Insulation advice please

[Dhalsim]

Hello all.

Wondering if anyone can help me with some insulation advice.

I have a 1890s Victorian terrace that had an existing DIY loft conversion done to it in the 90s.

Structurally its sound but the insulation is 55mm expanded polystyrene and is between the 75mm rafters and flush against the sarking felt therefore leaving no room for ventilation. I've now pulled down this insulation and plaster board.

Between the 75 mm (420mm centered) joists was filthy flattened mineral wool that I've now cleaned out as well.

So I have a blank insulation slate and going through the options to achieve as close to 0.16 W/m²K as possible. At floor level I will use 270mm mineral wool however my rafter level is a problem because of the lack of head room.

I think I only really have about 100mm below the rafters to play with. Because of the skinny rafters (75mm) and limited headroom I'm thinking of using a foil quilt product like superfoil SF40 (40mm) 25mm battens and 35 mm PIR board (which manufacturers tell me will achieve 0.18).

I'm aware of all the doubts and issues surrounding foil insulation.

As good as the manufacturers claims sound I'm aware there is a good chance it won't achieve this U value but I'm interested to know if anyone has actually used it to good effect?

If foil should be avoided can anyone advise on a different method of rafter insulation for my scenario?

many thanks in advance.

 

[Ian H]

What about adding 25mm to the rafters and having 50mm between and 50mm over the top (Celotex type stuff)?

 

[Blagard]

My own loft conversion retained the existing 75mm deep rafters and purlin on one roof slope but to increase rafter depth to 150mm (new rafters on another roof slope) I fitted some 75mm deep sections to the existing rafters, then fitted small section battens to act as a stop to maintain the gap I wanted and finally infilled with two layers of 47mm Celotex. Vapour barrier across the whole and then 9mm plasterboard + skim. I have to add that the purlin span and section was more than enough for the change and each roof structure should be looked at on is own merits.

 

[StephenStephen]

I used superfoil between rafters a few years ago and was surprised at how effective it was as a thermal insulator.
The downside was that it was expensive, at the time, compared to other forms of insulation, but I just didn't have the headroom for anything else.

 

[Dhalsim]

What about adding 25mm to the rafters and having 50mm between and 50mm over the top (Celotex type stuff)?

Thanks Ian. Yes considering that. it acheives a 0.28 W/m2K u value which will probably suffice and might have to be the solution. I did like the neatness of the wrapping foil and staple gunning as opposed to the cutting of loads of 25 mm celotex. But beggars can't be choosers.

My own loft conversion retained the existing 75mm deep rafters and purlin on one roof slope but to increase rafter depth to 150mm (new rafters on another roof slope) I fitted some 75mm deep sections to the existing rafters, then fitted small section battens to act as a stop to maintain the gap I wanted and finally infilled with two layers of 47mm Celotex. Vapour barrier across the whole and then 9mm plasterboard + skim. I have to add that the purlin span and section was more than enough for the change and each roof structure should be looked at on is own merits.

Thanks. Yes considered this too although worried about the extra weight of the timbers and will need to get a structural engineer involved to do the calcs which will bump the cost. The existing vertical stud wall that forms the non-party wall of the conversion drops down off the rafters before the purlin so no weight is taken by the purlin.

I used superfoil between rafters a few years ago and was surprised at how effective it was as a thermal insulator.
The downside was that it was expensive, at the time, compared to other forms of insulation, but I just didn't have the headroom for anything else.

Thanks. What thickness did you use? and in conjuction with any PIR or not? On a cold roof loft conversion?

 

[Doggit]

The problem with the foil, is that if you took a thermal imaging camera to it, it'd be warmer in the centre of the joists, and colder around the joists as the foil gets squashed, but would it still be effective overall; I've always been sceptical of it's claims, and would rather go with PIR.

You can get away with about an inch of ventilation under the sarking, and just put a 50mm batten on top of the joists. Then you'd set the 100mm pir to the top of the new joist height, and then go with 35mm of insulation across the joists plus the plasterboard. Finish off with a VCL, and you're fine. As to the dwarf walls, go with 60mm PIR (as it's still got the 100mm in the joists), and then the 35+ plasterboard across the top - this'll give you insulated storage space behind the dwarf walls. I didn't completely understand your posts, but it sounds as though the dwarf walls are there, but I'd remove them and do the job properly if they are.

 

[StephenStephen]

Thanks. What thickness did you use? and in conjuction with any PIR or not? On a cold roof loft conversion?

I think it was 40 if I remember rightly, no PIR. It wasn't in a loft conversion - it was in the main house - the ceiling at the top of the stairs, and the ceiling in a wet room. If I remember rightly it was (from the top) slate, batten, membrane, gap, superfoil, plasterboard.

 

[geraldthehamster]

When we insulated the first floor of our dormer bungalow properly, we were also faced with 3x2 rafters. I battened them out to 6 inches, and fitted 100mm Celotex between (which left 50mm air gap), and 25mm over. PB and skim. Also fitted ridge and soffit vents.

 

[Dhalsim]

The problem with the foil, is that if you took a thermal imaging camera to it, it'd be warmer in the centre of the joists, and colder around the joists as the foil gets squashed, but would it still be effective overall; I've always been sceptical of it's claims, and would rather go with PIR.

You can get away with about an inch of ventilation under the sarking, and just put a 50mm batten on top of the joists. Then you'd set the 100mm pir to the top of the new joist height, and then go with 35mm of insulation across the joists plus the plasterboard. Finish off with a VCL, and you're fine. As to the dwarf walls, go with 60mm PIR (as it's still got the 100mm in the joists), and then the 35+ plasterboard across the top - this'll give you insulated storage space behind the dwarf walls. I didn't completely understand your posts, but it sounds as though the dwarf walls are there, but I'd remove them and do the job properly if they are.

Thanks for this Doggit. I think I follow, but it's the rafters that are the issue. Not joists. At joist level I'm battening the 3x2" to 100 mm then laying 100 mm wool, then 170mm more wool across and using loft legs and chip board to board. That bit is the easy bit. The dwarf wall already has 57mm EPS so will keep that and add 100mm celotex to the back get a good u value (space not an issue here as I can go into eaves void). The dwarf walls fall in front of the 2 x 9" x 3" beams that hold the floor so dwarf wall can't go back any further. Will likely do as you say on the PIR, but see reply to Gerald below...

I think it was 40 if I remember rightly, no PIR. It wasn't in a loft conversion - it was in the main house - the ceiling at the top of the stairs, and the ceiling in a wet room. If I remember rightly it was (from the top) slate, batten, membrane, gap, superfoil, plasterboard.

Thanks for this Stephen

When we insulated the first floor of our dormer bungalow properly, we were also faced with 3x2 rafters. I battened them out to 6 inches, and fitted 100mm Celotex between which left 50mm air gap), and 25mm over. PB and skim. Also fitted ridge and soffit vents.

Yes this looking likely way to go although was thinking of 25 mm celotex first then 75 below, and across, the flush rafters and PIR board. Any reason I can't do this instead of the suggested battening (to extend) the rafters and adding the extra PIR in between? To me makes more sense to go across so that wood rafter thermal bridge is covered?

25 + 75 will give 100 mm a total u value 0.21. The 125 mm total will get to 0.17 but I think this is really pushing it on head height so will suffer the 0.21 and put an extra jumper on.

I didn't completely understand your posts, but it sounds as though the dwarf walls are there, but I'd remove them and do the job properly if they are.

Once I tidy up all the pulled down plaster and EPS I'll post pictures to help explain more. Many thanks for all your advice thus far.

 

[Doggit]

Sorry, meant to say rafters rather than joists; it's an age/dementia thing. Building regs require 125mm, so this obviously isn't being done under a building control notice - but it isn't the first, and won't be the last, but adding 3x2s to the joists isn't going to give you much strength, and adding the loft boards definitely isn't a good idea. If you've got 4x2 joists, then you're only going to 7x2s, and weak ones at that, adding loft legs, and it'll be a little unstable, but if you're insulating the rafters, then you don't need all that insulation in the floor. You'd be better off gluing and screwing 4x2s to the joists, and then adding 170mm of rockwool (being careful of the wiring), and then screwing the chipboard to the joists. If there's 9x3s supporting the dwarf wall, who did those. Are there purlins holding up the roof, or has someone taken out the original supports.

 

[geraldthehamster]

Yes this looking likely way to go although was thinking of 25 mm celotex first then 75 below, and across, the flush rafters and PIR board. Any reason I can't do this instead of the suggested battening (to extend) the rafters and adding the extra PIR in between?

No reason that I can see, no.

I don't think you need that insulation in the floor, if you're insulating at rafter level. As I understand it, the only reason for any amount of rock wool in the floor is for fire protection, and I'm not sure even that is necessary with sufficient plasterboard. You don't normally have thermal insulation between the floors of a building.

 

[Dhalsim]

Sorry, meant to say rafters rather than joists; it's an age/dementia thing. Building regs require 125mm, so this obviously isn't being done under a building control notice - but it isn't the first,and won't be the last,

I inherited a converted loft not to regs, I've looked into regularising and would involve starting from scratch, which is beyond my means. It is structurally sound and will get independent building surveyor to approve the upgrade. Just trying to bring it up to as good a standard as I can, keeping it habitable and safe, but within a 2.5k budget instead of 25k.

but adding 3x2s to the joists isn't going to give you much strength, and adding the loft boards definitely isn't a good idea. If you've got 4x2 joists, then you're only going to 7x2s, and weak ones at that, adding loft legs, and it'll be a little unstable, but if you're insulating the rafters, then you don't need all that insulation in the floor. You'd be better off gluing and screwing 4x2s to the joists, and then adding 170mm of rockwool (being careful of the wiring), and then screwing the chipboard to the joists.

I hadn't intended to extend joists, just 300mm loft legs and boards (with270mm mineral wool beneath), but this will only be for storage space not taking any significant weight. 4 x 2s on 3 x 2s will give 170 mm depth and i need 270mm and if i did that with timber I'd have to add 8 x2s ? which seems like a of a lot of timber, hence why I was turning to these loft legs.

If there's 9x3s supporting the dwarf wall, who did those. Are there purlins holding up the roof, or has someone taken out the original supports.

The 9x3's were from the original conversion, so i'm working within the framework of that original conversion. Purlins are still there and holding up the rafters.I'll do a sketch and post that. That should help.

Thanks for all your help

 

[Dhalsim]

No reason that I can see, no.

I don't think you need that insulation in the floor, if you're insulating at rafter level. As I understand it, the only reason for any amount of rock wool in the floor is for fire protection, and I'm not sure even that is necessary with sufficient plasterboard. You don't normally have thermal insulation between the floors of a building.

Insulation at rafter level ends at the dwarf wall. Then out to the eaves it is uninsulated cold boarded out storage space. So the mineral wall at joist levels is to insulate first floor rooms.

 

[Notch7]

Our local builders merchant has started selling an alternative insulation, which builders are finding quicker to fit in between joists and studwork walls.

It nay be of use for your application:

http://www.insulation-actis.com

 

[Doggit]

It looks interesting, but like any multi foil system, it'll be ineffective where it gets squashed and fitted to the supports. It'll be helpful when someone puts these products side by side, and tests them properly.

But I think you're main problem here, is that you've got two conflicting approaches going. You want to increase raise the height of the ceiling joists and put in loft legs to allow 270mm of rockwool, and then just use the loft for storage, but you're also putting insulation in between the rafters to keep the loft space warm. But if you're insulating the rafters, then you don't need to put 270mm of rockwool in the joists. In addition, you're insulating the rafters, but not sorting out the space behind the dwarf walls.

I would take the dwarf walls down, increase the height of the joists by gluing and screwing battens down. The adding 170mm rockwool right across the whole area and board the new floor, then insulate the rafters by increasing the depth, and adding the PIR, and then putting the dwarf walls back,and plasterboarding.

You do need to decide if you're creating a warm roof space, or a cold storage space, because at the minute, you'd got a mishmash of ideas that isn't going to work properly.