Help me choosing insulation for pitched roof loft conversion.

[Dannyboyski]

We are re-roofing our house... finally.

We are also converting the loft (when we get to it), we have a target u value of 0.18 W/m2k for the roof.

Rafters are currently 87mm x 55mm @ 400 centres, part of our structural work is to increase the rafters with a 50 x 50 batten on the underside (to allow removal of collar ties in the bedrooms for vaulted ceilings) so this would give us 137mm depth, but obviously I could choose to batten out further to allow for a greater thickness of insulation between the rafters (rather than under)

since we are taking the roof off anyway we seem to have a lot of options, we seem to have even more endless possibilities and combinations, of above, between and below.

Is there anyone out there that can point me in the right direction for a cost effective solution?

Many thanks in advance.

Dan

 

[cdbe]

Go to (or contact) one of the manufacturers websites. They have calculators that will give you the best (thinnest?) combination to meet your target. You may wish to consider using one of the super duper breathable roofing membranes that don't require a 50mm air gap. In terms of cost effectiveness, I would have thought that more between and less under would be cheapest and easiest to install.

 

[^woody^]

You need to find out what rafter depth your loft conversion will require first. This will be determined by structural requirements rather than depth of insulation.

You are unlikely to be able to over clad the rafters so in between and under will be necessary in any case.

Re-roofing will require a building regulation application, and insulation will need to be stated as part of that, and if that means alterations to rafters, then the application becomes more than the simple re-roofing one.

 

[Dannyboyski]

structural requirements are the extra 50 x 50 batten glued to the underside and fixed every 200mm. giving total rafter depth of 137mm Building control are onboard. I am able to over clad if desired. We're detached. BC are happy for the roof height to change a bit.

 

[^woody^]

structural requirements are the extra 50 x 50 batten glued to the underside and fixed every 200mm. giving total rafter depth of 137mm Building control are onboard. I am able to over clad if desired. We're detached. BC are happy for the roof height to change a bit.

You need planning permission to raise a roof surface, not building control's approval

50x50 battens add zero structural strengthening to a rafter. If you are removing internal supports then you need to endure that the exiting rafters are suitably spanned.

 

[Dannyboyski]

LBC say they don't run to planning dept for minor adjustments to roof height to accommodate insulation.

my SE has checked the rafters, and his recommendation was the battens. I presume he didn't spec the 50 x 50 battens for decorative reasons, so im hoping they will do something.

 

[noseall]

Is there anyone out there that can point me in the right direction for a cost effective solution?

Anyone reading this will be thinking why isn't the OP just fitting new, say 150mm x 50mm rafters instead of faffing about with a sub-standard cobbled solution...?

You may wish to consider using one of the super duper breathable roofing membranes that don't require a 50mm air gap

I don't believe such a thing exists. Whilst some are better than others, claims that they are a panacea for dispersing all things migrating moisture can throw at them, are misleading. Extreme cases - such as very cold external temp's coupled with excessive moisture internally, means that even the best membranes struggle.

It's for this reason we always supplement BM's with mechanical vents.

 

[DevilDamo]

It's for this reason we always supplement BM's with mechanical vents.

Do you not mean traditional eaves/soffit/over fascia vents in addition to ridge vents and/or tile vents?

 

[noseall]

Do you not mean traditional eaves/soffit/over fascia vents in addition to ridge vents and/or tile vents?

OFV's.

 

[cdbe]

Hmmm. I wonder if I should be worried? I've used Proctor Roofshield on my vaulted lean-to, with no top ventilation, it does have a 40mm airgap above the insulation and though not deliberately ventilated at the eaves there is an element of air gap. Although the MI's state that a vapour barrier is not required I will be fully taping both layers of celotex so I don't really see how any moisture can get in there anyway.

 

[Dannyboyski]

Anyone reading this will be thinking why isn't the OP just fitting new, say 150mm x 50mm rafters instead of faffing about with a sub-standard cobbled solution...?

Because the OP (me) is a self builder, and nobody has suggested that the solution is cobbled or sub standard.

my rafters are 6610 long @400 centres across the 11m roof. So I suspect new rafters would have a significant cost implication. Not to mention the fact that my chippy loves the tight grain on the Douglas fir originally used. He says you can’t get comparable stuff anymore. I appreciate advice, but if my structural engineer is happy with the proposed solution. Why would I rip it all out?

From much digging into different insulation options. It appears that having 150mm rafter depth is indeed the magic number.

 

[DevilDamo]

From much digging into different insulation options. It appears that having 150mm rafter depth is indeed the magic number.

When insulating at rafter level, I wouldn’t want anything less. 150mm rafters with 100mm Celotex GA4000 between and 62.5mm Celotex PL4050 under with the usual board joints sealed as VCL/ALB, skim plaster will achieve min. 0.18.

 

[^woody^]

but if my structural engineer is happy with the proposed solution. Why would I rip it all out?

One reason might be that SE's don't tend to have great knowledge of insulation and ventilation requirements, just structures. But if he is happy with that solution, has designed it, has allowed for other structural members being removed and the loft loaded, and building control have approved it, and all this is documented so as to enact his insurance if need be, and you don't think anyone will ever notify the planners, then that is fine and you have nothing to worry about.

What was the actual question again?

 

[Dannyboyski]

But if he is happy with that solution, has designed it, has allowed for other structural members being removed and the loft loaded, and building control have approved it, and all this is documented so as to enact his insurance if need be, and you don't think anyone will ever notify the planners, then that is fine and you have nothing to worry about.

What was the actual question again?

He did design it, he knows our intended purpose for the loft, Building control will be getting the justifications, etc etc all documented.
replacing the timbers would cost £££, adding a few bits of batten I can justify the price of for the proposed changes (collar tie removal to allow vaulted ceiling in the bedrooms.)

Apparently the uk has some of the least well insulated housing stock in Europe. Building control encouraged me to go with insulation above, apparently it's the most efficient. Although I'm still mulling over the decision.

original question: for insulating purposes, if I am battening out rafters from 87mm to 137mm is it sensible to batten out further to make meeting 0.18 u value easier / cheaper.

I think Ive decided that for my roof 150mm would be a useful place to be.

 

[Dannyboyski]

When insulating at rafter level, I wouldn’t want anything less. 150mm rafters with 100mm Celotex GA4000 between and 62.5mm Celotex PL4050 under with the usual board joints sealed as VCL/ALB, skim plaster will achieve min. 0.18.

Thank you for this Damo, I will check out the prices for this method.