Roof vs. Facade Exhaust for Heat Recovery - Which to Choose and When

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Roof vs. Facade Exhaust for Heat Recovery – What to Choose and When

When we talk about mechanical ventilation with heat recovery, most people focus on the unit's performance, pipe diameter, or the placement of air inlets in the rooms. The exhaust direction – that is, where the ventilation system "breathes" into the outside environment – remains surprisingly on the periphery of interest for a long time. Yet this decision can significantly influence how the entire system functions, how the house looks from the outside, what the installation and maintenance costs will be, and what its lifespan will be in the specific climatic conditions of the location.

In practice, I encounter two basic variants: roof exhaust (through the roof, so-called "chimney" solution) and facade exhaust (through the perimeter wall, usually at a certain height level on the facade). Each of these approaches has its logic, its strengths, and situations where it is, on the contrary, unsuitable. This article is not an advertisement – it is a practical technical overview that is intended to help you make an informed decision before you contact an installation company or include duct penetrations in the construction drawings.

Why the type and location of the exhaust matter

A heat recovery unit – whether centralized or decentralized – works with two air streams simultaneously: fresh air brought in from the exterior and stale air extracted from the interior. Both of these streams must pass through the building envelope somewhere. Facade or roof exhaust is precisely the place of passage.

At first glance, this seems like a technical detail. In reality, this "detail" affects:

Heat recovery efficiency – if recirculation occurs (extracted air mixes with incoming air), the real quality of the fresh air and the apparent efficiency of the system drops.
Moisture balance – condensate formed during heat recovery must drain somewhere; the location of the exhaust determines where.
Water ingress – a poorly designed exhaust is a common cause of structural dampness.
Noise – air exiting the unit is not silent, and if the exhaust is close to a window or terrace, it can be a problem.
Building aesthetics – grilles, duct penetrations, and chimneys are visible elements of the facade or roof.
Installation and future maintenance costs – roof exhaust requires access to the roof, while facade exhaust requires access from the outside.

Facade exhaust – principle, advantages and limitations

Facade exhaust is historically the most common solution in single-family homes. The duct passes through the perimeter wall at a selected height, with a grille or cap installed on the outside and connected to the heat recovery unit's ductwork on the inside. This approach is relatively simple to implement – it does not require access to the roof, special flashing, or roofing work.

Where facade exhaust works best

Facade exhaust is the ideal choice in these situations:

Brick houses with thick walls – drilling with a core drill is standard work, and the duct can be installed even retroactively without major interference with the structure.
Low-energy and passive houses with green roofs or other special roof constructions – any intervention in a flat roof covered with substrate is complicated, while the facade is more accessible.
Decentralized systems – a typical decentralized ventilation unit HRC E Master (pr. 160 mm) is designed precisely for direct passage through the wall; the duct can be installed directly behind the unit without long duct runs.
Renovations – when the roof is already finished and any work on it is risky or expensive.
Apartment buildings – each apartment has its own facade, where exhaust is logical and technically easy to implement.

Disadvantages and risks of facade exhaust

Facade exhaust also has its typical problems that recur in practice:

Air recirculation – if the exhaust and intake ducts are too close to each other on the same facade, stale air can return into the system. The minimum distance between the intake and exhaust should be at least 1.0–1.5 m depending on the configuration. Ideally, they are on different facades or at significantly different heights.
Dominant wind – in built-up areas or on exposed sites, strong wind blowing directly into the exhaust can increase pressure loss and disrupt air flow. Some systems have pressure compensation, but this still needs to be considered when orienting the exhaust.
Water ingress – if the grille or frame is not properly sealed and flashed (especially with insulation, this is particularly critical), water can penetrate into the insulation or wall structure.
Aesthetics – on a representative facade, a grille can be a disruptive element. The right choice helps – for example, an external aesthetic grille for IVAR.HRC pr. 160 mm is designed to visually blend with the facade.
Exhaust height and safety zones – the exhaust must not be too low (risk of vandalism, snow accumulation) or too close to windows, doors, or neighboring property. Local building regulations and sometimes the unit manufacturer's conditions apply.

Roof exhaust – when it makes sense and what it entails

Roof exhaust is standard in Scandinavian countries, but it is increasingly gaining ground in Central European practice – especially in new low-energy single-family homes with a centralized recuperation unit located in a technical room or in the attic space. Air is extracted vertically above the roof, where pressure and recirculation conditions behave significantly more predictably than in a façade solution.

Technical advantages of roof exhaust

Zero risk of recirculation – supply and exhaust are structurally separated on the same chimney (double-pipe system or two separate outlets with sufficient spacing), with the extracted air rising upwards and not being sucked back in at the bottom.
Better conditions for condensate drainage – condensate from the recuperation unit is usually drained at the bottom through a trap, but roof routing allows for shorter vertical sections and fewer elbows, reducing pressure loss.
Less susceptible to wind pressure – above the roof, aerodynamic conditions are relatively more stable than on a façade at a lower height (depending on the building configuration, but this rule is reliable in open countryside).
Preserves the façade aesthetics – no grilles on the representative façade, no risk of minor flaws in sealing after insulation.
Protection against vandalism and mechanical damage – roof exhaust is much less accessible.

Complications and costs of roof exhaust

A roof solution is not without compromises. From project and implementation experience, I know these points must be addressed in advance:

Penetration of the roof membrane – every hole in the roof is a potential source of leakage. The penetration must be professionally sheeted, flanged, and sealed; for shingle roofing, special flanged systems with gaskets are installed. If this is not done by an experienced company, the risk is high.
Longer duct = higher pressure loss – if the recuperation unit is in the basement or on the ground floor and the exhaust goes through the full height of the house above the roof, the duct can be 5–8 m or even longer. Every meter of duct and every elbow adds pressure loss, which must be compensated by a more powerful fan (and thus higher power consumption) or a larger duct diameter. See also our article on What duct diameter do I need for a recuperative ventilation system.
Condensation in the duct – long vertical runs in an unheated attic space can condense from the inside in winter if not properly insulated. The warm air being extracted rises, meets the cold pipe wall, and releases moisture. The duct must have thermal insulation and a slope towards the drain trap.
Access for maintenance – cleaning the roof exhaust, checking the grille or chimney requires access to the roof. Compared to a façade (reachable from a ladder or window), this is more complicated and costly.
Not always approved by the building authority – in houses in a heritage zone or in certain types of apartment buildings, any visible change on the roof may require the consent of the administrator or heritage authorities.

When to choose façade and when roof exhaust – practical decision logic

There is no single "correct" solution for all houses. Based on experience from many projects, I can offer the following practical criteria:

Choose façade exhaust if:

You are using a decentralized system (e.g., HRC units directly in the wall) – these devices are structurally designed for direct wall passage and roof installation would be technically complicated and unjustified.
You are doing a renovation and the roof is already finished or protected – any intervention in a finished roof costs money, time, and carries risk.
The unit is located in the basement or on the ground floor and the duct would be too long – a duct from the basement to the ridge of a typical single-family house can be 8–12 m, which is a pressure loss that a standard fan cannot handle without significantly increasing power consumption.
The façade is not visually prominent or it is a side/rear wall where the grille is not an issue.
You want lower installation costs – a façade passage usually costs less than a professionally insulated roof chimney installation.

Choose roof exhaust if:

You have a centralized unit in the attic or in a technical floor under the roof – the duct to the roof is short, technically clean, and efficient.
The house is located in an exposed area with prevailing wind and you do not want to deal with pressure compensation.
The façade is architecturally sensitive – a representative cladded or glazed façade is not suitable for visible grilles.
You want to minimize the risk of recirculation without having to address minimum distances and orientation of exhausts.
You are installing the system in a new house where roof work is still being done – the penetration can be built in before the roofing is laid at minimal cost.

Dimensioning and technical details – what not to forget

Regardless of where you place the exhaust, there are several technical rules that must be respected. Neglecting any of them can lead to noise, reduced performance, or premature wear.

Duct diameter and air velocity

Typical diameters for single-family homes are 63 mm, 75 mm, 90 mm, 125 mm, and 160 mm. Air velocity in the duct should ideally be between 2.0 and 3.5 m/s. At higher velocities, aerodynamic noise and friction losses increase, while at lower velocities, the duct is disproportionately large. For flows around 50–80 m³/h, a diameter of 90–125 mm is usually sufficient, and for larger flows, 150–160 mm. More details can be found in the article What duct diameter do I need for a recuperative ventilation system.

Connecting elements and seals in the ducting system are important components such as tongue-and-groove gasket, e.g. 63 mm (classic) or module connection for distribution box, e.g. 90 mm (classic), which ensure airtight and hygienic connections between duct sections – and their quality directly affects the efficiency of the entire system.

Pressure loss and booster

Long ducts, multiple elbows, and improperly shaped transitions can increase pressure loss to such an extent that the unit cannot deliver the required airflow. In such cases, a solution may be to insert a booster (e.g. 75 mm or 90 mm, classic) into the ducting, which locally helps to overcome pressure loss in long sections or before problematic elbows.

Thermal insulation of the duct in roof passage

Ducts running through an unheated attic space (loft, space between rafters, gaps in wooden frame) must be insulated. The recommended thickness of insulation for exhausting warm air in an unheated space is at least 20–30 mm of foam or mineral insulation. Without this, condensation in the duct can damage the insulation and cause biological growth (mold, bacteria).

Condensate siphon

Every heat recovery unit produces condensate – especially in winter, when the temperature difference between indoor and outdoor air is large. The condensate must be drained using a siphon, not just by gravity without a trap. A siphon prevents the entry of outdoor air (warm in summer, cold in winter) into the condensate tray and contamination of the heat recovery exchanger. This detail is surprisingly often neglected in practice – and it is one of the most common causes of odors from heat recovery units.

Minimum heights of façade outlets above ground level

The exhaust outlet should be at least 0.5 m above ground level (to avoid being buried in snow), and the intake should be at least 1.0–1.5 m above ground level (to avoid dust and surface contamination). For intake outlets, it must not be located in a place accessible to pets nor near a parapet where insects might sit. In practice, façade outlets for residential houses are typically installed at a height of 1.2–2.5 m above the finished ground level.

Combination of both solutions – hybrid approach

In practice, we sometimes encounter a hybrid solution: fresh air intake through the façade (near ground level, shaded from direct sunlight), and exhaust through the roof. This approach has its logic – exhaust air (warm, moist) naturally rises more easily due to the chimney effect, while cooler fresh air is drawn in through the lower-positioned façade. In summer, this also helps with natural circulation. The disadvantage is the use of two types of transitions and increased design complexity.

Such a solution is justifiable mainly in larger houses with a centralized unit, where the architect and HVAC designer collaborate from the beginning. In DIY installations or smaller decentralized systems, I do not recommend it without professional consultation.

Influence of surroundings and local conditions

The location of the house significantly affects which outlet will work more reliably:

Mountainous and windy areas – façade outlets on the windward side are problematic. Wind with a pressure of 50–100 Pa can significantly limit the airflow through a heat recovery unit designed for a total static pressure of 80–120 Pa. Roof outlets (or façade outlets on the leeward side) are much better in this case.
Garden areas with high vegetation – low façade inlets near hedges or grass can suck in insects, pollen, and mechanical dirt. The position of the outlet at least 1.5 m above ground level and the installation of a fine mesh (without increasing pressure loss) helps reduce this problem.
Dense urban areas and streets with traffic – intake outlets facing a frequently used road are unsuitable in terms of air quality. In this case, roof or courtyard façade outlets are clearly better.
Coastal areas (salty air environment) – corrosion of metal grilles and transitions is faster, so plastic or stainless steel components are recommended.

Practical examples from customer projects

To keep everything clear and grounded in reality, I am including a few specific scenarios that repeat in practice:

Case 1: Renovation of a masonry house from the 1980s

The house has an original pitched roof with tiles, new façade insulation, and plastic windows. The owner wanted heat recovery in three bedrooms and the living room without major construction work. Solution: four decentralized HRC units in the walls of the bedrooms and living room – each with its own façade passage (diameter 160 mm), with an aesthetic grille on the exterior side. A roof outlet would require rerouting through the newly insulated ceiling and modifying the protected tiled roof – which would be unnecessarily expensive for such a system. The façade is oriented toward the garden (south-west), and recirculation is not an issue.

Case 2: New low-energy residential house

A house with a flat roof, a centralized recuperation unit with a capacity of 350 m³/h located in the technical floor under the roof. In this case, a roof outlet was a logical choice – the distance from the unit to the roof was 1.2 m, the penetrations were built into the roof structure before the concrete pour, and a double-pipe chimney with a cap was installed from the beginning. The facade remained architecturally clean, which was one of the architect's requirements.

Case 3: Apartment building, individual flats

During the renovation of an apartment building with the replacement of windows with tight plastic ones, the need for ventilation solutions arose. Each flat received its own decentralized pair (supply + exhaust unit or a reversible unit). A roof solution was not feasible – access to the roof belongs to the building manager, and ducts would have to pass through neighboring flats. A facade outlet with aesthetic grilles was the only reasonable option. It was crucial to properly design the height and mutual position of supply and exhaust openings to prevent recirculation between flats.

Case 4: Mountain cabin

A cabin at an altitude of 900 m, prevailing south-west winds, occasional gusts and strong pressure. A facade outlet was tested – problems arose during every storm, as the wind pushed water into the penetration despite the wind baffle. Solution: a roof outlet with a double chimney cover, solid sheeting and thermal insulation. The problem of wind flooding was eliminated, and the system has been operating without failures since then.

Most common mistakes in the implementation of outlets

From practice, I know that mistakes repeat themselves. To avoid them, here is a list of the most common ones:

Too small a distance between supply and exhaust on the same facade – recirculation, odors, reduced air quality.
Lack of condensate trap – moisture in the pipe, biological growth, corrosion.
Uninsulated pipe in an unheated space – condensation, heat loss, dampening of the structure.
Outlet on the windward side without pressure compensation – reduced flow, noise, unstable function.
Insufficient sealing of the facade penetration during insulation – water seepage into the insulation.
Installation of a grille without insect protection – insects in the recuperation heat exchanger, clogged filters.
Neglected thermal insulation of the roof chimney – condensation in the chimney, dripping into the unit.

Legislative and normative requirements

In Slovakia, the design of ventilation and air conditioning systems is mainly governed by the standards STN EN 15665 (requirements for ventilation of residential buildings) and STN EN 13141 (performance testing of components/products for ventilation of living spaces). Regarding the placement of outlets, it is not allowed to exhaust air into enclosed courtyards, garages, or areas with fire risk. The supply outlet must be placed so that it draws in clean air – not from a space where exhaust fumes, exhausts, waste, etc., are present.

During renovations, it is necessary to verify whether an intervention in the facade or roof requires a building permit or just a notification. Most penetrations with diameters up to 200 mm in single-family homes fall under notification or are justified without formal proceedings – however, in areas with a zoning plan or heritage protection, always check this in advance.

Summary of the decision-making process

The decision on the type of outlet should follow this order:

Determine the type of system – centralized or decentralized. Decentralized = almost always facade.
Find out the location of the unit – the closer to the roof, the more advantageous a roof outlet is.
Assess the condition of the roof – new or renovated? A roof penetration is cheaper during construction.
Evaluate the facade – representative? Exposed to wind? Heritage protected?
Check local conditions – wind, vegetation, building density, proximity to the road.
Calculate pressure loss – for long roof duct runs, verify whether the unit fans can handle the required flow.
Consult the proposed solution with the installation company or the HVAC designer.

For more information on the overall system design, see our articles How to choose a mechanical ventilation system with heat recovery for a single-family house and How to properly design an air distribution system with heat recovery.

Frequently asked questions (FAQ)

Can I have supply and exhaust on the same facade?

Yes, but you must maintain a minimum distance – usually at least 1.0–1.5 m in the horizontal direction or a vertical difference of at least 0.8–1.0 m, with the exhaust being above the supply. Always check the specific requirements of your unit's manufacturer – some specify larger distances or even prohibit the configuration on a single facade altogether. If both outlets are on the corner of the building and, under normal wind conditions, are on different aerodynamic sides, it is acceptable even with a smaller physical distance.

What are the minimum heights of the outlets above the ground?

An exhaust (exhaust) outlet should be at least 0.4–0.5 m above the ground – to prevent it from being buried in snow. A supply (intake) outlet is recommended to be at least 1.0–1.5 m above the ground, to avoid drawing in dust, pollen from the ground or larger insects. In practice, both outlets in single-family houses are commonly installed at a height of 1.2–2.0 m above the finished ground level.

Do I need to insulate a roof outlet in summer?

Thermal insulation of the pipe running through an unheated attic is meaningful all year round. In summer, it prevents condensation of moisture from the outside on the cold pipe (in a climate-controlled interior, the pipe with cold incoming air is colder than the surroundings). In winter, it prevents condensation of warm exhaust air on the pipe wall. Insulation is therefore not just a winter measure.

What if the investor wants a roof outlet, but the unit is in the basement?

It is feasible, but it must be carefully calculated. A pipe 6–10 m long with several bends can add 50–100 Pa of pressure loss. Solutions include: a larger pipe diameter (reduces velocity pressure), fewer bends (curves instead of right angles), inserting an amplifier into the duct, or selecting a more powerful unit with higher static pressure. All of this must be calculated during the design phase, not during installation.

Can a roof outlet be a source of noise?

Yes, if the pipe is not properly secured, it can vibrate and transmit noise into the structure. Also, wind passing through the chimney cap can produce sound in strong winds. Solutions include anti-vibration clamps on the pipe, properly dimensioned caps, and in some cases, a noise silencer inserted into the duct. We elaborate on the noise of heat recovery units in the article Noise of heat recovery units – causes and solutions.

Does a facade penetration need special anti-icing treatment?

An intake penetration (through which cold outside air enters) can freeze in severe frost if the unit has only partial or no anti-icing protection. Modern heat recovery units have built-in bypass or preheating of the incoming air. If your unit does not have such protection, we recommend thermally insulating the facade penetration from the outside and ensuring that the grille on the outside is large enough to maintain the required flow even with partial frost.

Conclusion

Roof and facade outlets are not interchangeable in every situation – each has its typical area of application where it works best. A facade outlet is simpler, cheaper to implement, and ideal for decentralized systems and renovations. A roof outlet offers higher aerodynamic stability, zero risk of recirculation, and a clean facade – at the cost of higher installation costs and requirements for thermal insulation of the pipe.

The key is to make decisions based on specific conditions – the location of the unit, the condition of the roof, the orientation of the façade, the location, and the budget. If you are planning to design or implement a heat recovery ventilation system, we recommend also studying other related topics from our Knowledge Center – especially Centralized vs. Decentralized Heat Recovery Units and Step-by-step Installation of a Decentralized Ventilation Unit HRC. A professional decision made in advance will save you from problems, repair costs, and unnecessary frustration during system operation for the next 20 or more years.

Do you have a question about this topic?

Are you unsure or dealing with a specific situation in your home? Write to us – we are happy to help.