Faulty Vapour Barriers Causing Unsightly Streaks

What’s happening to this house. Something seems to have gone wrong and is causing those unsightly streaks. After you scroll down to view it, we’ll tell you what is happening, and why.

Insulated siding acts a barrier to reduce energy (heat) transfer to the outside, and manage energy bills. It also has a secondary purpose of reducing sound transfer, and facilitating the straight-line effect of siding that is its charm.

The vapour barrier used for damp-proofing is usually a foil, or a plastic sheet. Its purpose is to resist the transmission of moisture through walls, floors, ceilings and roofs. However, these retarders are only barriers to an extent, since they all have a degree of permeability, in other words a tendency to let some water through.

It follows that it is a false economy to choose a cheaper vapour barrier for a short term saving. Because if it fails, as appears to have happened in the above image, the siding must come off before the problem can be addressed. The Alberta Infrastructure Technical Services Branch has this to say on the transfer of humidity:


High Humidity Transfer from Building Interiors


Indoor relative humidity of 30% or greater can result in excessive condensation on or within the building envelope during the winter. Where feasible, provide lower humidity “buffer spaces” to separate  spaces  with  high relative  humidity  from  the building  envelope.

To make  such  separation  effective, design  partitions  and  mechanical  system  air  pressure  differentials to  minimize humid air transfer to the buffer spaces.

Where high humidity space cannot be “buffered” from the building  envelope,  the building envelope assembly will need to be designed to prevent surface condensation.


How Moisture Moves Through Building Cavities


There are three ways that moisture can travel into, and through a building cavity, until it permeates through the siding joints outside, and streaks down the walls as in the photo:

1 – Air currents may drive it into nooks and crannies

2 – It can diffused through permeable material like gypsum board

3 – It can travel as a passenger on the back of heat transfer

Air movement is generally the culprit in over 95% of cases. There is only one solution. Install a high-grade vapour retarder / air barrier before fitting the siding envelope. Savings on the energy bill will help cover the cost.


Speak To Us About Our Rigid Insulation Panels


R-value is a measure of resistance to heat flow through a given thickness of material. We use high performing polyisocyanurate rigid foam insulation sheathing systems. These offer the highest R-value per inch and reduce heat loss through studs too.

When comparing Valiant Exterior prices with other contractors, remember it’s what’s behind the siding that keeps you warm and your siding looking good. If cheap insulation fails, repairing the damage could cost considerably more than the ‘saving’.

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