Advice for loft insulation

[mg123]

Hi guys, I've been pondering on which option to take with regards my loft for insulation. House is a mid 30s build with some odd design features.
The roof joists (if you can call them that) are a mere 2 5/8 x 1 5/8 inches.
The ceiling joists are a little larger at 3 x 2 7/8 inches and they are surprisingly sold when walking on them (a third of the space the joists running left to right space sitting on 2 brick walls)
The spacing between joists is completely to ****, no consistency at all. It varies between 10 and 14 inch centres with no real logic applied that I can see.
The lack of depth in the ceiling joists makes the loft roll insulation a little less efficient as the 100mm depth would be compressed to the 76mm.
My thoughts are to use a foil bubble roll insulation up on the roof joists and staple them up about 30mm into the joist space and then add a second layer over the top of that. This would give 36mm air gap between the insulation and roof membrane and then 30mm between the 2 layers of foil insulation. The foil is rated to the equivalent of 65mm mineral/fibre insulation so should give me a 130mm equivalent. I could then run 100mm mineral/fibre between ceiling joists and then an extra 100mm (or 200mm) perpendicular over the top. With the compressed first layer this should get me to around 300mm equivalent.
Is there any issue with my approach or any better alternative methods? Money (as always) is tight so I'm looking to avoid PIR due to cost. Loft space is approx 42 square metres overall.
The reason I'm not just adding more mineral/fibre is that I'm planning to add a small amount of storage for some lightweight things (luggage, Xmas decs) up in the loft.
Appreciate any thoughts on the matter.

 

[Dibs-h]

I'm reading 2 things from your post: insulating the floor\ceiling joists and also the back of the rafter.

Assuming this is correct - you can get 75mm loft insulation rolls, so maybe use that in between the floor\ceiling joists and then go perpendicular with something thicker to get to your final thickness?

Rafters - I think this is where it gets a little complex. You need to know if your roof membrane is breathable. If it is - is your roof counter-battened or not. If it is breathable and counter-battened - you can fully insulate the rafters to their full depth.

If it is breathable membrane and not counter-battened, then you need to leave a void between the insulation and the low point of the membrane which might leave very little space for anything, as you might need about 2 inch of ventilation space.

If it is not breathable membrane - the same might apply, i.e. you may need to leave approx 2 inch ventilation space between the low point of the membrane and the face of the insulation.

I say 2 inch - but you could probably get away with 1.5 inch but I wouldn't go any less.

The membrane - if not breathable or counter-battened - should have been draped between the rafters on the outside and not pulled tight. This allows any run off to run down the face of the fabric and not be in contact with the back of the roofing laths and thereby ultimately rot them.

HIH

Dibs

 

[mg123]

Hi Dibs, appreciate your time and response thank you.
The roof does have breathable membrane, it was re-roofed only 2 weeks back. I realise I missed the perfect opportunity to sort the insulation out at the same time but it's my mistake and have to live with it.
The roof is battened but not counter battened, the membrane is pulled tight.
I haven't come across 75mm insulation rolls but I will endeavour to search them out.

 

[Dibs-h]

Hi Dibs, appreciate your time and response thank you.
The roof does have breathable membrane, it was re-roofed only 2 weeks back. I realise I missed the perfect opportunity to sort the insulation out at the same time but it's my mistake and have to live with it.
The roof is battened but not counter battened, the membrane is pulled tight.
I haven't come across 75mm insulation rolls but I will endeavour to search them out.

If you are going to go down the road of the foil bubble stuff - I'd be tempted to leave say a 40mm gap between the membrane and the outward face of the foil, then another lot straight across the face of the rafters. You'll get less than the 35mm gap between the 2 but a bit of a compromise.

The 75mm - I know Rockwool do it in slabs and it's more or less a "no itching" job.

 

[Lard]

I did an attic conversion years ago and used a combination of fibreglass and membrane on/within the rafters….I also fitted soffit vents etc etc….ok, it seemed to work (made it warmer) and I ended up renting the place out years ago and have had no complaints….

HOWEVER, moved into this bungalow and did a ‘better’ conversion, this time using a combination of 50mm PIR in-between the 100mm rafters (ie 25mm air space each side) and multi foil stapled to the rafter fronts.

For me the difference is incomparable….my point is that if you can afford to spend more then go with multifoil because it’s incredible.

Ensure you consider ventilation and air-gaps as per Dibs although if, as you say, you’ve just been re-roofed, you should already have, in my opinion, breathable membrane as well as ridge and soffit venting

 

[Dibs-h]

I had a look it up as it's been some years tbh since I did a roof (20 since the house & 12 ish for the shed).

This might be useful - https://www.silvatimber.co.uk/amfile/file/download/file/35/product/3810/

especially the pictures on on Page 5. Your setup looks to be like Figure 5 - but with no drape.

HIH

Dibs

p.s. the info relates to Tyvek (which I used) but the methods should be applicable to all\most brands.

 

[Doug71]

You could just do the ceiling joist, 75mm between them then 200mm perpendicular over the top. If you want an area for storage build a platform on some of the little plastic legs to raise it above the insulation so as not to compress it.

I'm no expert but you normally have a warm roof or a cold roof, not sure how well it works mixing the two

 

[Dibs-h]

I'm no expert but you normally have a warm roof or a cold roof, not sure how well it works mixing the two

Genuinely - what makes you say (and\or think) the 2 are being mixed?

 

[Doug71]

Genuinely - what makes you say (and\or think) the 2 are being mixed?

The OP was talking about insulating between and under the rafters and also insulating between and over the ceiling joist.

 

[Dibs-h]

The OP was talking about insulating between and under the rafters and also insulating between and over the ceiling joist.

I see where you are coming.

Looking at that Tyvek document I posted up - a cold roof is when it has no insulation between the rafters (it may or may not be ventilated as well). A warm roof is when it has insulation between between the rafters ( and possibly on top), which may also be ventilated or un-ventilated.

I think in the case of a cold roof - i.e. no insulation between\over the rafters, the only choice you'd have is something between\over the ceiling joists.

If on the other hand you had a warm roof - you could still use the between\over the ceiling joists. To my mind you end up "separating" the 2 entities (house - up to the ceiling and the loft). Use cheaper loft roll type insulation to reduce heat loss from the (habitable) house and use something else (lower U value) to keep the loft space from getting (really) cold. But unless the foil\bubble stuff was really cheap or free, or you were storing more than Xmas decorations or empty suitcases - I'd just stick with the roll stuff on the loft floor.

Hopefully not rambling. LOL

 

[Lard]

Just for info……a cold roof has the insulation below/between the rafters. This creates a void above the insulation that MUST be ventilated because it may contain water vapour that has condensed on the underside of the roof covering/vapour barrier.

Conversely, a warm roof is where the insulation is above the rafters, leaving NO air spaces to be ventilated. In this case, if you think about it, adding ventilation here will remove any ‘heat’ you’ve created.

In essence you can think of the ‘warm’ or ‘cold‘ description as relating to the ‘air space‘ itself.