Steico Flex wood fibre in short supply - what's the alternative

[Adsibob]

The build up of the loft space where my MVHR and hot water cylinder are going was going to be like this:

 

 

Steico Flex has been in short supply for months. We placed the order at the beginning of August and were told it was a 10-12 week lead time but we would have it in October. Today my builder got a call from the supplier who cancelled the order. Calling round today we are getting even longer lead times of 12-14 weeks (From now). My builder is in a rush and wants to use PIR instead of the 140mm Steico Flex. I'm not keen on this at all because this build up was designed (together with the external layers of the roof such as the sarking board) to give us a decrement delay of about 8.5 to 9.5 hours. Not as good as the ideal 11h, but still much much better for keeping the house cool than PIR. My builder says it's just the MVHR and hot water cylinder room so it doesn't need to be kept cool. There is not much linear metres of MVHR pipe in that room, as although all the pipes start and end there, they leave the MVHR room within 3m or 4m at most.

 

We have enough Steico flex for the the actual living spaces in the roof (where the build up is 160mm of steico instead of 140mm and we also have additional 40mm of hard wood fibre, so 200mm in total).

 

What should I do: just make the builder's life easy and go with PIR for the section of the roof that encloses the MVHR and cylinder room, or try and find an alternative supplier or alternative product? (I've already called 4 suppliers any nobody has Steico flex in supply.)

[PeterW]

Use Frametherm 35 instead - will be pretty similar and it’s a standard product. Never sure why architects specify random products at twice the price that are on ridiculous lead times. 

[Adsibob]

28 minutes ago, PeterW said:

Use Frametherm 35 instead - will be pretty similar and it’s a standard product. Never sure why architects specify random products at twice the price that are on ridiculous lead times. 

Thanks @PeterW, I had a look at Frametherm in a 140mm thickness and it has a thermal conductivity of 0.035 W/mK and an R-value (thermal resistance) of 4.00 m²K.

Steico flex has a similar conductivity at 0.036, and similar R value at 3.85. 

 

There is no data in Knauf's datasheet on Frametherm 35's specific heat capacity or density, which I think are the remaining inputs I would need to calculate decrement delay, but I remember that you used Frametherm in your build. But didn't you have a much thicker size that the 140mm I have space for?

 

At least comparing to the specs of generic glass wool on this website wood fibre would appear to have twice the specific heat capacity of glass wool and several times the density, suggesting it would give a much better decrement delay. 

Edited October 5, 2021 by Adsibob

[PeterW]

Ok but why are you concerned about the decrement delay on a service space..? Fluctuation of that space will be irrelevant as it is probably 8-12°C warmer than the rest of the building due to its contents and in reality you want to keep the thermal performance better than the decrement performance so density becomes irrelevant. 

[Adsibob]

3 minutes ago, PeterW said:

Ok but why are you concerned about the decrement delay on a service space..? Fluctuation of that space will be irrelevant as it is probably 8-12°C warmer than the rest of the building due to its contents and in reality you want to keep the thermal performance better than the decrement performance so density becomes irrelevant. 

I think you are probably right. I guess I was worried about the heat from the outside transferring to the MVHR room and then heat from there transferring to the adjacent living space. We have fairly thin metal stud partitions between the MVHR room and the adjacent rooms.

Also, on a hot summer's day, won't the excess heat in the MVHR room affect the temp of the air supply to the rest of the house, or is that not relevant because the air supply pipes are pretty short in that room - about 3m to 4m before the exit into adjacent rooms.

Edited October 5, 2021 by Adsibob

[PeterW]

So firstly you should be insulating the MVHR pipes to stop condensation etc as that is a concern.  
 

You should also have some sort of insulation in the studs to stop thermal and noise transfer, and that could be something such as the frametherm or slightly denser acoustic insulation panels. 
 

You have 65mm of PIR/PUR in that build up already, external heat effect is going to be minimal I would expect. 

[Adsibob]

7 minutes ago, PeterW said:

So firstly you should be insulating the MVHR pipes to stop condensation etc as that is a concern. 

I was told by the MVHR that the pipes should be within the thermal envelope. If i do that, why would I need to insulate them? Sorry, I'm sure I'm being quite thick about this. Also, what do you use to insulate them? Mine are 90mm radial ducting a bit like this: https://www.cef.co.uk/catalogue/products/4875483-fast-track-90mm-semi-rigid-ducting-50m-length?gclid=CjwKCAjw7--KBhAMEiwAxfpkWAMjqFn5zXL5HXettKD9Wlz_yypRWtKaaIs52jhED_tHs1zt_mUszRoCDnAQAvD_BwE&gclsrc=aw.ds

[Hastings]

There is Pavatex/Pavaflex wood fibre insulation. Made in the same factory as Steicoflex.

Both are breathable and wick moisture so check your design doesn't rely on that. They are generally used in combo with a techno VCL or none at all too. PIR if you use it best fitted with Gapotape otherwise is unlikely to insulate very well.

[Iceverge]

On 05/10/2021 at 12:22, Adsibob said:

I was told by the MVHR that the pipes should be within the thermal envelope. If i do that, why would I need to insulate them? Sorry, I'm sure I'm being quite thick about this. Also, what do you use to insulate them? Mine are 90mm radial ducting a bit like this: https://www.cef.co.uk/catalogue/products/4875483-fast-track-90mm-semi-rigid-ducting-50m-length?gclid=CjwKCAjw7--KBhAMEiwAxfpkWAMjqFn5zXL5HXettKD9Wlz_yypRWtKaaIs52jhED_tHs1zt_mUszRoCDnAQAvD_BwE&gclsrc=aw.ds

No need as the air inside will be almost the same as room temp. 

 

You must insulate the inlet and exhaust from outside however. These will contain air at outside air temp. 

[SimonD]

On 05/10/2021 at 11:03, Adsibob said:

The build up of the loft space where my MVHR and hot water cylinder are going was going to be like this:

 

 

find an alternative supplier or alternative product? (I've already called 4 suppliers any nobody has Steico flex in supply.)

 

I have to say I think that buildup is very weird as you've got layers of incompatible materials, thus you actually lose a lot of the benefit of the woodfibre board. Who designed this because it would be much better to have designed the buildup with the flexible woodfibre between rafters and then woodfibre boards on top of the rafters. This would give you the benefit of the materials, plus extra decrement delay and better hygrothermal performance of the envelope.

 

However, if you're still looking for alternative supplier, you could try Ty Mawr who sell the Schneider Holtz woodfibre products, or as has been suggested above, Pavatex Pavaflex. You could also try Hunton - huntonfiber.co.uk - a Norwegian company.

 

Good luck

[Adsibob]

2 hours ago, SimonD said:

 

I have to say I think that buildup is very weird as you've got layers of incompatible materials, thus you actually lose a lot of the benefit of the woodfibre board. Who designed this because it would be much better to have designed the buildup with the flexible woodfibre between rafters and then woodfibre boards on top of the rafters. This would give you the benefit of the materials, plus extra decrement delay and better hygrothermal performance of the envelope.

 

However, if you're still looking for alternative supplier, you could try Ty Mawr who sell the Schneider Holtz woodfibre products, or as has been suggested above, Pavatex Pavaflex. You could also try Hunton - huntonfiber.co.uk - a Norwegian company.

 

Good luck

It was designed by a RIBA architect and a RICS Surveyor. I’d be grateful if you could point out the incompatibility?

We are using wood fibre boards elsewhere in the house, but this specific part of the roof (which only bounds the staircase and the MVHR room) has limited headroom and so we need to be sparing with the wood fibre as it’s not as good with the U values as PIR so wouldn’t have passed regs. 
 

pavatex was my instinctive second choice but they are just as out of stock as Steico. Not heard of the others you mention, but I think I’ve settled for Knauf Frametherm 32 now, which isn’t as good as wood fibre for decrement delay, but is at least significantly better than PIR. It’s only for a small portion of the roof, so will have to do.

 

[Adsibob]

4 hours ago, Iceverge said:

You must insulate the inlet and exhaust from outside however. These will contain air at outside air temp. 

Thanks. But do you mean just the inlet?

[Iceverge]

inlet/intake + exhaust  are the cold side of the MVHR and duct to outside. the supply and extract are the warm side and duct to the house. 

[SimonD]

8 hours ago, Adsibob said:

It was designed by a RIBA architect and a RICS Surveyor. I’d be grateful if you could point out the incompatibility?

 

It's simply the buildup using PIR to the inside of the woodfibre. It makes no sense.  Beware professionals has become my motto following the self-build journey. Many RIBA and RICS peeps don't really understand how woodfibre actually works, why the usual recommend buildup other than it's supposed to be breathable, and then many of then don't even understand the full meaning of that term either. Sorry, I've been jaded

 

8 hours ago, Adsibob said:

wood fibre as it’s not as good with the U values as PIR

 

The good news on that one is that historically hygroscopic/capillary insulation materials like wood fibre, sheepswool, cellulose etc. have had their stated u-values down-rated due to the assumption that when they absord moisture they also insulate less well. Unfortunately this doesn't take into account that their behaviour in absorbing and releasing moisture tends to generate heat, and also that up to a point the moisture doesn't affect their thermal performance. You'll likely get better performance out of them than predicted, whereas it's frightfully common for insulation systems like PIR not to perform as predicted, but that doesn't always help with BC unfortunately.

 

8 hours ago, Adsibob said:

I think I’ve settled for Knauf Frametherm 32 now

 

Sensible decision with the buildup.

 

8 hours ago, Adsibob said:

decrement delay

 

Decrement delay is an interesting one for me. I designed my house for good decrement delay and it really works to a point when you have nice sinusoidal temperature curves through night and day. Last summer when we had the heat wave, the house, even the upstairs timber frame, maintained a nice cool temperature compared to outdoors. However, on day three of relentless heat, it seemed to just become saturated with heat and gave up. Suddenly the indoor temp shot up to just above outdoor temp. and then took some time to settle back down again.

 

[Adsibob]

2 minutes ago, SimonD said:

 

It's simply the buildup using PIR to the inside of the woodfibre. It makes no sense.  Beware professionals has become my motto following the self-build journey. Many RIBA and RICS peeps don't really understand how woodfibre actually works, why the usual recommend buildup other than it's supposed to be breathable, and then many of then don't even understand the full meaning of that term either. Sorry, I've been jaded .

 

Thanks @SimonD, but just to clarify, what issue might I face with this build up that combines 140mm of wood fibre with a much smaller amount of PIR? The RICS surveyor has been very good and introduced us to a lot of products that neither my architect nor I were familiar with, so is surprising that he might have got this one so wrong. What problem could I have from having this build up? (By the way, I haven’t shown the full build up - there is also a tyvek breathable membrane, Barton’s darling board and clay tiles.

[SimonD]

Don't worry about it too much, it's not going to cause you any damage or harm to your room. To explain:

 

Many people think that 'breathable' equates to vapour permeability and vapour diffusion but in the the case of woodfibre, this isn't the benefit as you do install a VCL to the inside of the woodfibre buildup. But this vcl needs to be one that is compatible with the properties of woodfibre.

 

Why? This is because the value of woodfibre is its capillary and hygroscopic capability that can absorb and take moisture through the envelope and out of the building, but it can also work in the opposite way to prevent overly dry air in the building during winter for example. This is why these buildups use osb or even clt as the vcl. If you add an impermeable or non hygroscopic/capillary material to the buildup this process is short circuited.

 

So where and when do you get best bang for buck with wood fibre? In well insulated and airtight buildings with low air change rate, preferrably 0.5 ach and below.

 

Also, in terms of decrement delay, the significant benefit comes not from the flexible wood fibre but the denser boards installed to the external side of the frame.

 

In your example case, there is a minor reduction of risk in that if your vcl and pir layer is breached or incomplete in any way, any excess moisture coming through due to vapour diffusion in this case, will be buffered by the flexi wood fibre whereas a mineral wool could not do this. However, given your situation and buildup, it is more cost effective and practical simply to use a mineral wool.

 

HTH

[BenP]

We opted for Rockwool Flexi between our studwork. Wood fibre board over to wrap it up. 

 

The flexi came in 50% of the cost of wood fibre batts or sheeps wool.