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The ventilation of old houses is made necessary or even essential because of the strengthening of their airtightness by the installation of insulating windows. A specific overpressure device is one of the solutions. Here are 5 questions on the subject.

5 questions about mechanical ventilation by insufflation (VMI)

Mechanical ventilation by insufflation (VMI) replaces the mechanical ventilation controlled (VMC) in homes whose configuration does not allow the passage of ducts in the attic, as well as in old houses with floor (s). This is a solution that still raises many questions.

How to lower the humidity level in an old house?

The insulation of a traditional house from the 1970s, heated with an oil boiler, was redone at the level of the attic and basement ceiling. A VMI has been installed to reduce the humidity level in the house. But it remains important (70%). Why is the humidity level difficult to lower?

A humidity level 70% is not catastrophic, especially in a house with good insulation, which eliminates the risk of condensation. Ten points less would certainly be preferable. There may be a problem with the operation of the mechanical ventilation by insufflation. It must be ensured that the interior doors are well relieved (day of 1 cm) to allow the circulation of the air towards its points of exit. Ventilation grilles had to be installed on the windows: it is necessary to check that they work in the good sense, because many companies pose inadequate grilles (designed for the VMC simple flux) which prevent the air to go out, the function grids of VMI being to let it out.

What are the advantages and limitations of a VMI?

In an apartment located in a building older than 40 years, mechanical ventilation by insufflation must be posed after the replacement of the wood exterior joinery with PVC. Is this ventilation system effective and appropriate? Does air insufflation risk introducing more fine particles?

The mechanical ventilation by insufflation is well suited to older buildings because the installation is simplified compared to VMC classic. The landlord makes a logical choice, as soon as the new windows no longer allow a natural renewal of air.
Concerning the "import" risks of fine particles, the system includes an air handling unit ensuring a fine filtration of air, while evacuating indoor pollutants. So there is no fear about it.

How to improve the circulation of air in a damp house?

In a 60-cm-thick breeze block house dating back to the 1960s, the ground-floor walls showed signs of hair rising. To limit the problem, a partition was destroyed, which allowed a better air circulation. The installation of a wood stove also allowed to clean the walls. In contrast, at 1st floor, there is condensation in the bathroom and traces of mold in the rooms, despite the presence of aerators at the top of the windows. Will the installation of a VMC or VMI solve these moisture problems? What is the cost of these facilities?

The moisture problems are complex to treat because their origin is not always obvious, several causes being additive: penetration of water by the facades or the roof, gutters and descents pierced or blocked... To determine the cause, it is necessary to proceed by elimination.
It is very rare that hair lifts reach the first floor, unless the interior and exterior cladding of the ground floor walls are too tight, and block the evacuation of moisture. It is then necessary to check the possible "external" causes: porous facades, cracks, infiltrations by the cover, by the links between joinery and structural work.
Regarding the condensationit can be caused either by insufficient ventilation, or by excessive water vapor production (bathroom), or by insufficient or absent thermal insulation (these causes are often combined). By acting on these three points it should be possible to solve the problems. Regarding the improvement of ventilation, the implementation of ventilation by insufflation (called "VMI") is well suited to old premises (where it is difficult to pass ducts) with significant moisture problems. Only drawback: the investment cost is higher than that of a conventional VMC.

Where to place the plant and the sheath of a VMI in isolated attics?

In attics insulated with cellulose wadding, a VMI must be installed. Where should it be placed and should a firewall be made around the sheath?

Mechanical ventilation by insufflation (VMI) is recommended in old buildings, whose attics are arranged and where the passage of the sheaths of a VMC would be too complicated. It consists of generating a air flow creating an overpressure that spreads in the house. The box, installed in the attic, is equipped with a heating system. However, it is not in direct contact with the insulation. As for the sheaths, they are not subjected to a rise in temperature which would be likely to ignite the cellulose layer. In any case, it must be classified C s2 d0 (according to the standard of fire behavior NF EN 13501-1), that is to say that in case of fire, it is consumed but not does not burn and emits no toxic fumes. So there is no problem to be done, especially if we use a professional installation, which will meet all safety rules.

How to properly ventilate an old house?

In an old house whose insulation was partially redone (rock wool in the attic floor, double glazed windows, etc.), some moisture problems appeared in the bathroom and in a staircase leading from the kitchen to the basement. To solve this problem and improve the ventilation in the housing, can we install a humidity-controlled VMC or a simple flow? Or is it better to set up a VMI? And what should we think of air handling units operating overpressure?

Installation of a VMC (controlled mechanical ventilation) is complicated to implement in an old house. In particular, ductwork is difficult to pass with the inevitable risk of degrading tapestries and paints. As for a CTA, air handling unit (other name of the VMI), operating overpressure and incorporating a function of air conditioner, it is indeed quite expensive to install. A mechanical ventilation by insufflation (VMI) is appropriate in this case, because it avoids these inconveniences. This system makes it possible to compensate for the shortcomings of natural ventilation by injecting a fresh air flow previously filtered and preheated in the housing. The evacuation of the exhaust air is carried out by the high ventilations of the humid rooms (kitchen, bathroom and toilet) and by the vents of the joinery of the dry rooms (stay, living room and rooms ).

It is also possible to consider a distributed mechanical ventilation (VMR), signifying the installation of separate extractors distributed throughout the house and in particular in damp rooms that can be managed collectively.

Firstly, install air dehumidifiers in the most affected rooms to sanitize them may be sufficient.

How to lower the humidity level in an old house?

The insulation of a traditional house from the 1970s, heated with an oil boiler, was redone at the level of the attic and basement ceiling. A VMI has been installed to reduce the humidity level in the house. But it remains important (70%). Why is the humidity level difficult to lower?

A humidity level 70% is not catastrophic, especially in a house with good insulation, which eliminates the risk of condensation. Ten points less would certainly be preferable. There may be a problem with the operation of the mechanical ventilation by insufflation. It must be ensured that the interior doors are well relieved (day of 1 cm) to allow the circulation of the air towards its points of exit. Ventilation grilles had to be installed on the windows: it is necessary to check that they work in the good sense, because many companies pose inadequate grilles (designed for the VMC simple flux) which prevent the air to go out, the function grids of VMI being to let it out.

Fix a condensation problem on the windows

In both rooms of the floor of a house, there is no air inlet. This produces condensation on the windows in winter and on sunny days it is very hot inside. This house dates from around 1900. The two windows of these rooms, double glazing, are less than 10 years old. The outer contour of the windows is granite. They are plastic, single leaf.
Dimensions of the windows: opening: 93,5 cm x 138,5 cm and glazing: 80cm x 124 cm. frame: interior dimension: 97.5 cm x 141.5 cm and outside on the granite: 93 cm x 141.5 cm. What must be done to create a circulation of air, for the room and the house? Is there a solution other than changing the openings and without drilling into the granite?

It is certainly not necessary to change the windows. We must simply ensure better ventilation of this accommodation. Several solutions are possible:
- to create a natural ventilation scheme which consists in setting up a circulation of the air between the rooms by means of the high ventilation grids in the rooms "dry" (living room, rooms...) and of low ventilation in the most humid (kitchen, toilets, bathrooms). Be careful, the air must be able to pass under the doors for this system to be effective; at least 1.5 cm must be left under the doors, planing them if necessary!
- install a VMC if the configuration of the premises lends itself to it (possibility of passing ventilation ducts in the attic),
- install a VMI (mechanical ventilation by insufflation) which is not to extract the stale air like a VMC, but to "push" the fresh air into the house via a central insufflation placed in the attic. This last solution will be both the simplest to implement and the cheapest.

How to create a ventilation system in a wet house?

Without the possibility to install a VMC, how to proceed to ventilate a very humid house with no ventilation? Are we condemned to open the windows to ventilate it?

A diagram ofnatural ventilation must be created, that is to say, a good air circulation between the rooms. In old houses and apartments, this ventilation was provided by a system of high and low grids (which remains an obligation in kitchens and bathrooms where there is a water heater or a natural draft boiler). A door trim of 1,5 cm by planing is still necessary.

It is possible to use a VMI, or mechanical ventilation by insufflation, which consists of putting the house under pressure by blowing fresh air, pushed outward by ventilation vents. Here too, it is necessary to ensure communication between the rooms, either by calibrating the doors or by placing grilles.

Is mechanical ventilation by insufflation a good choice?

In a house dating back to the 1950s in which PVC windows without ventilation were installed, mold was found in a room in which cardboard boxes were placed (at the height of the boxes) and along the wall or had been created. a dressing room. It has been proposed the installation of a VMI contra-indicating the insulation of the wall which, allegedly, worsen the state of condensation. What to think?

The VMI or "mechanical ventilation by insufflation"works in the opposite way to a conventional VMC, since it consists in putting the parts in overpressure, generating a flow of air that propagates in the house, without sheath or particular ducts.It is installed when a VMC does not It is not possible to see how the insulation of the wall in question would "worsen the state of condensation", whatever the side of the wall.An insulated wall is by definition a less cold wall, therefore less subject to the condensation.

Efficiency of mechanical ventilation by insufflation

This is an old house, very poorly ventilated. The installation of mechanical ventilation by insufflation (VMI) is envisaged. Is it a good solution, really effective? Why is this system underestimated?

The VMI is an unknown system because it concerns the renovation and is of less interest to the industrialists than the VMCs that have until now been associated with new homes. It is rather a good solution but must be done with great care. It requires some interior design work, including the "blasting" of the doors, which involves sawing the lower part to facilitate the flow of air from one room to another, to the bars of ventilation, to place on all the windows of the house.

On the same topic

  • Questions answers
    • Can we install a ventilation grille on a PVC window?
    • VMC's problems and Housebrico's advice to solve them
    • How to solve moisture problems due to lack of ventilation?
    • How to ventilate an African house?
    • Can we connect the primary exit of WC and that of VMI on the same exit on the roof?
    • VMC installation in our attic
    • Installing a VMC
  • DIY tips
    • The ventilation of the house
    • Ventilation and air treatment: extractors, controlled mechanical ventilation (VMC), dehumidifiers...
    • How to install a central air treatment?
    • Thermo-acoustic doubling improving air quality
    • Condensation on walls and windows

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