WUFI

Hi,

What is the maximum relative humidity for mineral wool?

My roof is :
Ventilated roof void
WRB
300 mm mineral wool
150 mm mineral wool between timber joists
Vapor barrier
12'5 mm plasterboard
3mm skim plaster

1- I placed an air infiltration (5 mm) in the outermost side of the 300mm insulation . Is it right to place the moisture source in this position?

2-With this my relative humidity in the last centimeters of the 300 mm insulation is maximum 85%. Is it right to analyse the last centimeters of that insulation layer for moisture analysis?

Normally it is said the typical threshold value of 80% but reading on internet it is said 90-95% as you see in the screenshot.

3-Can I consider my roof in risk due to mineral wool at 85% rh?

4- what other measure can I apply to my roof to make it safer?

Hi,

1- I placed an air infiltration (5 mm) in the outermost side of the 300mm insulation . Is it right to place the moisture source in this position?

Yes

2-With this my relative humidity in the last centimeters of the 300 mm insulation is maximum 85%. Is it right to analyse the last centimeters of that insulation layer for moisture analysis?

The outer side in the mineral wool is the right position, but as the mineral wool is not harmed by moisture, we usually evaluate the risk of condensation. Please see the "Guideline for assessing condensation problems in hydrophobic mineral fiber":
https://wufi.de/en/service/downloads#co ... assessment

Normally it is said the typical threshold value of 80% but reading on internet it is said 90-95% as you see in the screenshot.

That does not apply here, as there is no big risk of mould growth in the mineral wool itself - the table says: "on the material", not inside - so if you evaluate centimeters? you will get a completely different mean value. If you like to evaluate according to this table you have to assess the outermost element of the calculation grid of the mineral wool - not centimeters. Our mould model (WUFI Bio) is validated mainly for the interior surface and can be used on the warm side in the construction but with care. We do not recommend to evaluate the mold on the cold side in the construction.

3-Can I consider my roof in risk due to mineral wool at 85% rh?
4- what other measure can I apply to my roof to make it safer?

See 1 and 2.

Christian

Hi

Thanks for your reply.

I follow your explanation until the point 2. So I added the 1cm because I already read this procedure. But thanks anyway

But I don't understand the first part of the comment after

1- "Normally it is said the typical threshold value of 80% but reading on internet it is said 90-95% as you see in the screenshot"
Could you put an example please about what you mean to analyse the in the mineral wool?

Or do you mean that the only way to assess risk mould growth or water run off on mineral wool is analysing the last rectangle/ bar grid by WRB for relative humidity (90-95% limit) and using 1cm thick ( outermost centimetre ) procedure ( link you sent me) when we use the water content ( instead of relative humidity) as variable to analyse the mineral wool ?

Moreover , thank for your advice again but I know that WUFi bio and vtt ( correct me if I am wrong) is just for internal surfaces so I only analyse if there is risk of mould growth in plasterboard and behind vapor control layer.

2- If my roof is 31degrees inclination,

Following British standards,

What is the maximum water content in g/ m2?

Drop formation and draining on inclined surfaces 51 – 250 g/m² :
70 g/m² inclination of 45°
150 g/m² inclination of 23°

Let's say that my roof would be safe the water content is below 120 g/ m2 because each degree inclination from 45 to 23 degrees is 3'6 g/m3 if my roof is 31 degrees ( from 23 to 31 is 8 degrees that is equal 30 g/m2). Is it right?

Regards




Regards

Hi Alberto,

1- "Normally it is said the typical threshold value of 80% but reading on internet it is said 90-95% as you see in the screenshot"
Could you put an example please about what you mean to analyse the in the mineral wool?

The table you are showing says in the header "...studies on mould growth ON building materials". If you evaluate centimeters of mineral wool as you mentioned, the water content you are assessing is inside the mineral wool, not on the surface.

Or do you mean that the only way to assess risk mould growth or water run off on mineral wool is analysing the last rectangle/ bar grid by WRB for relative humidity (90-95% limit) and using 1cm thick ( outermost centimetre ) procedure ( link you sent me) when we use the water content ( instead of relative humidity) as variable to analyse the mineral wool ?

Yes, exactly, outermost element of mineral wool or also WRB for relative humidity.

Let's say that my roof would be safe the water content is below 120 g/ m2 because each degree inclination from 45 to 23 degrees is 3'6 g/m3 if my roof is 31 degrees ( from 23 to 31 is 8 degrees that is equal 30 g/m2). Is it right?

Generally yes, but unfortunately the standard does not tell clearly if the surface where the water runs off is in contact with an other material or not. I guess its a smooth surface, where the water will run earlier, than if the surface directly is in contact with mineral wool, where the water will be hold by the fibers longer before it runs off. We usually use the 200 g/m² given in EN 13788 for evaluating dew water formation in mineral wool. This value also corresponds with results from measurements we did for vertical facades. But with the values from BS 5250 you should be on the save side.

Merry Christmas and a Happy New Year
Christian

Thanks a lot once again.

1- Then to analyse the relative humidity on the mineral wool , there are two possibilities:

- outermost grid element of mineral wool insulation or

- innermost grid element of WRB

Is it right?

2- what happens if using the RH analysis, I am in the save side but using 1cm procedure ( water content) , my mineral wool is in risk of mould growth.
Because for one roof in london with no WRB, no vapor barrier and unventilated air cavity, the 1cm procedure says that the mineral wool (with unventilated air cavity above M.Wool) is in the safe side (80g/m2) and I think it is not right.

3- for the table of BS in the first row "fine mist, no dropping of liquid water" below 30 g/m2,

is it for vertical or inclined surfaces? Or is it the most strict condition to avoid moisture issues in the assembly independently if the surface is vertical, horizontal or inclined?

Regards

1- Then to analyse the relative humidity on the mineral wool , there are two possibilities:
- outermost grid element of mineral wool insulation or
- innermost grid element of WRB
Is it right?

Yes, but this position is interesting for dew water only . We do not recommend to evaluate mold on the cold side, as the models are not thought to be used there.

2) See above. We recommend to evaluate the dew water, not mould.

3) In my eyes the inclination is not relevant here, as the mist will form on all surfaces. But nothing will run off and that is what shall be avoided.

Christian