Mixing wood fibre sarking with PIR in pitched warm roof

[DonnaP123]

Advice needed! For our woodcrete extension we have built a warm pitched roof with 80mm wood fibre sarking boards over the rafters. We planned to put approx 190mm flexible wood fibre bats between the rafters, but now realise many of the rafters are not deep enough to accommodate this. 
So my question is, is it okay to put PIR between the rafters (as would need less depth than wood fibre for the equivalent U value), and how much would we need to reach a U value of 0.15 overall, including the wood fibre sarking already there? I can’t find an online calculator that allows me to mix wood fibre with PIR!
 

We would lose the breathability of the wood fibre but should still hopefully benefit from not overheating in the summer (which was the main reason for using it) given we have 80mm wood fibre above the rafters, so I don’t think there is an inherent problem with mixing the two, but could be missing something?

Edited March 13 by DonnaP123

[Temp]

OK so the target U value is 0.15.

That's an R value of 1/0.15 = 6.7 needed.

 

Wood fiber has a Thermal conductivity = 0.041 W/mK

R value of 80mm would be..

= (1/0.041) * 80/1000 = 1.95

 

So the PIR needs to provide an R of..

6.7 - 1.95 = 4.75

 

The Thermal conductivity of PIR is about 0.022 W/mK

The thickness in mm you need would be..

T = 4.75 * 0.022 * 1000

= 104.5 mm

 

However I'm not sure it's safe to put any insulation between the rafters without a condensation risk analysis..

 

The term warm roof has two different meanings that get confused. One refers to the temperature of the loft (I prefer warm loft for this).

 

The other refers to the temperature of the structural members (rafters). If the insulation is entirely above the rafter then we call this a Warm Roof because the rafters are on the warm side of the insulation where condensation cannot form. Conventional roofs have all the insulation between or below the rafters putting them on the cold side. To prevent condensation in this case they must normally be ventilated, typically with a 50mm ventilated void.

 

If you have some insulation above and some between the rafters then this is a hybrid roof frowned on in some quarters. The insulation between the rafters lowers the temperature of parts of the rafter potentially increasing the risk of condensation. Some insulation can be installed between rafters but you really should get a condensation risk analysis done to work out how much. I have a feeling it won't be as much as you need.

 

You could try approaching a PIR manufacturer. Explain you have 80mm wood fiber above rafters and would like to fit some PIR between but need a condensation risk analysis done to work out how much is safe. They might be able to do one free for you to try and get the business.

 

Note I'm not an expert on condensation and the breathability of the wood fibre might make a difference to the above.

 

Could you put a whole lot more wood fibre on top of the existing 80mm to make it a proper warm roof?  I estimate you would need a total of.. T = 6.7 * 0.041 * 1000 = 275mm wood fibre, Less the 80mm you already have is another 195mm. Or you could possibly put the 105mm PIR on top? This would complicate fixing tile battens.

 

Your BCO might have strong views about this. Did he approve the original hybrid with wood fibre both above and between? 

 

What do others think?

Edited March 14 by Temp

[SteamyTea]

3 hours ago, Temp said:

What do others think

I think you have summed it up quite well.

 

Dew Point (condensation temperature) can be estimated quite accurately with this formula.

 

T_{dew °C} = T_{(internal °C)} - ((100 - RH_{(internal %)})/5)

 

The temperature gradient can be considered linear, so the middle point will be half the temperature difference.

 

Using the typical ΔT of 20°C and an overall roof thickness of 0.25m and an internal relative humidity level of 60%.

 

T = 20 - ((100 - 60)/5)

T = 20 - (40 / 5)

T = 20 - 8

T = 12

 

As the roof is 0.25m thick, and the ΔT is 20, that is 80°C.m^-1 (20°C / 0.25m)

Dividing by the dew point temperature, 12°C

 

80 [°C.m^-1] / 12 [°C dewpoint] = 6.7 m^-1

1 / 6.7 [m^-1] = 0.15 m

 

So in that situation, there is a condensation risk at 150mm and extending outwards.

You can see why the insulation is preferred on the outside of the rafters.

 

Now the real answer is how often do you get those conditions.

I had a quick look at 2023 weather data for my location and the percentage of time when there was a greater than 20°C temperature difference was 0.36%.

 

From my understanding, a warm roof is treated just like an ordinary wall (and why wouldn't it) in that you still need to fit a vapour control layer, this reduces the higher (generally) temperature and RH air migrating into the structure.

 

(it is very early in the morning and I may have made a mistake, but shall let other find it)

 

Edited March 14 by SteamyTea

[Susie]

I got both Kingspan and recticel to do a risk analysis for me on my architects  hybrid roof.  He wanted 80mm above and 120mm between rafters which I thought was the wrong way round so got it checked. It turned out Kingspan said it’s just about ok but recticel said no and they said the other way round is best or all above and gave me a few options they both did the analysis for free but recticel product is often cheaper for no obvious reason other than it’s not the brand name of choice for architects as far as I can see. 
my decision in the end is all above, less risk. 
they both ask for your postcode and give you graphs and data back, our data is collected from a point further inland than my home. 
im not sure how future proof the calculations are though, especially for me quite near the coast with mist that can linger in the shoulder months, I can have 24hrs of mist/fog.  I have noticed the changes over the last 8 years in all seasons, 

I gave BCO my results from recticel and they were happy with my decision, to change to warm roof rather than hybrid. 
 

[SteamyTea]

3 minutes ago, Susie said:

im not sure how future proof the calculations are though, especially for me quite near the coast with mist that can linger in the shoulder months, I can have 24hrs of mist/fog.

Mist is condensed water vapour, so needs to be treated differently. Needs to be treated as rain, not vapour.

 

Below is the data near me for last year.

At temperatures below 16°C the RH is pretty consistent at around 86%.  That accounts for 79% of the time (don't tell emmets that it is really cold and damp in Cornwall).

Say climate change raises the temperature by 1°C, from 12°C up to 13°C, the RH will reduce by 2%, to 84%, really nothing in reality.

 

[SimonD]

7 hours ago, Temp said:

Did he approve the original hybrid with wood fibre both above and between? 

 

The proposed build up using wood fibre is a standard and acceptable way to install wood fibre, but it isn't well understood here in the UK - trust me, I had to submit loads of technical documents so my BCO and team could learn about these systems!

 

For the OP, I would personally avoid using mixed layers of insulation that behaves differently and look at other ways to make it work. One way to do this is to use counter battens below the rafter and put in a layer of the wood fibre there.

 

3 hours ago, SteamyTea said:

So in that situation, there is a condensation risk at 150mm and extending outwards.

 

One of the behaviours of wood fibre and other natural materials that often gets overlooked is the ability of the insulation to buffer the moisture.In the modelling for my house, the roof showed a small amount on condensation risk at the outer layers, but as the system was analysed by the manufacturer's technical team, it was assured that this small amount, possible in Jan/Feb would be satisfactorily buffered. But this is obviously looking at a buildup that uses the same materials throughout.

Edited March 14 by SimonD

[SteamyTea]

2 minutes ago, SimonD said:

the ability of the insulation to buffer the moisture

Generally it buffers as a liquid, not a vapour, so the risk is the same.

Having said that, I am of the mind that a few grams of liquid water does not pose a long term problem.  If it did, all newly build masonry buildings would rot the timbers in no time.

[Susie]

4 hours ago, SteamyTea said:

Mist is condensed water vapour, so needs to be treated differently. Needs to be treated as rain, not vapour.

 

Below is the data near me for last year.

At temperatures below 16°C the RH is pretty consistent at around 86%.  That accounts for 79% of the time (don't tell emmets that it is really cold and damp in Cornwall).

Say climate change raises the temperature by 1°C, from 12°C up to 13°C, the RH will reduce by 2%, to 84%, really nothing in reality.

 

Thanks, So I better stop telling the other half its all in his head the feeling of being in constant high humidity because most of the time we probably are, I told him once that bit of the barometer was most likely broken as it didn't change much, he can't see the dial now without his glasses and the novelty has worn off.

[SteamyTea]

53 minutes ago, Susie said:

I told him once that bit of the barometer was most likely broken as it didn't change much

Probably rusted because of lack of movement.

 

16°C and above only happens 21% of the time, and some of that is at night.  And we had a really hot spell last year.

[DonnaP123]

Thanks everyone! Food for thought….dew point analysis and seeing if we can find a way to make wood fibre alone work.

[Iceverge]

5 hours ago, DonnaP123 said:

Thanks everyone! Food for thought….dew point analysis and seeing if we can find a way to make wood fibre alone work.

 

How thick are the rafters and I'll push something cheap and buildable though ubakus.  Or you can do it yourself. https://www.ubakus.de/en/u-wert-rechner/?

 

Also what's the planned build up above the woodfiber? This is extra important to know if you plan on a material of very low vapour permeability such as foil faced PIR. 

 

I'm not a fan or PIR between timber rafters. It's slow and wasteful to fit properly. Expands and contexts differentially to timber so is a recipe for gaps long term. It's poor for noise and fire Vs minerals wool. 

 

 

 

 

[Redbeard]

On 13/03/2024 at 23:05, DonnaP123 said:

we have built a warm pitched roof with 80mm wood fibre sarking boards over the rafters.

 

I have not had a comprehensive read, so apologies if I am countering anything you have already said, but I assume your roof is finished, with tiles/slates/whatever already on. If not, can you increase the depth of rigid WF?

 

As per others' comments it looks like, strictly, your roof is 'the wrong way round', in that, crudely, the 'conventional wisdom' is that you have more above the rafters than between (66.6%/33.4% is sometimes suggested). However I have seen some details (sorry, cannot remember where, but I have a feeling a colleague did this) using WF sarking and fulll-fill Warmcel (recycled newspaper) in deep I-beams, so clearly some people have different ideas.

 

If you decide to 'go for it', why not just put counter-battens on under the rafters to achieve the depth you need, so you do rafter depth in one direction and between counter-battens in the other, which has the virtue of each layer covering the last layer's gaps?

[Iceverge]

It can be done safely IF you are fastidious with airtighness. 

 

Almost nobody is, hence the fear of hybrid roofs. 

 

You only get issues when you have more moisture getting in Vs getting out. 

 

Airpaths through the structure are the real risk when drafts carry moist internal air into the structure to below the dew point and it condenses. 

 

I have never ever heard of a structure failing purely through diffusion (vapour passage through materials) so it doesn't matter where the airtighness layer goes IF it is comprehensive. 

 

 

 

 

 

[DonnaP123]

On 17/03/2024 at 08:29, Redbeard said:

 

I have not had a comprehensive read, so apologies if I am countering anything you have already said, but I assume your roof is finished, with tiles/slates/whatever already on. If not, can you increase the depth of rigid WF?

 

As per others' comments it looks like, strictly, your roof is 'the wrong way round', in that, crudely, the 'conventional wisdom' is that you have more above the rafters than between (66.6%/33.4% is sometimes suggested). However I have seen some details (sorry, cannot remember where, but I have a feeling a colleague did this) using WF sarking and fulll-fill Warmcel (recycled newspaper) in deep I-beams, so clearly some people have different ideas.

 

If you decide to 'go for it', why not just put counter-battens on under the rafters to achieve the depth you need, so you do rafter depth in one direction and between counter-battens in the other, which has the virtue of each layer covering the last layer's gaps?

Thank you - you are correct, the roof is built with the sarking above the rafters, tiles etc on top. We couldn’t increase the ridge height to have a full warm roof so have compromised with the hybrid version. 
 

Counter-battening would work and is a good idea, thank you.

[DonnaP123]

On 17/03/2024 at 13:37, Iceverge said:

It can be done safely IF you are fastidious with airtighness. 

 

Almost nobody is, hence the fear of hybrid roofs. 

 

You only get issues when you have more moisture getting in Vs getting out. 

 

Airpaths through the structure are the real risk when drafts carry moist internal air into the structure to below the dew point and it condenses. 

 

I have never ever heard of a structure failing purely through diffusion (vapour passage through materials) so it doesn't matter where the airtighness layer goes IF it is comprehensive. 

 

 

 

 

 

Excellent advice! And I guess I WILL have to be extremely fastidious with the VCL and just make sure we get it right! We will also have MVHR which should (I guess) also help to mitigate excess moisture. Or at least it can’t hurt.

[Redbeard]

11 hours ago, DonnaP123 said:

And I guess I WILL have to be extremely fastidious with the VCL

 

I would suggest an 'Intelligent' membrane such as Pro Clima's 'Intello'. Expensive but good if it does what it says on the tin - and I have worked on the basis that it does!