Loft conversion - roof insulation

[CoachCarly]

First time posting here but had some good advice from other threads, thank you. I am NOT a builder so I need all advice at SIMPLE level.

Had a loft conversion done on 2 bed bungalow - Due to limited cash - conversion getting done in stages, all the structural work is now done. Next job is insulation, boarding and final fix on electrics and plumbing.

I am trying to source some cheaper materials now but I am confused about how much and what kind of insulation I need to put in the roof prior to boarding. We are also tight for head space to trying not to lose too much.

builidng inspector said (I think) 100mm insulation, batton, 50mm gap and board our rafters are 80mm . So I get that we have to make the rafters bigger during the process.
I Had a search on the internet and it would appear that Celotex and Kingspan have a great U rating and are thinner than rock wool? More expensive Which would you reccomend. I really want to save money but done want to make life difficult for myself

Not sure does the inspector want 100mm depth of something/anything or a decent u rating????

Also how do I measure the roof space, I now have a large dormer. Not sure how to calculate the square metre.
How far down do I go with the insulation - it would seem sensible to go past the upstands to the floor but I noticed another post said not to...

Can you advise on good price and place to purchase what ever insulation you would reccomend

Many thanks in advance
Caroline

 

[CoachCarly]

Have I not asked correctly ??

 

[mointainwalker]

CC

Some of the questions you are asking are simply not sensible.

You have identified that PIR offers better U-values than fibre-wool and that it is much more expensive.

Since we do not know your budget, how important lost head-room is, or even what part of the UK you live in, how can anyone advise you ?

Check with the building-inspector exactly what he requires in what type of insulation. This will then give you joist-sizes for that type and you can then decide"

1) Bigger joists, cheaper insulation, less head-room or
2) Smaller joists, dearer insulation , more head-room.


Sorry, can;t answer other points.

 

[DevilDamo]

I am confused about how much and what kind of insulation I need to put in the roof prior to boarding.

Isn't this information specified on the working/building regulation drawings, which would have obtained approval?

builidng inspector said (I think) 100mm insulation, batton, 50mm gap and board our rafters are 80mm . So I get that we have to make the rafters bigger during the process.

That sounds like a construction I used to specify (prior to the regulations changing recently). The BCO is assuming your rafters are 150mm deep, which would maintain a 50mm gap with the 100mm insulation below (i.e. flush with the bottom of the rafter). You would then fix a wallboard (or similar) to the underside of the rafters and that would also make up the u-value/thermal requirements.

However, as you only have 80mm deep rafters... then you would have to look at an alternative. I assume you have trussed rafters? As mentioned before, your architect/technician should be able to provide an alternative or even speak to your BCO but they would or should direct you to the drawings... if you had them done.

I Had a search on the internet and it would appear that Celotex and Kingspan have a great U rating and are thinner than rock wool? More expensive Which would you reccomend. I really want to save money but done want to make life difficult for myself

There are many similar products on the market and Kingspan and Celotex being the most established. I always specify Celotex boards as they are bought in sheets and can be cut and fixed to suit (i.e. between rafters). Kingspan boards also work in a similar way. On some jobs, the contractor has replaced the Celotex board with one called Xtratherm, which is available from Buildbase and I'm sure other builders merchants. It's cheaper than Celotex and has very similar, if the same properties.

Not sure does the inspector want 100mm depth of something/anything or a decent u rating????

The u-value would be determined on when the building regulation application was registered with the local authority. So when was that? The regulations changed on 1st October 2010 and it sounds like your proposals will not be checked against the new (increased) thermal regulations. I'm assuming the application was deposited and registered within the last two years? And... the u-value will also be dependant on the element (i.e. wall, pitched roof, flat roof, etc...). Again, this should all be specified on the drawings... the insulation requirements and u-value's.

Also how do I measure the roof space, I now have a large dormer. Not sure how to calculate the square metre.

Just out of interest... for what reason are you wanting to calculate this space? If it's to do with planning, then you will need to calculation the volume (cu.m) and you can do that via the Planning Portal...

http://www.planningportal.gov.uk/uploads/volcalc/volcalc.html

I can't see why that calculation would be required for Building Regulations. Unless you're trying to calculcate the amount of insulation required?

How far down do I go with the insulation - it would seem sensible to go past the upstands to the floor but I noticed another post said not to...

Again, this should be noted on the drawings. It depends if you are choosing to run the insulation from the rafters and continue down past the vertical stud walls to the eaves and then lap to the insulation within the wall. Or you would run the insulation from the rafters, down the vertical stud walls and then across the floor to the insulation within the wall. It really depends if you're able to provide the necessary insulation requirements within the vertical stud walls as they're normally 100mm thick, which may not be wide enough to accommodate the insulation. And if you did run the insulation down the vertical walls, it would eliminate the possibility in providing doors/access panels through to the eaves, which could be used for storage.

I think I've answered everything?

 

[freddiemercurystwin]

Isn't this information specified on the working/building regulation drawings, which would have obtained approval?

I like your optimism DD!

 

[DevilDamo]

It's worrying isn't it

 

[geraint]

50mm cellotex between the rafters . tri iso over the rafters held on with nails... then batten over then plasterboard.. that should hit the u values.. dont even attempt to put a screw near the tri iso....

 

[CoachCarly]

Thanks DD for your notes
Sorry I was not clear, that was an indication of my understanding..

I am doing this through permissible developments do did not need planning permission. Architect only did basic drawings for the structural engineer for the dormar, so not much info..
I managed to find out some more info so I am feeling more informed.

I spoke to someone from another area and he said he has been told to do 80mm and 70 of insulation !! so my 100 does not seem so bad now .

Building inspector is a bit anal so have to do as he says or I will not get signed off

Thank you all for answering my vauge question I really appreciate it
Caroline

 

[freddiemercurystwin]

Building inspector is a bit anal so have to do as he says or I will not get signed off

No, if the insulation is not thick enough it doesn't comply, simples! Or one rule for you another for everyone else?

 

[CoachCarly]

[No, if the insulation is not thick enough it doesn't comply, simples! Or one rule for you another for everyone else?

WHAT ??? It would appear that different areas of the country are applying different regs or mimimums .. Not one rule for me!

 

[freddiemercurystwin]

Pray tell, what U value have you been asked to achieve?

 

[CoachCarly]

I haven't !
BI has said to put 100mm in and advised 75mm then 25mm. but has reminded me that it is my responsibilty to meet the regulations.
I am struggling to find out more on the internet.
I think I may have to ask a company to come out to quote just so I can find out what they say?

 

[JohnD]

75 and 25 is probably so the 75mm will fit in the space between the timbers. Fibreglass will squash between the timbers easier than rigid foam and will take up minor irregularity.

Has the roof got a breathable membrane under the tiles?

 

[CoachCarly]

No we have no membrane,
I was wondering if I could just put in the 100mm however someone said that the 75mm would go between the rafters then the 25 goes accross all, perhaps to insulate the rafters too????


also not sure if the 50mm air gap starts at the tiles or at the batten.

I am wishing I could afford a specialist !!

 

[Deluks]

75mm would go between the rafters then the 25 goes accross all, perhaps to insulate the rafters too????

That works. As you have no felt or membrane then you will have approx 50mm between insulation and underside of tiles. Good enough. If you ever get the roof redone, make sure a breathable membrane goes under the new battens, not standard slaters felt.