Mildew on air tightness tape underneath window - advice wanted on potential causes and solution

[Omnibuswoman]

We recently noticed a build up of mildew on the air tightness tape at the base of our largest window (triple glazed, aluclad, 2.2 by 2.3m). The air tightness tape felt damp to the touch, but without any clear cause. I have just cut through the tape to see what is happening behind it.

 

There is a layer of green fabric that laps under the window which the air tightness tape covers. The builders used this green fabric to weatherproof the house -it sits on top of the timber frame beneath the wood fibre board insulation. The green fabric strip under the window feels very cold and possibly damp, and the air tightness tape overlying it was damp to the touch. Is this interstitial condensation? Is it possible that there is cold air coming in under the window, and creating condensation on the tape where it meets the warm air in the house?

 

If so, what is the solution... should I inject a bead of expanding foam on top of the green fabric, underneath the bottom edge of the window frame, and then seal it back up with airtightness tape?

 

Or should I go outside, and investigate what has been done on the outside of the frame when it was installed?

 

Photos attached. The first photo shows the mildew on the tape. The second photo shows the bottom of the window frame after I pulled off the fabric of the air tightness tape (I couldn't remove the adhesive layer), and also shows the green fabric that comes in under the window.

 

Many thanks...

 

[Redbeard]

Sorry to hear this. Is that OSB in pic 2 a full-thickness thermal bridge, bridging inside to out of the wall? If so that could be part of your answer. I take it that you are in mid-build (?) without the ventilation system being 'fired up' (?). If that supposition is right then the general atmosphere may/will be more moist than you'd normally expect, and coupled to a thermal bridge (?) that could 'do the business' re mould. Must say I have never had this happen, but I have never done a New build (except an extension) and that horizontal detail, in my refurb works, would normally be 25-40mm rigid WF. Sorry if this does not help.

[Nickfromwales]

I’d say the tape which is exposed to both ice cold and room temp is now a place for condensation to form. 
 

My money would be on the frame not being fully foamed 360°, preventing cold and drafts getting to the rear of the tape.

 

Look from outside and see how well (usually not) the unit was sealed up from weather, and report back.

 

Dont forget the time of year! 

[Nickfromwales]

Is the frame thermally broken? Or are these standard units? 

[Iceverge]

I wouldn't panic, The window and door frames, especially the sills will be the thermally weakest part of the house.   There's only so much you can do to insualted them as you need to deal with water shedding as a primary concern. 

 

Therefore they're the the coldest and the first place to gather any moisture. As soon as you get some heat into the house and some mechanical ventilation I wouldn't be surprised if the issue disappears. 

 

 

[BotusBuild]

Knowing the build, some info:

The house is already rendered on the outside.

The heating is already in place.

Ventilation may not be complete (@Omnibuswoman can confirm or deny this)

 

@Omnibuswoman - let me know if another head/pair of eyes "on the ground" will help

[Omnibuswoman]

Thanks @BotusBuild, @Iceverge, @Redbeard and @Nickfromwales for your replies. Botus is right, the MVHR is not yet commissioned, and the humidity in the house sits around 65%. We use the German method of 'shock ventilation' regularly, but the humidity remains relatively high as the area we are in has a rather damp climate.

The house has been heated for the last year and sits around 20-21C almost all of the time. I've been closely monitoring this recently via a Shelly H&T, and found that it fluctuates very little with the lowest reading being 19C. 

 

In answer to the questions:

@Nickfromwales, I'm not sure entirely what you mean by thermally broken... the window is a non-opening one. I also suspect that there is foam missing from underneath which is why I was mulling on the option of injecting expanding foam beneath the frame, however I was worried about potentially trapping moisture and making the problem worse... I will go outside tomorrow to have a look at it from the outside. There should be a strip of compraband filling any gap beneath the frame, so I will look for that.

 

The window also has a lead skirt outside of it as the porch roof sits just below the bottom of the window. It may be that this is acting as a cold bridge.

 

@Redbeard the OSB is only on the inside of the window frame. It doesn't cross the thickness of the wall to the outside. The OSB feels damp, but I think that is from the condensation on the green fabric.

 

I will investigate further tomorrow and report back. @BotusBuild, I will give you a shout if I need a second pair of eyes on the issue, thank you

 

 

 

 

 

[Nickfromwales]

14 hours ago, Omnibuswoman said:

I'm not sure entirely what you mean by thermally broken... the window is a non-opening one.

Standard window and door frames are solid material, wood or alu, or PVC etc. A thermally broken frame will have a layer of something (cork often used in wooden windows) as a means of separating the exterior and interior halves of the frame.

 

14 hours ago, Omnibuswoman said:

I also suspect that there is foam missing from underneath which is why I was mulling on the option of injecting expanding foam beneath the frame, however I was worried about potentially trapping moisture and making the problem worse... I will go outside tomorrow to have a look at it from the outside. There should be a strip of compraband filling any gap beneath the frame, so I will look for that.

Pull out the compriband and set it to one side.

 

Use kitchen roll etc to remove any obvious moisture.

 

Get some Illbruck 330FM foam and seal up under the threshold. Have you used a foam gun before? On amazon you can buy disposable tips which give you a means of getting the foam much deeper into the cavity, LINK, which you fix onto the gun by pushing them on and then tape in place with a good few turns of pvc electrical tape so that it doesn't fall off 'mid squirt'.

 

Any residual moisture will be absorbed and dissipated over the following few days, so don't worry about 'trapped' moisture, it'll find it's way outta there by itself.

[Omnibuswoman]

I have been out to see what the finish looks like… when the porch was built (recently) the guy squirted some foam under the window and added a strip of trim (possibly an offcut of a composite decking board from the look of it). 
 

 

So, the question is should I get the foam gun narrow tip and squirt in some expanding foam from the inside to stop air ingress? 
 

Thanks, M

[Nickfromwales]

Yup. Zero chance of getting anything in from outside. Seems an ok job so prob just needs the air to stop moving about under the unit and BINGO.

 

Have you used a foam gun before? It’s such fun (not) if not?

[Omnibuswoman]

I have had that pleasure  I have a Faithfull teflon coated foam gun, so will dust that off and will order the narrow tip you recommended. Will update once complete. Thanks for your help!