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?

I have alos came across of insulated loft boards
https://www.specifiedby.com/ecotherm-insulation/eco-loft-insulation-boards

so I do not see any gaps left, so need for them?



many thanks

FreeBear

werdas said: 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 this gap is open to the outside, then no, it will not be efficient. You'll have cold air circulating, which will draw any (slightly) warm air out from above the (fibreglass ?) insulation between the joists. The PIR insulation boards will also end up being much colder and prone to condensation during the winter months unless there is a vapour control membrane built in to the ceiling (highly unlikely).

Remove the PIR insulation, put down 200mm of fibreglass between your loft legs, and then board - Compressing the insulation by ~25mm is not going to impact significantly on its performance. Don't waste your money on insulated loft boards.

aoleks

what we did, and I appreciate this might not be the cheapest option, is to remove everything that's not "meant" to be there. we emptied the loft to the point all we had were the joists (100mm) with insulation in between (synthetic wool), not boarded. at that point we got a local company to install structural joists on top of the existing ones, raising the space between the joists to 300mm. they added an additional 170mm insulation between, to make it 270mm total. left a 30mm gap and then boarded up from corner to corner.

what you end up with is 270mm all over the place, as recommended, a 30mm ventilation that prevents squashing the insulation, a structurally raised loft that you can walk on, boarded with high-quality, weather resistant boards (tongue/groove). perfectly insulated, you can walk/dance on without worrying about the ceiling and looking neat and tidy. aside from the insulation benefits, it's now a perfect, clean storage space.

werdas

loft heigh is not that great, so would like to keep as loft boards as low as possible, ( would not like to go for loft legs option)
I have PIR boards, so I do not mind using them, justy need to know how correclty to put them, 
A: the way other part of the loft been done
B: with out any air ventilation
C: ? if there another way?

FreeBear

Cut the PIR boards to fit between the joists (I use a sharp serrated kitchen knife on the stuff). Any gaps can be filled with expanding foam (use a gun for better control). Don't insulate over the top of the joists with the PIR - This will allow the joists to "breath", and you'll be less likely to trap moisture in the timber.

100mm of PIR is equivalent to 200mm of fibreglass which is an acceptable thickness even if current recommendations is 270-300mm.

werdas

FreeBear said:

Cut the PIR boards to fit between the joists (I use a sharp serrated kitchen knife on the stuff). Any gaps can be filled with expanding foam (use a gun for better control). Don't insulate over the top of the joists with the PIR - This will allow the joists to "breath", and you'll be less likely to trap moisture in the timber.

100mm of PIR is equivalent to 200mm of fibreglass which is an acceptable thickness even if current recommendations is 270-300mm.

hi, thanks, I understand 
but why they sell
https://www.specifiedby.com/ecotherm-insulation/eco-loft-insulation-boards
even B&Q
?????

werdas

FreeBear said:

werdas said: 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 this gap is open to the outside, then no, it will not be efficient. You'll have cold air circulating, which will draw any (slightly) warm air out from above the (fibreglass ?) insulation between the joists. The PIR insulation boards will also end up being much colder and prone to condensation during the winter months unless there is a vapour control membrane built in to the ceiling (highly unlikely).

Remove the PIR insulation, put down 200mm of fibreglass between your loft legs, and then board - Compressing the insulation by ~25mm is not going to impact significantly on its performance. Don't waste your money on insulated loft boards.

hi, the gap of the beams laid across the joist is facing walls, one internal one external, gap is not facing edge of the roof slope, hope I explained clearly?

werdas

please see picture

werdas

please see picture