Loft insulation ceiling level and rafter level - insulation depth.

[.Ox.]

Hello Guys

I’m looking at turning my loft into clean/dry storage - as well as a space to run a NAS and a few NUCs.
This will not be a living space, as we don't have the headroom.

I currently have 270mm of insulation at ceiling level, with the loft boarded out. I'm now looking at insulating between and below the rafters.

The roof is slate tiled, with no membrane/sarking, lots of ventilation between the tiles etc.

I have extended the rafters to 100mm, with the plan of fitting 50mm Celotex between the rafters, leaving a 50mm airgap. Using aluminium tape to seal any joints etc.

I am then planning on fitting 50mm over the top of the 50mm between the rafters, bringing the insulation to 100mm.

I understand that it may not hit the recommended U-values, but this room is just for storage, and running a few devices in the loft. The insulation may then be covered with plasterboard eventually, but this may not happen right away.

Do you think 100mm will be suitable and be enough to reduce the chances of any interstitial condensation?

 

[eviled]

you have a cold roof (type of roof design not description of temperature)

your roof should be cold and well ventilated

top up your ceiling level insulation but don't put any in the rafters or you'll get damp issues and no real benefit anyway

((there are so many fundamental issues with what you've written that show a lack of knowledge that could be damaging))

 

[Rroff]

you have a cold roof (type of roof design not description of temperature)

your roof should be cold and well ventilated

top up your ceiling level insulation but don't put any in the rafters or you'll get damp issues and no real benefit anyway

((there are so many fundamental issues with what you've written that show a lack of knowledge that could be damaging))

Been puzzling me a bit having just moved into a new house (not new build - 1970s) - the floor of the loft is fully insulated but there are a dozen or so bales of insulation up there (enough to redo it like 3-4x over - probably more than that) placed on the roof supports - not sure if someone had maybe been planning on insulating the rafters at some point or something or whether they were left up there for convenience when the property was built or something.

 

[MassiveJim]

you have a cold roof (type of roof design not description of temperature)

your roof should be cold and well ventilated

top up your ceiling level insulation but don't put any in the rafters or you'll get damp issues and no real benefit anyway

((there are so many fundamental issues with what you've written that show a lack of knowledge that could be damaging))

I don't understand why the OP can't do what has been suggested.
The method he has described, whilst being under the recommended U value (which would be achieved with 150mm iunsulation in most cases) is still the correct method.

There should be no damp if a 50mm airgap is left between tiles and insulation.

 

[JBuk]

Hello Guys

I’m looking at turning my loft into clean/dry storage - as well as a space to run a NAS and a few NUCs.
This will not be a living space, as we don't have the headroom.

I currently have 270mm of insulation at ceiling level, with the loft boarded out. I'm now looking at insulating between and below the rafters.

The roof is slate tiled, with no membrane/sarking, lots of ventilation between the tiles etc.

I have extended the rafters to 100mm, with the plan of fitting 50mm Celotex between the rafters, leaving a 50mm airgap. Using aluminium tape to seal any joints etc.

I am then planning on fitting 50mm over the top of the 50mm between the rafters, bringing the insulation to 100mm.

I understand that it may not hit the recommended U-values, but this room is just for storage, and running a few devices in the loft. The insulation may then be covered with plasterboard eventually, but this may not happen right away.

Do you think 100mm will be suitable and be enough to reduce the chances of any interstitial condensation?

This is how my dormer room has been insulated except the gap was 75mm , so left 25mm for ventilation , there’s also a small gap from the eaves to create an airflow

This method was one I read into and my joiner also told me that’s the way he’s been taught to do it , I’ve never had any problems

 

[Entai]

I don't understand why the OP can't do what has been suggested.
The method he has described, whilst being under the recommended U value (which would be achieved with 150mm iunsulation in most cases) is still the correct method.

There should be no damp if a 50mm airgap is left between tiles and insulation.

As @eviled says the roof design type (cold roof) is one where you do not, and should not put insulation between the rafters.

The design of that kind of roof space, means you need constant cross roof ventilation from side to side and from eaves to ridge.

Putting insulation between the rafters, even when leaving air gaps will kill all the flow of ventilation needed, and condensation, mould and worse will build up.

Loads more info here

https://www.tradingdepot.co.uk/info/building/superglass-cavity-insulation/pitched-roofs-insulation/

 

[eviled]

Maybe I was a little harsh but there will be no heat in the actual loft space so sticking expensive sheet of insulation to the rafters will do absolutely nothing.

270mm of decent loft roll will give pretty good U-values, use a top up roll to give excellent insulation

I've put my mates alarm, smart things and NAS in a small vented enclosure in his loft to protect it

 

[the_r_sole]

Maybe I was a little harsh but there will be no heat in the actual loft space so sticking expensive sheet of insulation to the rafters will do absolutely nothing.

Apart from regulate the temperature in the loft so it's not going from extremely hot to extremely cold - and good for storage!

what the op is proposing is pretty much how you would go about a loft conversion - op if there is no ventilation left in there after insulating I would suggest putting in some fixed ventilation to prevent any humidity build up...

 

[Entai]

Apart from regulate the temperature in the loft so it's not going from extremely hot to extremely cold - and good for storage!

what the op is proposing is pretty much how you would go about a loft conversion - op if there is no ventilation left in there after insulating I would suggest putting in some fixed ventilation to prevent any humidity build up...

Although again it entirely depends upon the type of roof you have.

We had a cold roof, and then went for a loft conversion.

The entire roof had to be disassembled, all the tiles taken off and stripped right back, and the proper linings put in place before the roof could be re tiled.

Only then, was it was of the correct construction to allow insulation to be put between the rafters and boarded out.



Had it been a warm roof construction at the start, then we could have insulated and boarded out without having to dismantle the entire roof, and saved us a few thousand in the process.

People think a roof is a roof, and a loft is a loft, there are different types and different types need treating accordingly if converting or boarding out even if just for storage, unless you ant to create more problems for yourself years down the line.

 

[the_r_sole]

Although again it entirely depends upon the type of roof you have.

We had a cold roof, and then went for a loft conversion.

The entire roof had to be disassembled, all the tiles taken off and stripped right back, and the proper linings put in place before the roof could be re tiled.

Only then, was it was of the correct construction to allow insulation to be put between the rafters and boarded out.



Had it been a warm roof construction at the start, then we could have insulated and boarded out without having to dismantle the entire roof, and saved us a few thousand in the process.

People think a roof is a roof, and a loft is a loft, there are different types and different types need treating accordingly if converting or boarding out even if just for storage, unless you ant to create more problems for yourself years down the line.

If your existing roof covering is knackered then yes, you'll need to sort that out before anything else, but as long as you have enough ventilation and in the right places it's pretty straight forward - that's speaking as an architect who has worked on a lot of loft conversions over the last decade...

 

[Entai]

If your existing roof covering is knackered then yes, you'll need to sort that out before anything else, but as long as you have enough ventilation and in the right places it's pretty straight forward - that's speaking as an architect who has worked on a lot of loft conversions over the last decade...

Well our roof was structurally fine and solid so no reason to change anything for that, it was not as you say knackered, and would have lasted many decades had we not wanted a loft conversion.

However there was only an old fashioned impermeable sarking felt membrane beneath the tiles.

This had to be swapped out for a more modern breathable membrane, so that after insulation was put in and boarded out, any water vapour trapped between the insulation and tiles would be able to escape through the breathable membrane and tiles to the outside and not just build up between the insulation and non-permeable membrane, causing damp and condensation issues in the future.

 

[the_r_sole]

Well our roof was structurally fine and solid so no reason to change anything for that, it was not as you say knackered, and would have lasted many decades had we not wanted a loft conversion.

However there was only an old fashioned impermeable sarking felt membrane beneath the tiles.

This had to be swapped out for a more modern breathable membrane, so that after insulation was put in and boarded out, any water vapour trapped between the insulation and tiles would be able to escape through the breathable membrane and tiles to the outside and not just build up between the insulation and non-permeable membrane, causing damp and condensation issues in the future.

note the difference between "covering" and "structure"... I get it, you're an expert, I'll leave you to it

 

[niko]

as long as there is sufficient air vents, both at soffit level and closer to ridge there is no need to have a breathable membrane. the air flow from to to vents is ok. breathable helps minimise the amount of vents needs for good airflow

 

[eviled]

There will be no soffits in a slate roof with no membrane

Mess this up (like your plans currently do) and it will definitely be a red flag on a survey if you ever sell

Everyone here is chipping in with limited knowledge and understanding and it worryingly bad advice

In fact don’t ask a pc building forum, get professional advice

 

[fastwunz]

There will be no soffits in a slate roof with no membrane

Mess this up (like your plans currently do) and it will definitely be a red flag on a survey if you ever sell

Everyone here is chipping in with limited knowledge and understanding and it worryingly bad advice

In fact don’t ask a pc building forum, get professional advice

Building surveyor here - I’m also uncomfortable with it. You risk interstitial condensation by trapping warm air against cold thus forming on the timbers and resulting in wet rot.

I’m not saying it’s likely but roofs such as yours MUST breath so be very careful dude