Adding insulation to an uninsulated 1995 conc tile roof, what product and how much is needed?

[difflock]

About 175m2 to do, 120mm deep rafters at nominal 16" centres.

Floor joist voids already glass fibre/hairy molly filled and flooring chipboard over throughout.

I have legitimately acquired 1.1mm thick Lamilux which I intend to finish with internally. It is impervious, bright, reflective and wipeclean.(The spiders will not like it!)

So PIR or XPS? Or are there affordable alternatives, like a bloke was saying sheeps wool insulation should now be cheap, since the fleece is worthless to the farmer(but its not, at the purchaser end, that I can see) or Sisalwool, which also appears to be expensive.

Rafterlok PIR also appears to be too bloody dear, for what it is.

Btw I dislike the thought of polystyrene, due to its potential flammability.

I can fit 70mm between the rafters, but is it worth fitting another 50mm across the hill, to the underside of the rafters, before affixing my lamilux to finish.

[GarethM]

Is it a usable loft space or a roof space 

[difflock]

Gareth,

A good question, and well presented, so it deserves an answer🙂

It is really only for storage, though I have a proper set of stairs to access it, and the space is fully "walk through".

So glorified storage.

Cheers

mth

 

Edited March 9 by difflock

[GarethM]

I'd watch a few Robin Clevett videos on YouTube. That and skill builder, gives a good idea of the principle.

 

Insulation isn't bad, just needs doing properly.

Personally I'd start with increasing the floor insulation if you've got enough headspace.

[Botty Cough]

Adding weight to an existing roof can be problematic and also sealing up airflow can also cause big trouble.

Do your research 

[difflock]

7 minutes ago, GarethM said:

I'd watch a few Robin Clevett videos on YouTube. That and skill builder, gives a good idea of the principle.

 

Insulation isn't bad, just needs doing properly.

Personally I'd start with increasing the floor insulation if you've got enough headspace.

The proper stairs would complicate that floor raising somewhat, plus anyway the cold draught would simply roll down the stairs into the rest of the house, though there is an internal panel door between the bottom of the stairs and the landing proper,  hence my intention to insulate at the rafters. 

[Botty Cough]

3 minutes ago, difflock said:

The proper stairs would complicate that floor raising somewhat, plus anyway the cold draught would simply roll down the stairs into the rest of the house, though there is an internal panel door between the bottom of the stairs and the landing proper,  hence my intention to insulate at the rafters. 

You will need to make them stronger.

[difflock]

8 minutes ago, Botty Cough said:

Adding weight to an existing roof can be problematic and also sealing up airflow can also cause big trouble.

Do your research 

Botty,

I did state that I would fit the 70mm deep PIR between the 120mm deep rafters, therby leaving the requisite 50mm airflow gap beneath the roofing felt. Yes?

I will knowingly "risk" the minimal additional weight of the insulation, knowing the general overdesign and factors of safety involved in building regs.

To our knowledge or intention this dwelling will not be sold, well in our lifetime. After that??

Cheers,

Marcus.

[GarethM]

Having watched a lot of videos.

Tightly fitted between, preferably using spacers behind to prevent movement.

 

The best one I saw used gappo tape to seal the joints, then a further layer of thinner insulation 90 degrees opposite.

 

Obviously ventilation is a consideration afterwards tho, not a builder but the moisture needs venting somehow.

[Botty Cough]

29 minutes ago, difflock said:

Botty,

I did state that I would fit the 70mm deep PIR between the 120mm deep rafters, therby leaving the requisite 50mm airflow gap beneath the roofing felt. Yes?

I will knowingly "risk" the minimal additional weight of the insulation, knowing the general overdesign and factors of safety involved in building regs.

To our knowledge or intention this dwelling will not be sold, well in our lifetime. After that??

Cheers,

Marcus.

If you do that make sure you have ridge tile vents and soffit vents to allow flow between the insulation and roof tiles.