loft insulation advise please

[werdas]

hello
my upstairs bathroom renovation is finished now
so time to insulate the loft

my rafts are spaced 60cm apart
raft height 10cm
currently there is 100mm insulation between rafts ( house was build 1990) might replace some of insulation

I should be straight forward, putting legs 175mm, 1700mm insulation across, board it . and that is it.

HOWEVER
2/3ds of my loft been done in a different way, and Im not sure if this was done correct:

100mm insulation between rafts
60mm height timber across the rafts, then
10cm PIR insulation boards on top of 6cm timber
and then loft boards

so in theory there must be a gap between pir insulation and isulation layer between rafts, I guess it should be 6cm gap- for air ventilation/circulation? to prevent moist?

is this insulation is efficient and correct?


if so shall I do same way on the uninsulated part of the loft? or do I go for loft legs etc?

many thanks

 

[Bonni]

Kingspan (or similar brands) is about 3 to 4 times that of loft insulation. So if you could get up 100mm Kingspan in, that'll equate to 300mm loft insulation.

Flat roofs have 120mm in, so the more you can get in, the better.

 

[werdas]

Kingspan (or similar brands) is about 3 to 4 times that of loft insulation. So if you could get up 100mm Kingspan in, that'll equate to 300mm loft insulation.

Flat roofs have 120mm in, so the more you can get in, the better.

Thanks

so the principal how current pir boards are layed with 6cm gap is fine? if so I will just do the same for the rest of the loft

 

[John D v2.0]

No you shouldn't have gaps in the insulation layer, you would need to pack some mineral wool in the 6cm gaps. Otherwise you would likely have air movement in the gap and negates your insulation

 

[werdas]

No you shouldn't have gaps in the insulation layer, you would need to pack some mineral wool in the 6cm gaps. Otherwise you would likely have air movement in the gap and negates your insulation

i understand for multi insulation layers, but does this applye for insulations layers and PIR insulation ? I thought gap prevent condensing

Thanks in advance for you insights

 

[John D v2.0]

If you have decent air barriers against both layers it would be ok, but that's unlikely to be the case, so you just have two chances to lose your heat via air movements

 

[werdas]

ok so if I understand correclt I will but 100mm PIR boards across rafters ( between them there is 100mm insulation layer) and on top of PIR board I will put loft boards
see picture attached

 

[John D v2.0]

Sounds reasonable to me.

 

[werdas]

Sounds reasonable to me.

great , thanks

 

[werdas]

had some feedback from other people, now not convinced, due to risk of condensation ......
on other hand there are products:
like this
and even b&Q does something like this

 

[foxhole]

had some feedback from other people, now not convinced, due to risk of condensation ......
on other hand there are products:
like this
and even b&Q does something like this

You can’t get condensation unless you allow moist air movement, so no gaps, no air, no moisture, no condensation.

 

[werdas]

thanks, but there still will be air gap, in my understanding , unless insullation layer needs to bee topped up and suppressed ( which is not good)
see picture