Timber Frame Construction Analysis

[leecor88]

Hi guys,

I would like to start off with a big thank you for helping Brendan and I with an introduction to WUFI. I am basing my dissertation on the performance of timber frame constructions in the Irish climate (as I probably mentioned before), and currently using WUFI as a means of testing the various types of construction.

At the moment all three construction types are working out fine which is a bit of a problem as I thought one would fail! Where I think I am going wrong is with my moisture source. I am testing these wall types allowing for a certain amount of moisture to get into the construction as a result of rain during construction, driving rain, moisture from inside as a result of penetrations in membranes etc. Should the source be continuous across the insulation/Stud layer? Any advice from experience would be greatly appreciated.

Also when exporting data in ASCII and TXT format and opening in excel, to make the results into a graph, do you just select all the data and it will produce an accurate graph? It seems easy enough but I am just checking that I'm not overlooking anything.

Looking forward to your response with many thanks,

Lee Corcoran

[Calina Ferraro]

Hi Lee,

First off, you should be aware that the timber studs in the wall can create 2D effects that WUFI Pro (1D) won’t demonstrate. If you need more accurate simulations, you should be using WUFI 2D. According to the Fraunhofer’s testing, if there is insulation on the outside of the studs (i.e. thermal bridging is eliminated and the studs are kept warm), the 2D effects are mitigated enough that a 1D simulation gives a good approximation. However, if you have cold studs (i.e. insulation between the studs or on the interior only), you should be using WUFI 2D.

Moisture Source
In response to your questions, ASHRAE 160 requires allowing 1% of the driving rain to penetrate behind the membrane. This should be applied as a moisture source in the outer few mm of the build-up just behind the membrane. WUFI will simulate the ingress of moisture through the material based on the diffusion calculations (and/or capillary transport if it is relevant for the material).

Exporting Data/Graphing in Excel
When you export “Courses” the first column in the text file is the time, the remaining columns are whatever values you selected to export in the order that they are listed.

When you export “Profiles” the first column in the text file is the location within the build up (measured from the left/exterior surface) and the remaining columns are whatever values you selected to export, in the order that they are listed.

To graph them, import them into Excel and select the columns you wish to graph. If you’re not sure that your graphs are accurate, you can compare the ones you generate in Excel to the Quick Graphs in WUFI. After all, the values should be the same in both. Excel just gives you the option of formatting and combining graphs.

Hope that helps! If you need anything clarified, just let us know.

~Calina

[Daniel]

One additional information: for timber frame constructions the most critical conditions normally occur in the cavity on the cold side of the insulation - not in the studs.

Therefore WUFI can also be used for the normal situation - only in case of timber frame with additional interior insulation the most critical point moves to the previous interior side of the studs and should be evaluated by WUFI 2D.

best regards
Daniel

[paulprice]

Daniel,
I assume you mean in the insulation closest to the ventilated cavity rather than in the cavity itself?


Lee,
I had a go at many varying timber frame buildups with the 1% rain leak keeping u-value at 0.22 and using many varying buildups as wufi is really good at multiple cases. Definitely arrived at failures or poor water content readings, in outer insulation just inside outer plywood or osb for example.

[leecor88]

Thanks for some great advice and feedback!

Just to clarify one thing. In one case I am using a Breathable wall i.e. no OSB or breather membrane. The moisture content in the insulation reaches a safe level and has a consistent pattern over 5 years, however a monitor point on the coldest point of the insulation tells me that the RH stays between 75 and 80%. Would this cause problems in a real life situation?

Thanks again and I will be sure to thank you guys in my dissertation for all the advice.

Regards,

Lee

[Daniel]

For my understanding: You always use a ventilated facade but sometimes without sheathing or even membrane on the insulation?

The most critical point is normally on the cold side of the insulation in winter - but in the exterior sheathing (OSB or whatever). The diffusion transport can easily reach this position and than accumulates due to the more diffusion reatarding sheathing or membrane. If you have no sheathing or membrane you cannot get any moisture accumulation at this point.

On the exterior side of the construction we normally use a limit of 95 % RH during winter (at low temperatures) but within the moisture sensitive materials - for the mineral woll insulation also condensation water under certain limits and for short periods can be accepted.

In summer its better to remain below around 80 % due to the mold growth risk.

best regards
Daniel

[Calina Ferraro]

Lee,

You have to be very careful when looking at the moisture content graphs. It makes a difference whether the material is hygroscopic or non-hygroscopic. Only hygroscopic materials absorb moisture out of the air based on humidity levels. Non-hygroscopic materials remain dry until condensation occurs and then moisture is deposited on them in the form of droplets.

As a simple example of this, think of the way paper or cotton (both hygroscopic) becomes very limp on humid days because they absorb the moisture out of the air. On the other hand, something plastic (i.e. non-hygroscopic), it will hold the shape and feel just the same regardless of the humidity levels.

This means that the water content of hygroscopic material will vary with humidity levels, while the water content of non-hygroscopic materials is effectively 0 until condensation (100% RH) is reached, and then it jumps when the water droplets condense out of the air.

So that’s real life, now let look at how WUFI deals with it…

WUFI calculates the water content of materials based on the Moisture Storage Function (defined in the material properties). In order to be able to perform the simulation, WUFI requires a smooth moisture storage function for all materials. This function does not exist for non-hygroscopic materials, so as a calculation tool, WUFI creates a default moisture storage function anytime one is not defined. This default moisture storage function is just a calculation tool, so the outputs from it (i.e. water content) should not be interpreted as representing real conditions.

What does this mean for you?

Most insulation materials are non-hygroscopic. If you’re not sure, check the material properties of the layer. If a moisture storage function is defined, it means the material is hygroscopic and the water content graphs are realistic. If no moisture storage function is defined, then WUFI is using the default function and the water content graph for this particular layer should be ignored because the water content is effectively 0.

Note that in these non-hygroscopic layers, the RH values are still real and accurate and these typically represent the critical values that you should be looking at for mould growth, rather than water content.

For more information on this topic, read the WUFI help file for the Moisture Storage Function.

~Calina

[leecor88]

Note that in these non-hygroscopic layers, the RH values are still real and accurate and these typically represent the critical values that you should be looking at for mould growth, rather than water content.

From what I have found out during my research of fully breathable timber frame walls is that the choice of insulation is critical and must be hygroscopic. Therefore I am simulating the case with Cellulose insulation. Is it right to say that as a result of the hygroscopic properties of the insulation that condensation will not be a problem? I should also mention that there is a softwood fibre board replacing where the OSB would normally be on the external side of the timber frame. In theory any moisture in the construction should be absorbed by the insulation and the under pressure in the cavity will cause any moisture to be drawn outwards.

I think this system is very interesting which is why I am basing my dissertation on it, although it is yet to stand the test of time in the Irish climate.

Many thanks again Calina and Daniel for taking the time to answer and steer me in the right direction, I really appreciate it.

Lee

[paulprice]

Calina,

Although, as you say, water content is not a good guide for insulations as some are hygroscopic and some are not, I have been thinking the water content of the wood sheathing (plywood or osb or panelvent) is a reasonable guide in that timber decay is likely if water content goes above 20%. For example if the inner 1mm of wood sheathing – ply, osb or panelvent – goes over that value, especially if for most of the year, then it indicates that the wall is not drying out to an acceptable level.

Is this a fair way of judging the wall?

Also, in regard to Lee's question, is it fair to allow leakage into a timber frame wall when there is always supposed to be a rain-screen (masonry, timber cladding or render) with ventilated cavity to prevent this. Putting the 1% leak into the simulation makes most sheathing-to-the-outside build-ups that I have tried fail in terms of wood moisture content unless there is panelvent on the outside rather than osb.

Thanks for your feedback, Paul

[Calina Ferraro]

A couple of things:

Lee,

In response to your question: “Is it right to say that as a result of the hygroscopic properties of the insulation that condensation will not be a problem?” – be careful with your interpretation.

It is true that with hygroscopic materials you are unlikely to get actual condensation (i.e. 100% RH). Hygroscopic materials pull moisture directly out of the air turning it into liquid water within the pores as the RH increases. As the vapour is pulled out of the air, capillary action in the material tends to carry the liquid water away to a drier location. As such, unless the material is completely saturated, you won’t generally hit 100% RH in a hygroscopic material.

However, because the water content increases and decreases with RH within a hygroscopic material, at a very high RH there may be a very high water content (this depends on the shape of the moisture storage function). This means that you can have sufficient moisture to cause problems of mould or rotting timber even if you don’t have actual condensation or 100% RH conditions.


Paul,

Your method of assessing timber is correct. Create a thin layer of the sheathing board at the critical location (typically the cold side) and monitor the water content within this layer. The German DIN standards state that rot occurs if the water content is over 20%-M. We have been told that according to Fraunhofer’s test results, rot is unlikely to occur at this percentage if the temperature is less than 10C, but it would be up to you to judge how conservative you want to be and which standard to apply.

In regards to you question about leakage, the 1% comes from ASHRAE 160. My understanding is that this is not to assess ‘typical’ conditions, but rather is to ensure that the wall is robust enough to dry out if there is some sort of failure (e.g. damage to the rainscreen or a leak from a roof gutter). Again, it would be up to you to decide which standard to use and if this is a “fair” way to assess the robustness of a wall.

~Calina