A Disastrous Case Study: How Replacing Windows Alone Can Lead to Mold

An image is worth a thousand words.

In our previous articles, we have cautioned that an energy retrofit should be approached with an integrated, whole-building approach, and by a professional with experience in calculating and simulating thermal envelopes.

The photos below show what happened to one family’s home when they decided to do an independent partial energy retrofit, without consulting a qualified professional. In this case, they decided to replace the old windows of their house with newer, high performance windows, hoping that they would be able to reduce their energy consumption.

These photographs are courtesy of our friend and colleague Damiano Chiarini of Studio Rinnova, who joins us in emphasizing the importance of an integrated approach to energy retrofits in order to avoid disasters like this one. He was called in to consult the building owner on possible solutions after this attempt to upgrade the building resulted in widespread mold and condensation throughout almost every room. 

Before the works took place, the house had no mold problems at all. It had old windows, that were most likely anything but airtight. Old windows, allowing for considerable air leakages, meant that the house remained ventilated (however cold).

The retrofit works consisted solely of replacing the windows and did not include any improvement of the thermal envelope or the ventilation.

The outcome, as you can see, is dramatic: mold appeared quickly around windows, on the walls and on the ceiling. The thermal envelope of this house is so poorly constructed, that not only can you identify the concrete beams inside the ceiling slab, but you can also clearly “read” the wall masonry, and see exactly where the mortar beds are. It’s just like looking to the house with a infra-red camera.

In a previous article, dedicated to mold and surface condensation problems, we had a chance to explain the causes of this phenomenon.

Mold everywhere: Who is to blame?

The building occupants? Perhaps. Many times, clients don’t fully understand the ventilation needs of their home, especially when airtight windows are to be installed. When clients decide not to install mechanical ventilation, it means that they are shouldering the responsibility of properly ventilating the home by manually opening windows on a daily basis and using fans when necessary. This is an easy thing to intend, but difficult to carry out properly. Unfortunately in Italy, less than 1% of homes are equipped with mechanical ventilation. However, in our culture which is accustomed to natural ventilation of buildings, there are many many cases (especially in the humid climate of Emilia) when traditional methods simply won’t cut it anymore. As building technology develops, so does our need to properly manage and operate our buildings. Client awareness and education is always an extremely important factor in residential design.

The contractor? Did the contractor install the windows to be too airtight? No: external openings should be installed airtight. Improperly installing windows to allow for intentional air leakages is a waste of new windows and an insult to the installer. The more appropriate question is: Didn’t the contractor know that this was going to be the inevitable consequence of replacing the old windows, without addressing the rest of the envelope? Was this the first time ever he replaced old windows? Or maybe he overlooked informing the client about the problem in order to get the job?

Was there a professional designer involved? Either there was, and he/she did a  very very very poor job, or there was not one hired. Unfortunately, many times the design professional’s fee is seen as an added, unnecessary cost. Clearly it is not, as we can see from the photos.

And finally: is it fault of the Italian Government, to grant tax breaks for energy saving that also allow for such incomplete, disastrous results? You can read our thoughts on that here.

What about energy savings?

It’s likely that the owner is still going to receive economic benefits from the saved energy. New, better performing windows and less air leakages result in a cheaper energy bill: mold does not affect the energy balance in this way. It only affects your health and comfort… only!

Once again, we want to underline the fundamental divide that separates energy savings from comfort. In another article, for example, we described how Italian regional energy certificate does not guarantee the real quality of a building.

What are the health consequences?

Exposure to mold can contribute to diseases of the respiratory system, especially when it is present in such dramatic quantities. Exposure can worsen symptoms of asthma and allergies as well, causing stuffiness, wheezing, and difficulty breathing. People with immune-compromised diseases and chronic lung illnesses can contract serious infections from exposure to mold. Babies and children can develop asthma and other respiratory problems if exposed to mold from an early age.

Shouldn’t the walls be allowed to “breathe”?

Many people ask this question when confronted with the concept of airtight windows. They wonder if mechanical ventilation is really necessary, since they’ve heard that properly constructed walls allow the house to “breathe”.

A quick explanation: That concept is what is referred to as the structure’s “transpiration”, the ability of a gas to pass through a material because of difference in pressure.

Yes, a certain amount of water vapor will effectively pass through the envelope structure. This is even happening in the case of the building shown in these pictures. The reality of structure transpiration, however, is that even the most transpirant thermal envelope in the world would not be able to carry out the amount of water vapor produced inside a house on a daily basis (from cooking, showering, cleaning, breathing, etc). For this reason, structure transpiration cannot replace ventilation. We are going to publish more information about this in the future (you can sign up for our newsletter if you’d like to stay informed).

Couldn’t the windows be installed with holes to allow air to pass through?

Unfortunately, this non-solution is still suggested by many, ignorant (pardon me) professionals. However it is just that – a NON-solution.

What is the point of investing money to remove the old windows, buy newer, better ones, and install them poorly on purpose? Why should you spend money on high-performance windows, only to compromise on comfort and energy savings just because you are afraid of mold? Rather than installing new windows in a way that negates any benefit they have and allows your home to fill up with cold air, is it not better to just keep your money and your old windows?

Solving the mold problem:

With this article, we may get accused of pushing for the hiring of design professionals (like ourselves, surprisingly). So be it.

In cases like the one in the photos, as with any energy retrofit project, you need a competent professional who can calculate improvements for the thermal envelope, for the thermal bridges, and for the level of health and comfort inside the building.

Often, it just makes no financial sense to persist with partial works (for example: adding just the external thermal insulation) that result in minimal energy savings and no certain solution for a mold problem. A correct deep energy retrofit should be comprised of several works that have to be planned as parts of a whole approach, with a clear and consistent vision.


  1. Reblogged this on duanetilden and commented:
    Case study of an improperly designed energy retrofit project where airtight windows and other measures were taken without an integrated approach. The failure to upgrade the building envelope resulted in mold formation due to persistent condensation inside the building walls & ceilings.

  2. Informative case study. Sometimes the wrong professionals can be hired to perform construction tasks without knowing the design components or the applicable codes. The dew point in the wall resulting in condensation and mold was a combination of poor insulation and lack of proper ventilation.

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