Why are PIV’s popular in Europe but not the US?

I saw this device in my youtube feed recently and wondered, what is this and what does it do?  This installation is very curious because it’s taking air from a vented loft (attic) and forcing it into the rest of the house.  The maker of the video specializes in roof replacement and repairs, and started talking about condensation and mould in homes…yes mould with a “u”, meaning the English accent indeed was from the UK.  

“Damp” in the US is a word to describe clothing when it doesn’t completely dry in the clothes drier.  “Damp” in the UK is used more to describe weather and homes.   For sure, both have to do with moisture, but because of the predominant masonry construction in Europe, “damp” is the condition of homes that have too much moisture in the walls in general, and “rising damp” is the moisture that comes from the ground and is wicked into walls.  It’s a big problem, because we know that puts the home at big risk of mold.

American homes are overwhelmingly wood-framed, which can also get damp and moldy when continuously exposed to moisture, but in general 1) we don’t have as much rain as northern Europe and 2) wood framing dries out more quickly than entire walls made of concrete block or brick.  This is why “rising damp” is not in our vocabulary but very well known in Europe.  Accordingly, Europeans have devised several ways of avoiding “damp” and “rising damp” in their homes.  (UK-based “SkillBuilder” could be considered a “damp doctor” and this video with an expert in damp-proofing is very good.) 

1. Take care to see that the landscaping or earth touching the foundation is not built up too high so that liquid water is in contact with the wall above the damp-proof course (see the next point), or broken/dirty gutters are pouring water on the wall when it rains.  If this happens, you could consider adding a French drain or gravel trough to redirect water away from the foundation.

2. A “Damp-proof course” (DPC) is a layer of plastic, engineered bricks, or slate (slate was used before the advent of plastic) built low into the wall around a home, so that the moisture cannot wick up into the rest of the wall.  Recognizing the relationship of moisture coming from the ground and mold growing indoors, damp-proof courses (DPCs) became mandatory in London in 1875 following the Public Health Act.  (A Brief History of Damp-proof Coursing)

3. “Damp-proofing rods” are good for renovating old homes to protect from rising damp, whether or not a DPC has been installed.  This video demonstrates the placement and use of these rods, which are made of fiber but infused with a water-repellant liquid which then infuses the entire layer of mortar where they are placed.  Many builders prefer them to injectable cream for ease of use.

4. Hygroscopic salts can often contaminate the brickwork or mortar and then after construction can be liquified and migrate through the brickwork, changing the appearance of finishes such as plaster.  If this happens, don’t wash it off but knock it off with a stiff brush. Salts will attract more moisture if left on the wall.  

5. Positive Input Ventilation (PIV) systems:  We’ve written a lot about the benefits of having a slight positive pressure in homes to keep out PM2.5, VOCs and other pollutants.  PIV systems, like a continuous fresh-air ventilation system, “enforce” positive pressure by bringing outside air inside via a filter and distributing it via new or existing ducts to various parts of the home. Often one “damp-proofing measure” such as “having the house double-glazed” (replacing single pane windows with double-pane windows) will move moisture from condensation on the windows to mold elsewhere, like a pantry or closet, so you’ll need to consider the impact of major changes you make in your home’s envelope.  Because the climate is so different in the UK, SkillBuilder Roger Bisby even goes so far to say that airtight homes are a terrible idea in the UK (but we are still positively in favor of them in the US). 

PIV systems have been around for a while in Europe, and Drimaster by Nuaire is a well-known brand of PIV systems in the UK.  PIV is discussed at minute 15:40 and after of this video.   Damp is a problem when cool surfaces in the home cause condensation, but some of these systems have a slight (approximately 400 watts) heater so that the incoming air can increase moisture retention.   It all depends on psychrometrics, however!  If your home is cool, and you bring in warm air from the outside, this warm air as it cools will drop moisture onto cool surfaces–cool air cannot hold much moisture!    If your home is warm, however, and you bring cool air from outside, you will be “drying out” your home, because as the cool air warms, it has the ability to hold more moisture.  (Cold air is relatively “dry” air).  

Basic PIV systems work in the UK climate, but you may need to go to the NorthWest to simulate the same climate and benefits in the US.  OR, you may need to substitute your PIV for an HRV (heat recovery ventilator) or an ERV (Energy Recover Ventilator).  

Now here’s the difference: PIV’s produce Positive Pressure in your home.  HRV’s and ERV’s typically provide Neutral Pressure in your home.  This is because basic HRV’s and ERV’s push the same amount of air into your home as the air they exhaust out.    If you have a relatively “leaky” home, the idea of positive pressure is a good one because it bumps up the pressure inside just enough so that air is always going out of your home, not in.  However, if you live in a hot or humid climate and have a leaky home, it’s safe to say that you’re going to want to condition that incoming air so that it’s not adding humidity to your house!   That’s why, if you have a leaky home AND you live in a hot and humid climate, you will want to select an ERV (only ERV’s manage humidity) with variable speed fans/blowers so that you can adjust the incoming fan speed to be greater than the outgoing fan speed.   Here is an awesome article about these types of units and how to balance an ERV, to make it a form of PIV (maintaining positive pressure in the home). 

Guess what?  ERV’s not only help with condensation and mold problems (Damp), but they also keep PM2.5 out of your home when there’s wildfire smoke, or urban pollution during winter temperature inversions.  More and more studies are confirming the harmful effects of PM2.5 on all parts of our bodies.  A 2026 study investigated the effects of PIV systems on indoor PM2.5 concentrations in 24 homes across New Zealand. Following PIV installation, mean indoor PM2.5 concentrations decreased across all homes, with reductions ranging from 38% to 62%. (Positive pressure ventilation systems and indoor air quality: PM2.5 outcomes in residential buildings)

Therefore, if you are building a new home, we recommend:

1. FIRST, find an architect, builder and HVAC contractor who know airtight enclosures, how to specify techniques and equipment for in YOUR CLIMATE, and how to build them.  It’s so important, time-wise and budget-wise, to get these things right from the beginning, before 1 shovel of dirt is disturbed.  Check out our article on the benefits of Passive House construction–planning is key.

2. If you are indeed building an airtight home, you will not need positive pressure ventilation.  You need balanced (neutral pressure) ventilation.  Understand first-hand how the architect, builder and HVAC contractor will keep the home airtight and ventilated with fresh air.  Ask how the humidity will be mitigated.  Ask how the bathrooms and kitchen will be ventilated, still preserving your balanced ventilation.  Ask what the target blower-door rating will be and how they expect to achieve it.

3. Understand first-hand any air purification systems in your home work.  Is it mainly MERV 13 filters on the ERV or HVAC?  MERV 13 should be the minimum rating specified in any new home, and with it, adequate filter size to avoid over-taxing your HVAC.  Check out our article for making sure you have enough return filter area for a high-MERV filter like MERV 13 (for 1” filters; other factors will come into play with thicker filters). 

In summary, if you are aiming for an airtight home, you won’t need one of those PIV systems in your attic in the US.  Even if your US home is leaky, more than likely it needs an ERV which is balanced to the positive side.  If your climate is similar to the UK, however, PIV’s could be handy in the winter.  The end of the lesson is that wherever you live, be sure to factor in all the nuances of your climate and home construction before making changes.

Photo by Tamas Kolossa on Unsplash