Wood fibre and PIR?

[Haddock78]

Hello all,

 

we recently moved into a very neglected 1950s chalet style house, in the far south of Cornwall. The in-roof upstairs bedrooms are eccentrically insulated with a random mix of old glass wool, loose polystyrene sheets and - mostly - nothing. 

Knowing nothing much about insulation or building regs a few months ago, I now know a little more but feel ever more confused... Our rafters are 100mm deep at 600mm centres (ventilated roof). I'm assuming there is no real option in  the sloping ceilings apart from PIR to get to, let alone exceed, building regs (I think  at least 150mm is going to be needed)?

As we have more space behind the dwarf walls I was wondering if there was any mileage in fitting wood fibre insulation in this area? I've noted the various comments/topics on thermal mass/decrement delay as well as the claimed ability of wood fibre to store moisture. Or is it pointless to mix the two and we should stick to PIR for the whole thing?

Many thanks for any light you can shed.

 

[SteamyTea]

19 minutes ago, Haddock78 said:

in the far south of Cornwall

That will be the Lizard then.

 

Can you change the roof to a warm roof?

[Haddock78]

29 minutes ago, SteamyTea said:

That will be the Lizard then.

 

Can you change the roof to a warm roof?

Well almost - a smidgen north of the Helford.

I'm not sure about changing it to a warm roof. I assumed that as it is (probably) non-breathable felt I would have to leave the full 50mm below? Sorry if I'm misunderstanding what you are getting at. 

[Iceverge]

Welcome welcome.

 

Any pictures to share?

 

Fundamentally you need

1. A continuous layer of insulation with a

2. vapour control layer (airtightness too in           this case) inboard and 

3. space for ventilation outboard. 

 

Once that is done you're looking at

1. cost Vs

2. ease of install Vs

3. fire performance Vs

4. decrement decay Vs

5. embodied carbon V

6. u value for the various insulations. 

 

These won't matter a jot unless you get the fundamentals sorted first. 

[Haddock78]

Hi Iceverge,

 

yes, I've definitely read so much stuff on here and elsewhere to get the message about points 1,2 & 3!

Some photos attached (random foam/tape/plasterboard to temporarily block up holes where old cupboard was removed).

 

My thought for conventional buildup was for:

Sloping Ceilings - 50mm PIR between rafters/100mm PIR under/VCL/15mm Plasterboard

Dwarf Walls - 50+mm PIR over studs in eaves/100mm between studs/VCL/PB

 

My alternative was to use the same for the SC, but change the DW to:

PB/VCL?/100mm Woodfibre Batts between studs/100mm WF Batts over/80mm Woodfibre board to hold the batts outside of the studs in place?? My rough calculations suggest a comparable U value (~0.15) and about 1/3 more expensive than the PIR option.

Comments I've seen suggest WF doesn't need a VCL on the inside face, but wondering if I could continue PIR VCL along top of WF and then down outside to meet VCL under/over insulation laid in eaves. At present the eaves have glass wool, but mostly destroyed by previous plumbing work access and full of dead flies. Quite keen to replace it as well.

 

[SteamyTea]

Is your place a Cornish Unit?

[Haddock78]

2 hours ago, SteamyTea said:

Is your place a Cornish Unit?

No, we had it checked alongside the survey and it isn't. 

[SteamyTea]

5 minutes ago, Haddock78 said:

No, we had it checked alongside the survey and it isn't. 

Good, or not. If it had been, would just say demolition and rebuild.

Have you considered re-roofing?

May work out easier and cheaper, and you can add in PV if suitable.

Edited January 5, 2022 by SteamyTea

[Haddock78]

6 hours ago, SteamyTea said:

Good, or not. If it had been, would just say demolition and rebuild.

Ha, yes! Probably if we had the money that would probably be the best option here anyway...

 

Tbh I'd assumed that reroofing would be out of our budget and doing it diy/pieceal was going to be the only way. Maybe its something we should revisit: We'd like less ugly and better insulated dormers, the soffit and fascias are shot and pv was definitely on the plan. ?

[Iceverge]

Re roofing to a warm roof might be tricky being semi-detached as you would need to add insulation thickness above the rafters. 

 

I assuming the budget isn't going to stretch to a big bang job I think your bit by bit approach is fine. 

 

How much head space can you afford to loose? Do the rooms in the roof suffer from overheating on sunny days?

[Haddock78]

It's actually detached ( I think Steamy Tea was just using a picture of a typical Cornish Unit house, nothing like ours thankfully...!) with a pretty steep pitched roof. One single and one double dormer at the front and a flat-roof extension - of indeterminate age - midway along the rear. 

 

We think the tiles were replaced in the last 20 years and they seem in ok condition apart from not having been cleaned in a long time. Most of the ballpark reroofing figures appear to be about replacing the tiles. I've no real idea about the practicalities and how much it would cost to keep the tiles but replace the rafters/battens/felt to provide more space?

With the present plan of doing the insulation 'between and under' we are looking at losing 115mm on the sloping ceilings. Not ideal but not the end of the world, we're midgets anyway.

We only moved in in October, so can't say for sure about overheating - but the temperature definitely fluctuates significantly with the weather between mild and cold/windy conditions.

 

[Iceverge]

Sorry for the delay. 

 

Personally I would do the below. Note the service cavity allows for an unbroken airtightness barrier. Maybe put the woodfiber in 2 staggered layers to really do an ace job. 

 

 

 

If you're stuck for space or cash use PIR instead of the woodfiber. Remember U value is only one piece of a complicated picture and woodfiber would be my choice. 

 

 

 

 

For the eves space if you're really in the mood for some hardship you continue the method right to the wall plate. 

 

Otherwise just put 100mm Rockwool or similar batts between the uprights, 100mm woodfiber over. VCL and then 50mm insulated service cavity with 15mm soundblock plasterboard to finish. 

 

 

 

 

 

 

 

 

[Iceverge]

I forgot to say you'd need to lay some insulation on the horizontal portion of the eve space behind the dwarf walls. Rolled out mineral wool or blown cellulose would be fine. 

 

If you do plan on redoing the roof anytime in the near future I would just put 50mm mineral wool between the rafters, vcl and then a 50mm mineral wool insulated service cavity. 

 

The when the roof was redone top up the mineral wool insulation between the rafters to 100mm , put 100mm woodfiber over the top, membrane, batten counterbatten and tiles. 

 

 

 

[Haddock78]

On 09/01/2022 at 18:56, Iceverge said:

I forgot to say you'd need to lay some insulation on the horizontal portion of the eve space behind the dwarf walls. Rolled out mineral wool or blown cellulose would be fine. 

 

If you do plan on redoing the roof anytime in the near future I would just put 50mm mineral wool between the rafters, vcl and then a 50mm mineral wool insulated service cavity. 

 

The when the roof was redone top up the mineral wool insulation between the rafters to 100mm , put 100mm woodfiber over the top, membrane, batten counterbatten and tiles. 

 

 

 

 

Apologies for the slow reply. Thanks for putting that together, certainly helps to visualise it and some food for thought. We're still waiting to get someone round to talk through what is possible, and at what cost! Though I would guess we would have to get to the 0.15 in the present building regs? No idea how much headroom we would have to potentially lose before it is considered a justification for achieving the less stringent U values?