roof insulation for a stone pent shed

[samhay]

Somewhat usual story - I am slowly converting shed into a workshop.
Difference is that the shed is a brick and stone pent style, created by knocking through the old outside toilet and coal shed at the bottom of the garden. Yes it is as small as you are probably thinking, about 10' x 5'.

Having put a decent DPM down under a floating wooden floor, I now have a nice dry shed. Next step is the roof. This was replaced a few years ago and is traditional slate with battens and, what appears to be a breathable (not felt), membrane over the 4 x 2 roof joists. I would like to board out the ceiling (underside of joists), so figure this is the time to put in any insulation. Major priority is sound proofing rather than heat insulation, but some of both would be nice.

Venting is a problem and the reason for the post.
Access to the barge board is restricted and the ridge board is fixed against the high wall with coping stones and lead flashing above, so no obvious way to get any ventilation through the roof space.
If I fix insulation between the eves, what's best practice?
1. use a 70mm insulation board and leave a 25 mm gap on the cold side, relying on the membrane to allow the space to breath.
2. push insulation board or fibreglass insulation hard against the membrane, so there is no gap. I see mention that the requirement for a gap can sometimes be relaxed in this case. I guess slate, which allows reasonable airflow, may be such a case as long as the membrane is breathable?
3. something else?

In any case, I am planning on putting a membrane (or taping the joints if using e.g. Celotex) on the warm side under matchboard, but would welcome any suggestions.

Thanks,
Sam

 

[MikeG.]

So you want to insulate between the rafters, and not horizontally at tie level? If so, then you should leave a min 25 airspace above the insulation below the membrane, with 50mm being ideal to allow for the sag of the membrane. With a breathable membrane there is no need to have further ventilation, particularly with an intermittently used building, but contrary to what you wrote, the airflow under slate isn't great so you shouldn't take liberties. Your rafters, however, are only 100mm deep, so if you use 50mm of fibreglass between (to leave 50mm gap over), you will need further insulation on the underside of the timbers. As acoustic insulation is more important, you could batten out with horizontal 50x50 battens to the underside of the rafters, at 600 centres, with mineral wool between, and then your ceiling boards under that fixed to the battens. If you aren't going to use OSB, then a vapour barrier, not a breathable membrane, is desirable between the insulation and the ceiling boards.

That doesn't give you an awful lot of thermal insulation, frankly. However, it will be the best you can do acoustically, and be sort-of adequate thermally. Otherwise, 50mm of Celotex under the rafters will improve the thermal performance but decrease the acoustic damping.

If trying to cut fibreglass between rafters sound awkward, it's because it is. You should use cavity batts, which are stiffer than loft insulation on a roll. Make sure you cut them just slightly oversize so that they stay jammed in place until you can batten out below, but if you're struggling, staple some garden netting under the rafters to hold the fibreglass up.

 

[samhay]

Thanks for quick reply.
Interesting about airflow under slate and I can see how that could be a problem in retrospect.

Head height is not great, so I am reluctant to reduce this with insulation under the rafters. I guess that leaves me with 50-70 mm between rafters (depending on how much liberties I take), which isn't great but is also infinitely more than there is currently.

I guess Celotex or similar will give better thermal insulation but will also turn the roof into a drum skin. Any thoughts on the Rockwool RWA45 insulation, or similar? 50mm acoustic bats between rafters might be the least worse option. Thermal insulation will be rather old fashioned, but small spaces warm up fast, so I don't see that as being a deal breaker.

Was thinking vapour barrier rather than membrane on the warm side, honest. Could stretch to space blanket if that is likely to make any real difference to the thermal insulation?

 

[MikeG.]

......Head height is not great, so I am reluctant to reduce this with insulation under the rafters. I guess that leaves me with 50-70 mm between rafters (depending on how much liberties I take), which isn't great but is also infinitely more than there is currently.

I guess Celotex or similar will give better thermal insulation but will also turn the roof into a drum skin.

Exactly.

Any thoughts on the Rockwool RWA45 insulation, or similar?

I've never specified it, but if it does what it says it does (which it will), then you'll get marginally better sound deadening and marginally poorer thermal insulation compared with cavity batts. The benefits will be marginal, too, because the depth is so thin.

Could stretch to space blanket if that is likely to make any real difference to the thermal insulation?

Controversial subject, and it won't help anyway because they require battening out either side, so will use up 50+mm of headroom anyway.

Your choice of matching board for the ceiling isn't going to help you (think drum skin, again). When I've a moment later I'll find an acoustic board which will work better acoustically.

I have to say that soundproofing the roof is going to be a wasted effort if you have a single glazed window and uninsulated unsealed door, or any other gaps. Sound breaks out through the tiniest gap (in fact, the waves actively wrap around edges so gaps focus sound, concentrating it), so gap sealing is going to achieve more in terms of sound deadening than all your work in the roof.

 

[samhay]

>The benefits will be marginal, too, because the depth is so thin.
+
>I have to say that soundproofing the roof is going to be a wasted effort if you have a single glazed window and uninsulated unsealed door, or any other gaps.

Yes, I have a single-glased window and an uninsulated unsealed door, although it is heavy and fairly tight to the door frame.

It sounds like I may be better off prioritising thermal insulation then. In which case I'll do some careful measurement of the membrane sag and decide what depth of insulation board I can get away with.

I'm familiar with the spcialist acoustic plaster boards, from a previous life. They are heavy and a bit of a pig to install in the ceiling, so I think I'll stick with the plan of softwood matchboard and give up on any real sound insulation.

Thanks again.

 

[samhay]

Finally got round to doing this over the last couple of weekends, so figured I'd put the thread to bed.

Ended up using 11mm OSB fixed to the rafters with 50mm fibreglass insulation between rafters. Not what I was originally planning, but OSB and insulation were available locally and my source of matchboard dried up. It was going to be significantly more costly to use insulation board and I would have had most of a board left over, so figured the difference in insulation was probably not worth the expense.

I used Knauf Dritherm slabs, which went in much easier than I was expecting. I was dreading this stage, but it really wasn't a problem as cutting them a little oversize meant they happily stayed where they should until the boards went up.

Getting sheet material on the ceiling, however, was a major faff. After some head scratching, I realised I would have to defy the laws of geometry to fit a board the full span of the ceiling. Thankfully I realised this before installing the insulation, so I put a row of noggins in and ripped the sheets in half. Still not fun putting half-ish sheets of board in by oneself, but much more manageable.