Good morning,

I hope you are all well.

Please bear with me as I may not get all the terminology right. I've been a long-time lurker on this forum, following excellent advice given by members.

I'm planning a warm roof retrofit, with sarking/rafters exposed internally (for an existing room in roof - which was converted many moons ago, and currently without insulation). The idea is to improve the property, which we are planning on staying in (it's been painful enough renovating to this point, so I don't plan on moving any time soon!). Price is a factor, as is relative ease of installation.

The reason we're not insulating from within is because I'm very tall, and just want to make the most of what little space we have upstairs (whilst dodging rafters).

My plan is, starting from the inside is as follows. The roof is a 35 degree pitch (if important), south facing on one side.

1) Existing 3-in Rafters (Traditional Cut Roof with Purlin)

(External)

2) T&G WISA Sprice Flooring as Sarking (18mm - to make installation easier - to be exposed from the inside, so this is also a cosmetic choice, as well to give good racking strength/still be simple to install)

Insulation is still undecided:

Either:
3a) 110-120mm PIR (for price/not decrement delay - I've got a reasonable price on the un-foiled variety)

or

3b) 100-160mm Wood Fibre - Sarking Board (for improved decrement delay - not looked into the price yet, but seems pricey)

4) Counter Battens
5) Battens
6) Pantiles

Eaves to ridge ventilation above insulation.

There are few dormers to factor in, plus a balcony cut out of the roof from one dormer.

We can add some insulation in between the rafters, if absolutely needed, but wanted to avoid this if possible. We definitely cannot add anything on the room-side of the rafters (again for headroom).

I've not yet worked out the nitty gritty, like fixings and membranes. Though have been making my brain hurt by reading through steico/super-7/celotex etc technical documents!

But am wondering if something like above could be feasible (with a few changes probably).

Many thanks,

Jonathan

Posted By: Pile-o-Stone

Thank you for getting back to me.

I'm not sure why you're going with woodfibre on top of PIR?

Hi, no sorry, it was the option of either PIR or woodfibre, one or the other not both. I've been melting my brain, reading about decrement delay and other properties of insulation, trying work out, given that it's only a little bit of insulation, what would be better to go with.

Having exposed rafters will looks great, especially in an older building

I was hoping so. I'm glad someone agrees with me.

Our roof is always the last one in the neighbourhood to lose its snow.

That's reassuring. Good to hear a bit of first hand experience.

As Paddy said, and just to add:

If you are looking for a U value of say 0.15 W/m2K, using the lambdas Paddy quoted, the insulation thickness needed would be approx

PIR 0.022/0.15 = 150mm
Woodfibre 0.034/0.15 = 230mm

Pro rata for different U values, many on GBF talk of U=0.1 or less.

At those thicknesses of woodfibre, issues like weight and 'nail ability' start coming in.

Alternatively you could build an even thicker roof, with I-joists or some kind of truss, and fill it with something light and cheap like cellulose or mineral wool?

Incidentally, are you in Scotland? Using planks for sarking seems to be trad here. I like it too!

Posted By: GreenPaddy

So 60mm of PIR will do what 100mm of wood fibre will do, heat loss wise, but be 150 time less vapour open.

Thank you, that's useful.

Are you replacing the roof anyway (due to its poor condition) cause this is a very expensive route, irrespective of the insulation?

Yes, unfortunately it is in a poor state, plus needs some extra work doing to it (extra dormers to be installed + replacing rotten woodwork galore), rotten battens from leaking tiles etc. I've already been up and replaced a reasonable amount of battens/felt as a previous emergency measure an am aware that if I don't pull the trigger we'll spend more time fixing than replacing.

just asking if you're aware of the cost impacts.

Not fully costed, but aware that it's going to cost a reasonable amount. We're in a situation where we look at the place as a house for life, and are also too deep to turn back... I've done most of the work myself: ripping up floors/insulating between joists, replacing half the ceiling joists in the place, digging up around 16 tonnes of of floor to install wet underfloor heating/insulation (wouldn't recommend that with a bad back, carrying concrete back in, in gorilla tubs was pretty miserable).

Posted By: WillInAberdeen

PIR 0.022/0.15 = 150mm
Woodfibre 0.034/0.15 = 230mm

That's useful, thanks.

Pro rata for different U values, many on GBF talk of U=0.1 or less. At those thicknesses of woodfibre, issues like weight and 'nail ability' start coming in.

What thickness of PIR would give 0.1... out of interest? (to overcompensate).

Nailing/securing issues: That is something I've come across. I'm wanting to get as much insulation up there as possible, whilst not breaking us too much financially and/or completely overhauling the structure of the roof (your idea below is a good one).

Alternatively you could build an even thicker roof... and fill it with something light and cheap like cellulose or mineral wool?

I hadn't thought of that. We've used a lot of Rockwool so far, and I like it a lot (for sound deadening and interim ceiling insulation to keep us comfortable & warm (the cats, mainly) whilst I pull the rest of the house apart). Not sure how cheap it is at moment though... seems everything is getting dearer!

Incidentally, are you in Scotland? Using planks for sarking seems to be trad here. I like it too!

No, but envious of that building technique, I noticed a lot of it in Belgium too. We're down in Yorkshire.

Posted By: GreenPaddy

If you are going for a new sarking layer then I would consider installing false rafters above the sarking and laying a thicker layer of insulation between the false rafters... perhaps rockwool batts at nearer £6/m2 for 100mm depth with draped felt, battens and tiles over

Thank you. I'm trying to imagine how this would work. I like it, because Rockwool batts are easier to fit in bays than Celotex or similar. Would there be a condensation risk for this? (namely if adding PIR on-top of the batt filled 'false rafters' as well?)

you could make provision for future EWI

I like the idea of provisioning for future wall insulation. It is certainly an option... if needed. Although I have spent an inordinately long amount of time carefully removing badly drilled bricks/matching bricks/reinstating bricks/repointing bricks/brick acid-ing bricks. Thinking smugly how I'll never have to paint again... it'll pain me to cover it up.

With it being a dormer bungalow I am thinking we could go for the option of 'over-compensate insulation on the roof', so these suggestions are all really helpful in getting my brain going!

With your build up using a continuous insulation layer over the new boarding, you would get approx the following Uvalues...

- 100mm PIR@0.022 = 0.18
- 100mm EPS@0.32 = 0.24
- 100mm EPS@0.40 = 0.28

step up to 125mm PIR gets you to 0.15, which is a much more satisfactory Uval, for all the work you're doing.

I've used continuous 100mm PIR before, then 45x75 over the top, mirroring the rafters, fixed with structural screws by Heco (Topix) or Timco. Also 45x100 on edge fixed directly to the rafters, at ridge and eaves like a boarder for the insulation

Then membrane and battens (I actually used boarded sarking, but that's Scotland).

I would not use any insulation that is water absorbant.
I would not add timbers and insulate between (more work, and 12% of area would be timber not insulation)
Foam between all the insul board joints to give a monolithic insul layer.

I understand your desire to keep every inch of head room, but I would be very tempted to board over the rafters with your finish, between the purlins, then fill between the rafters AND over, as per above. If you laid 75 PIR between the rafters, and 100mm PIR over (continuously), you would get to Uval approx 0.12. That starts to make it all worthwhile. Remember, your roof looks at the sky, so is subject to much greater heatloss, than vertical surfaces like walls, so need to go that extra bit further insulation-wise.

Post note Edit: - VCL in there too, close to the internal as poss. For simplicity, maybe loop it over and between the outside of the rafters. Then push PIR boards into it between rafter. Alternatively underside of rafters, but then there's a gap at each purlin - one to think about.

I'd need a passport and some sun block to go that far south

Seriously, I would suggest you decide upon your preferred insulation method, and then get a structural engineer to confirm the fixing method/design, which will also cover building control requirements. I have yet to meet a domestic architect who could advise well on insulation practise (our esteemed FTom excepted )

If you're still really stuck, but want to go down the continuous insulation over rafter route, PM me, and I'll send you some details, and also around veluxes, that I've used, passed building warrant, and still ontop of a house after 6 years.

+1 for between and over the rafters. 75mm between and 100 or even 150 over in your case. We have done this on two places now where we didn’t want to increase the height of the roof plane too much. In our case it was foil-faced PIR 90 or 100mm between and 100mm over, fixed through counterbattens with twist fixings/ helical nails (ours were 165mm length to go through the CB & insulation and into the top of the rafter, spacing is set out on the internet to suit your specific case) Then breather membrane, tile battens and tiles.

If increasing the height is less of a consideration, then it looks like you are on the right lines with the Viking House route.