Insulation PIR vs Wood Fibre

stamas01

Hi all,

So I bought a period property in Scottland that was built around 1900. I want to insulate the internal walls (I cannot insulate the outside as it is in a Conservation area). I did my research, and I am now considering two options.

Option 1: Celotex GA4100 General Purpose PIR Insulation Board - 2400 X 1200 X 100mm
Option 2: STEICO Therm Wood Fibre board

PIR:
Advantages:

• Much better R-Value (the higher, the better): According to the Celotex datasheet, a 100mm insulation achieves 4.5 R-Value (m².K/W) .
• Easier installation, as I can use a plasterboard over it.

Disadvantages:

• According to Scotland-specific research (Guide to Energy Retrofit of Traditional Buildings), PIR-type insulation must only be used if a complete condensation analysis confirms that it can be used. Unfortunately, for that reason, the study excludes PIR insulation and does not compare it to the other options.
• Given the wall type, we must maintain a well-ventilated air gap behind the insulation. As it is not trapped air, does it not somewhat decreases the insulation capability compared to wood fibre, which does not require an air gap?
• If it is not airtight, there is a severe risk of condensation.

Wood fibre:
Advantages

• Breathable. This approach does not require ventilated air gap.
• It does not have to be airtight, and if cracks appear over time, it won't cause worry.

Disadvantages:

• Much lower R-Value (the higher, the better): datasheet states it achieves 2.5 R-Value (m².K/W) at 100mm.
• Hard to install as I have to lime plaster over it.

Our wall is about 600mm "mass masonry wall" and is internally finished with lath and plaster. Given that plaster is not breathable, I can't see much difference between PIR with airgap and lath and plaster or is there? Also, I find it difficult to translate the R-value difference to the money spent on heating during the winter. Can someone comment on this as well? Based on your experiences and expertise, would you choose the PIR or wood fibre option, and why?

Thanks for all the help!

theoretica

If you have traditional lath and plaster, with lime plaster, it is breathable.

stamas01

theoretica said:

If you have traditional lath and plaster, with lime plaster, it is breathable.

It certainly looks traditional. So that is one of my questions answered. Thanks for taking the time

Eldi_Dos

With the proviso that it depends what the walls have been painted with over the years with the age of the property.

There was a fashion at one time to varnish under the dado rail to give a wood effect, also if a room had a range and was used as a cooking and laundry area the walls could have been treated so they are not breathable. you really need to strip back to bare plaster to be sure.

stamas01

Eldi_Dos said:

With the proviso that it depends what the walls have been painted with over the years with the age of the property.

There was a fashion at one time to varnish under the dado rail to give a wood effect, also if a room had a range and was used as a cooking and laundry area the walls could have been treated so they are not breathable. you really need to strip back to bare plaster to be sure.

Thanks for that. I will remove the lath and plaster anyway to install the new insulation. I was just confused why PIR is so dangerous while lath and plaster are not. But I learned that lath and plaster are traditionally done using lime-based plaster, so it is breathable. But of course, it depends on what you paint it with.

Eldi_Dos

stamas01 said:

Eldi_Dos said:

With the proviso that it depends what the walls have been painted with over the years with the age of the property.

There was a fashion at one time to varnish under the dado rail to give a wood effect, also if a room had a range and was used as a cooking and laundry area the walls could have been treated so they are not breathable. you really need to strip back to bare plaster to be sure.

Thanks for that. I will remove the lath and plaster anyway to install the new insulation. I was just confused why PIR is so dangerous while lath and plaster are not. But I learned that lath and plaster are traditionally done using lime-based plaster, so it is breathable. But of course, it depends on what you paint it with.

Sorry I just meant to remove any paper and rub down plaster before you can be sure of its breathability, with a property of that age.

FreeBear

A quick comparison of PIR and wood fibre boards -

PIR - 0.023 W/m²K

Wood fibre - 0.037 W/m²K

Another one to add to the mix - Cork at 0.043 W/m²K - Still breathable like wood fibre, and is best with a lime finish.

A rough fag packet calculation suggests that 100mm of wood fibre or cork will get you close to the 0.3 W/m²K target given for English building regulations - Not sure what Scottish rules mandate.. Assuming a similar target figure, I'd go for wood fibre with a lime finish to maintain breathability of the walls. Don't forget to wrap the insulation around the window & door reveals. You will be restricted on the levels, so iy might be worth using 25mm of PIR in those areas (or Aerogel if you can stomach the price).

One tip - fix a strip of 12mm plywood above the windows, say 150mm high, and extending 150mm either side (use extra mechanical fixings to secure it to the wall). This will provide a sturdy surface to fix curtain rails to. If you are planning on wall units in the kitchen, more plywood on the walls would be prudent.

I've been insulating the solid brick walls here. But it is only the upper half that is solid, lower half is cavity. The exterior has a thick layer of cement render (original coating), so breathability is out of the window. Using 65-75mm of Kingspan PIR boards internally finished off with plasterboard & ply in a couple of spots. Still have a couple of rooms to do, but the house is already much easier to heat and feels warmer during the winter months.

If this house had been solid brick top & bottom, I would have gone for wood fibre.

stamas01

Thanks, @FreeBear for taking the time and writing this all up for me. I appriciate it! I forgot to mention that my house is stone-built. Would you still go with the wood fibre?

FreeBear

stamas01 said: I forgot to mention that my house is stone-built. Would you still go with the wood fibre?

Scottish Granite by any chance ?

Not that the type of stone makes that much difference - I'd still go for wood fibre board.

stamas01

FreeBear said:

stamas01 said: I forgot to mention that my house is stone-built. Would you still go with the wood fibre?

Scottish Granite by any chance ?

Not that the type of stone makes that much difference - I'd still go for wood fibre board.

I have no idea. The Home Report only stated that it is stone built XD. I attached a pic if it is any help.

Eldi_Dos

The railings on the side wall I think are quite rare and of interest, they look to me to be the originals.

During WW2 railings and lots of gates were cut down and taken away to be smelted, they might have got a dispensation for them with the drop off the end,which could have been deemed dangerous in the dark.

If you look at the coping stones on front wall you may well see marks where original railings where in wall, these marks usually have mortar over them. If so it is good practice to keep the mortar in order to stop water puddling there and getting into coping stones and freezing which can crack some stone.

I have always referred to facings on that style of house as sandstone but do not know if that is the correct description.