I will be converting my loft as part of my victorian refurbishment. The rafters are in good condition (plenty of ventilation!) as are the original slate roof tiles which are mounted on battons over the rafters. But no roof felt. I would like to insulate using woodfibre batts between joists. What layers would I need?

Not sure on your build but when I insulated my Victorian roof for a conversion in hindsight I wish I had insulated over the rafters at the same time I had the new roof...

No breather layer needed

Posted By: modernvictorianThat's good. I think. I am imagining the scenario of a tile coming off and rain soaking the woodfibre batts...

Yes it will - and any snow blown under the tiles will also also soak the insulation. Driven rain can also get to the insulation and cause problems. It rather depends how well fitting the tiles are.

insulation batts ate difficult to put between the rafters without gaps at the edges wool insulation (either from stone or glass NOT INO sheep) is easier to install without bypass gaps at the edges

What's the layer underneath the tiles in the illustration? I can't think what it might be in modernvictorian's context?

Also, what's the point of the 'drape' in the Mento, given that the whole thing is on a slope? Or even if it isn't on a slope?

Posted By: Mike1That's a layer of Pro-Clima Intello

I don't understand. I'm talking about immediately underneath the tiles. I'd expect Intello to be on the warm side, under the rafters, where the 'dotted line' is.

The central drape is no doubt to encourage any water to run down to the eaves

But it's running across, the roof not, down it?

Posted By: Mike1The section is from gable-to-gable (left-right), so any water would run down to the eaves. Maybe the interlock of the tiles is misleading?

Ah yes, that's what threw me. I thought it was upslope, but now I see that doesn't make sense with the rafters either

I still don't understand what the layer immediately under the tiles is though? Is that a tile batten?

Hello MV, forgive me if I am teaching you to suck eggs, but I do not know how much you already know.

You talk of woodfibre (henceforth WF) 'batts' between rafters. I assume you mean the 'flexible fluff', not the rigid WF. If the former, yes, that's fine as far as it goes, but remember that conventional wisdom says that you need a 50mm ventilation gap between the top of the rafter and any insulation (there are arguments for accepting a 25mm gap where there is no membrane at all, but I usually leave 50 as I am a pessimist).

Most WF has a lambda ('k') value of about 0.038-0.044W/mK, so let's take a working figure of about 0.040 for ease of calculation. 200mm of WF @ 0.040 would give an R value of 5m2K/W. Assume a 'base case' U value of 2.0W/m2K for the existing roof, giving an R value of 0.5m2K/W, so the R value for the whole sandwich is around 5.5. The reciprocal is 0.181818181.... so you could argue that that *just* creeps through the 0.18 requirement of Part L1B of the Bldg Regs - or you could argue that it doesn't, because my calcs do not take into account the 'timber fraction' - the amount of the 'sandwich' which is rafter, not insulation, in which case, increase to 225mm and you have probably cracked it.

OK, assume your rafters are 75mm (most Victorian ones I come across are, though the one I am working on at present has 100mm and a rare few have even bigger section). You can, with a 50mm ventilation gap, get 25mm between the rafters (can you get flex WF in 25mm - I perhaps doubt it). Therefore that's 200mm underneath. You could use rigid WF and 250mm fixings, but I would then advise you to retain a structural engineer, as the loadings will be massive. The alternative is a 'Larsen Truss'-type arrangement ('spaced stud') - effectively a secondary rafter, but that will still add weight to a timber section which would be deemed too small today.

I have not even considered headroom issues.

I have to say that's why, although I regard it with distaste, I often use PIR.

I hope that helps, or tells you what you already know, in which case, sorry!

Posted By: WillInAberdeenMV, neither our previous nor current Victorian houses have any felt under the slates. The slates have been there for many decades without leaking, and there is no sign of rain or snow getting in, even when a few slates blow off each winter, though there's plenty of 'ventilation' as you said! So we didn't disturb the slates to add membrane when we converted the loft..

Will - I am pleased to hear this. The loft is in great condition. Even the old insulation looks pretty clean. I keep being told I need a roof membrane but as you say I am reluctant to change something was has worked well for the last 100 years or so.

Posted By: Nick ParsonsHello MV, forgive me if I am teaching you to suck eggs, but I do not know how much you already know.

Preparing for first renovation project so only what I've read! Grateful for any advice.

The reason I am interested in wood fibre vs PIR is because of the supposed heat buffering effect due to lower thermal diffusivity values. I imagine this might be beneficial in a hot loft in the summer. Also I don't want to fill my house with petrochemicals if I can help it. However the additional thickness required could be problematic.

My architect has calculated for PIR I would need 150mm between rafters plus 50mm insulated plasterboard to give a final U-value of 0.17.

For wood fibre 150mm Pavaflex between rafters plus 100mm Pavatherm- combi to give a final U-value of 0.17.

The rafters don't seem deep enough for this if they are 75mm. Hmmm. I need to get up there and check, Can I get away with not using a 50mm gap at the top of the insulation given I have no roof felt?

Ultimately if I have to go PIR then so be it.

Posted By: modernvictorianCan I get away with not using a 50mm gap at the top of the insulation given I have no roof felt?

You really do need a membrane under the tiles, no matter what. If the insulation does get wet it won't be as effective, and it could stay wet for some time if the weather's bad.

The only advisable way to reduce the 50mm gap is to use a technical membrane as above, unless you strip the tiles and put it above the rafters.

No membrane = vapour permeable membrane and in terms of vapour transfer is probably a lot better

In the past I have stuffed some lofts, skeiling, dormers and roofs full of insulation with no ventilation and never see any problems, many had old style 1F sarking with string

Have seen plenty of problems with polythene or perforated polythene membranes and one or two with mouldy breather membranes in contact with quilt