW Posted October 12, 2018 Share Posted October 12, 2018 So if you go earthwool over PIR between the battens you won’t meet building regs for the roof uValue with just 100mm - would need to be double. On the multifoils - and this is my personal opinion - they are a waste of money and pointless at the point you are looking to install. They work as an airtight layer inside but that’s it. I would go with the following slate batten standard breather membrane 150 joists 120mm PIR in two layers 50mm PIR internal layer taped as VCL counterbatten plasterboard Its also a warm roof build up too... Link to comment Share on other sites More sharing options...
Declan52 Posted October 12, 2018 Share Posted October 12, 2018 If you go with a breathable felt you don't need a 50mm gap. You can get the higher density earthwool that is better for this type of job. You can fully fill the gap if you counter batten the roof. https://www.knaufinsulation.co.uk/products/earthwool-frametherm-roll-32 Link to comment Share on other sites More sharing options...
the_r_sole Posted October 12, 2018 Share Posted October 12, 2018 (edited) . Edited September 26, 2019 by the_r_sole Link to comment Share on other sites More sharing options...
zoe61 Posted October 12, 2018 Author Share Posted October 12, 2018 38 minutes ago, PeterW said: So if you go earthwool over PIR between the battens you won’t meet building regs for the roof uValue with just 100mm - would need to be double. On the multifoils - and this is my personal opinion - they are a waste of money and pointless at the point you are looking to install. They work as an airtight layer inside but that’s it. I would go with the following slate batten standard breather membrane 150 joists 120mm PIR in two layers 50mm PIR internal layer taped as VCL counterbatten plasterboard Its also a warm roof build up too... Am I correct in saying Building regs state Roof = 0.20W/m². If so then when I input the values into the Simple U-Value Calculator I get a U-value of 0.15- have I done something wrong? You state 120mm PIR in two layers is this in between the rafters? Seems like a really horrible job Link to comment Share on other sites More sharing options...
zoe61 Posted October 12, 2018 Author Share Posted October 12, 2018 30 minutes ago, the_r_sole said: Who is sizing the roof structure? 150mm seems pretty light.... the way to size the insulation is to know what u-value you are aiming for - off the top of my head we usually go with 180 total depth of pir in room in the roof type Structural Engineer. She actually sized as 125, I increased it! Link to comment Share on other sites More sharing options...
ADLIan Posted October 12, 2018 Share Posted October 12, 2018 Is this new build house or extension? For new build expect U=0.15 or better. For extensions BR require 0.18 or better +1 to most of the above, especially multi foil! I would not bother with Frametherm 32 (extra cost for minimal improvement on U-val especially timbers at 400mm c/c) - go for 35 version. Fully fill the rafter space with this, breather membrane, counterbatten, batten & slate with 50mm PUR under rafter. Note this is not a warm roof. It gives a warm roof space but to be a warm roof construction the majority of the insulation must be above the rafters. Link to comment Share on other sites More sharing options...
A_L Posted October 12, 2018 Share Posted October 12, 2018 24 minutes ago, zoe61 said: Am I correct in saying Building regs state Roof = 0.20W/m². If so then when I input the values into the Simple U-Value Calculator I get a U-value of 0.15- have I done something wrong? Not necessarily, 0.15 is better than 0.2, but any serious calculation of U-value must include the timber present in layer three. With at least a 15% timber fraction it will make a significant difference. Link to comment Share on other sites More sharing options...
Cpd Posted October 12, 2018 Share Posted October 12, 2018 4 hours ago, PeterW said: slate batten standard breather membrane 150 joists 120mm PIR in two layers 50mm PIR internal layer taped as VCL counterbatten plasterboard Peter. With this build up where would you utilise the 30mm gap ? (The difference between the 150mm deep joists and the 120mm pir) if the gap is below the membrane then you will need to cut battens to act as a stop to maintain that gap. Personally after doing my roof this way I would not recommend to anyone..... I found it a really messy and a really time intensive job. I detailed it very very carefully and will have Pretty much NO air-gaps between the layers or around the sides of the inserted insulation but it was a crazy amount of work. Just finishing the 25mm sheets over the joists today...... i would always recommend getting the insulation blown in as it’s far superior, great at sealing air gaps and has a good decrement delay. Badly done with infilled PIR leaves most of the insulation useless as air can circulate around it, and having done it well I know it takes to long to be cost effective for most builders / roofers to do it this well. My build up is slate full sarking board breather membrane 50mm vented gap, from soffits to ridge. 50mm PIR butted up and sealed (foamed and taped) to full length batons running down the inside of each 220mm deep rafters and butted up to the sarking. 120mm PIR inserted and fully sealed and taped. This came level with the inside of the rafters 25mm sheets of PIR over the rafters. Still to do. Osb over everything plaster board over everything I don’t intend to have a service void as I have designated it to be like this. Link to comment Share on other sites More sharing options...
PeterW Posted October 12, 2018 Share Posted October 12, 2018 1 minute ago, Cpd said: Peter. With this build up where would you utilise the 30mm gap ? (The difference between the 150mm deep joists and the 120mm pir) if the gap is below the membrane then you will need to cut battens to act as a stop to maintain that gap. Use battens below the membrane and create the void there. Saves on counterbattens but some of the other insulation types are full fill and the breather membranes can cope with this - the Cromar ones we use are certified for full fill and no air gap. If cost is no issue then I would go with sarking board every time and full fill that but make the joists 200mm. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now