Insulation Overview advice

[pilgrim]

I've tried reading through a few of the topics here and making sense of what other people have done with insulation.

In my build I have 100mm rafters between the ceiling plaster board and the felt layer of the roof. (the ceilings cut through the top corners of the upstairs rooms.

I have 100mm studs between each room and 170mm between the floor and the ceiling of the ground floor.

 

So I was thinking of putting 75mm PIR board insulation between the rafters leaving the 25mm gap between the board and the felt for airflow to stop condensation. (I think I have understood the warm and cold roof correctly)

adding 100mm of knauf acoustic roll mineral wool insulation between the internal walls and floor gap.

 

I was recommended to use the PIR board insulation by someone as it has a better U value, but I have seen a lot of comments here arguing against using it. so any advise for or against it in my situation would be appreciated.  

Thanks

[joth]

A diagram might help?

 

From part L1B you need a U value of 0.18 W/m2K at a pitched roof, or 0.16 if insulating between ceiling and a cold loft. Your 75mm of PIR only delivers 0.3

I think building regs would require 120mm PIR minimum (Our passive house retrofit we took it up to 300mm of wood fibre board)

 

@Iceverge had some interesting thoughts on PIR for (pitched) roof insulation here

 

Here's our retrofit build up

 

 

[AliG]

Are you refitting an existing roof or building a new one? 100mm rafters would be highly unusual in a new roof.

 

Assuming you are just improving insulation in an existing house then I would put 75mm between the rafters and 50mm across the rafters below this using 62.5mm PIR backed plasterboard. This gives a 0.19 U-value. If it is a new build then you will have to meet the regs as @joth says and there should be some kind of drawing showing what is planned which will have been given to building control.

 

As for the acoustic insulation between rooms that will only help with sound and not heating costs. 100mm is probably unnecessary in walls but it depends how much you want to reduce sound transmission. Sealing the top and bottom of the walls to avoid gaps that sound can travel through will probably be of as much or more benefit than just fitting sound insulation.

[joth]

2 hours ago, AliG said:

If it is a new build then you will have to meet the regs as @joth says and there should be some kind of drawing showing what is planned which will have been given to building control.

You need to build building regs on a renovation too, if more than 50% of the element (e.g. roof pitch) is being renovated, or if more than 25% of the entire envelope is being done.

From the sounds of the project it will easily hit both those criteria 

If you're bringing a house back into service after 20 years of dereliction, and claiming the zero-rate VAT discount for doing so, now really is the time to get it up to spec for insulation as the insulation will be cheaper with that discount, and it will cost a lot more in making good to retrofit it after this opportunity.

 

 

https://www.energy-saving-experts.com/wp-content/uploads/2017/04/Part-L1B-and-what-you-need-to-know-to-get-your-building-to-pass.pdf section 6.1

https://www.atamate.com/atamate-blog/uk-building-regulations-made-simple-part-l1

Part L1B: How much renovation can we do to a residential building before the guidelines apply?

Britain is full of older buildings that do not conform to the energy efficiency standards set out in Part L1 but do occasionally need repair, and there's scope to do a fair amount of work on a building before hitting the threshold above which it must meet the defined standards. If repair or renovation work hits any of the following, the building must conform to the Part L standards:

• Major renovation, defined as replacing more than a quarter of the building's surface area.

• Renovation of up to half of the area of either a wall, floor or roof that forms part of the dwelling's thermal envelope, which separates the dwelling from either the outside or another part of the building that is likely to be at a different temperature.

[AliG]

Thanks @joth I hadn't seen the other thread.

 

This clearly needs building control sign off then. Not just for the insulation, but the electrics, smoke alarms etc.

 

I am guessing @pilgrim that you haven't done that. I know it probably seems like a hassle and it will keep costs down, but it will be worth it in the end to have it properly signed of and they might even notice some things that you haven't thought about.

[pilgrim]

Thank you for your help.

I had not realised that I would have to deal with building control so I will contact them.

I have just done a quick and dirty sketch of what I was talking about if that helps make things a bit clearer. There is also a room in the roof conversion, which has approx 100mm fibreglass insulation all around it. I am also planning on replacing this.

@joth Does this help at all?

 

 @AliG I have plenty of height on the ceilings so I will add the extra 50mm below the rafters. Between the walls and first floor ceiling I was only really hoping to deaden the sound transfer not too worried about heat transfer between the rooms upstairs. And yes I totally agree there are a lot of things that seem like extra hassle that I hadnt considered doing before I started but as you say it makes complete sense to do these other jobs now while I have a blank canvas, this will be my house im not just flipping it so I want to everything right and to the best standard that I can. Do it once, do it right as they say!