WUFI

Hi,

I tried to simulate a roof with the weather file from Goteborg

My roof is the next:

Metal siding
22 mm groove and tongue board radial spruce råspont
40 mm unventilated cavity
30 mm woodfibre insulation
300 mm mineral wool
0'2mm PE foil
12'5 mm gypsum board

0% fraction rain adhering

but I got this error.

"Selected climate for left surface does not contain information about long wave atmospheric counterrradiation. User-defined cloud index 0.66 will be used"

1- What is supposed that I have to do? Click on continue?

2- Do you think that the groove and tongue board will cause moisture problems/ mold growth in this roof with unventilated roof? Does it make sense to install groove and tongue radial spruce ( material in Fraunhofer database) in that roof as I defined in the assemble in exactly that position? What is the goal to put this type of material below aluminium siding and above unventilated or ventilated cavity?

3- would it make sense to put a wrb above the woodfibre insulation with the risk or having mold growth in that interface between both materials? Or is it better a water repellent above spruce radial material?

Thanks in advance

Hi,
this is not an error, but a warning. If there is no atmospheric counterradition in a climate file, it has to be estimated based on the cloud index and other parameters to calculate the explicit radiation balance. For further information see the WUFI Program help, see "Long-wave Radiation Exchange".
1) click on continue or use another climate location which contains atmospheric counterradiation
2) this groove ant tongue sheathing usually does not cause moisture problems, they are often more suitable for this positions, as they tolerate moisture somewhat better than wood-based materials. What causes moisture problems is usually the construction itself (combination from vapor barriers, insulation etc) and not single materials. The sheathing below the aluminum or also at other positions is usually required for the bearing of the loads or to keep the insulation in place
3) yes, this is quite common to use weathering barriers with low sd-vlaues at this position, also to drain dropping secondary dew water (which may occur during the outer side of the ventilation gap is colder than the intruding air).
Christian

Thanks for your reply

1-What is the difference between groove ant tongue sheathing and wood based Materials?

2-Would the woodfibre insulation be enough to cope with external stress like external wind or moisture due to its properties ( hygroscopic, breathable and windproof)? Or does it need a wrb above to be protected?

3- would this be the most normal or typical assembly for this type of roof ( of course simulation need to be done)?

Metal siding
22 mm groove and tongue board radial spruce råspont
40 mm ventilated cavity
1mm WRB
30 mm woodfibre insulation
300 mm mineral wool
1mm isover vario vapor barrier
12'5 mm gypsum board

Do you recommend to put another sheating board between wrb and woodfibre insulation? which one?

If I put ventilated cavity then in the wufi model, should I put only the next? or the whole assembly including all layers?

30 mm woodfibre insulation
300 mm mineral wool
1mm isover vario vapor barrier
12'5 mm gypsum board

Which are the ACH range for strong, normal and low ventilation for roof as we can see in the a value absorptivity table for eaves, middle and ridge?

Why is the a value absorptivity dependent on the ventilation rates and the position between eaves and ridge as it is mentioned in the article Hygrothermal Simulation of ventilated pitched roofs with effective transfer parameters ?

4- should I place an inflitration source in the outer side (0.005m) of the woodfibre insulation?

Hi Alberto,

1-What is the difference between groove ant tongue sheathing and wood based Materials?

Wood based materials are made from eg. wood chips and some kind of glue like (fibreboard, chipboard, oriented strandboards, plywood and veneer laminated wood). They have a lower moisture content limit than solid wood, because moisture softens the adhesive, usually results in strong swelling and softening. According to the German standard for wood protection DIN 68800 the maximum allowed water content is 20 percent by mass for solid wood and 18 percent by mass for wood based materials.

2-Would the woodfibre insulation be enough to cope with external stress like external wind or moisture due to its properties ( hygroscopic, breathable and windproof)? Or does it need a wrb above to be protected?

As far as i know the hygroscopic, windproof insulation boards do not necessarily require a weather barrier.

3- would this be the most normal or typical assembly for this type of roof ( of course simulation need to be done)?

yes, that is a way it can be build. If this is the most typical, i can not answer

Do you recommend to put another sheating board between wrb and woodfibre insulation? which one?

no

If I put ventilated cavity then in the wufi model, should I put only the next? or the whole assembly including all layers?

If I remember right the model for ventilated roof is for tiled roofs and not validated for metal roofs. You can try both.

Why is the a value absorptivity dependent on the ventilation rates and the position between eaves and ridge as it is mentioned in the article Hygrothermal Simulation of ventilated pitched roofs with effective transfer parameters ?

The absorptivity mainly is used in here as it is a parameter, which is easy to change in WUFI. The idea is to get a critical representative temperature on the WRB without to handle the real ventilation, which is most time unknown to the user. It is one way of simplification. There may be other methods as well.

4- should I place an infiltration source in the outer side (0.005m) of the woodfibre insulation?

The infiltration source usually is placed below the second airtight layer. So here the vapor barrier is the first and and the WRB is the second. So yes, that would be the position.

Christian

Thanks for your answer. I really appreciate it

The absorptivity mainly is used in here as it is a parameter, which is easy to change in WUFI. The idea is to get a critical representative temperature on the WRB without to handle the real ventilation, which is most time unknown to the user. It is one way of simplification. There may be other methods as well.

Could you briefly explain those methods please? I ignore them

. As far as i know the hygroscopic, windproof insulation boards do not necessarily require a weather barrier.

In my case I have woodfibre insulation . Is it different to woodfibre insulation board? Then is your answer "no" for woodfibre board but "yes" for woodfiber insulation? Based on your previous answer Ponit 3

Thanks in advance

The absorptivity mainly is used in here as it is a parameter, which is easy to change in WUFI. The idea is to get a critical representative temperature on the WRB without to handle the real ventilation, which is most time unknown to the user. It is one way of simplification. There may be other methods as well.

Could you briefly explain those methods please? I ignore them

There may be other methods, but I do not have any in mind at the moment.

In my case I have woodfibre insulation. Is it different to woodfibre insulation board? Then is your answer "no" for woodfibre board but "yes" for woodfiber insulation? Based on your previous answer Ponit 3

I had the same in mind. If your insulation has the specifications you mentioned (hygroscopic and windproof) you can use that without a weather barrier.

Christian

thanks Christian