Retrofitting air-to-air cooling in a Passivhaus – maintaining airtightness

[Novice]

We have been living in our certified Passivhaus since it was completed in August 2017. It's a relatively large house (approx. 400 m²) and overall has proved extremely comfortable and energy efficient.

However, in recent summers we have increasingly experienced overheating in the main living areas. Despite using night purging and running the MVHR in summer/bypass mode, there are periods when we struggle to bring the internal temperature back down.

We are therefore considering installing a couple of air-to-air heat pump/split AC units – probably one serving the main living area and another on the top floor.

My main concern is maintaining the integrity of the Passivhaus airtight layer when installing them.

The wall construction is shown in the attached drawing. The airtight layer is 12 mm SmartPly/OSB on the inside of the 300 mm twin-stud timber frame, with a 38 mm internal service cavity before the plasterboard.

Obviously the installation will require refrigerant pipework, condensate drain and probably cabling to pass from inside to outside. I don't want an installer simply drilling through the wall and filling around the services with expanding foam.

Has anyone installed split AC/air-to-air heat pumps retrospectively in a Passivhaus or similarly airtight timber-frame building?

In particular, I'd be interested to know:

• How did you detail the penetration through the OSB airtight layer?
• Did you use proprietary airtight service grommets/collars or tapes around the pipe bundle?
• Is it better to install a larger airtight sleeve through the wall and pass all the services through that?
• How was the sleeve/penetration dealt with at the breather membrane and external cement board?
• How did you deal with the condensate drain without creating an air path?
• Would you pressure-test the house again afterwards to confirm the airtightness hasn't been compromised?

I'd be very interested to hear from anyone who has done something similar, particularly on an existing Passivhaus.

[ProDave]

51 minutes ago, Novice said:

running the MVHR in summer/bypass mode

Bypass mode, as it's name suggests, bypasses the heat exchanger.

 

In a hot summer, that will result in HOT outside air entering via the mvhr with no cooling effect.

 

Leave it in normal mode.  Then outside air that is hotter than inside, will be cooled by the cooler extracted air.

 

I have struggled to find any time when it is actually advantageous to use bypass mode.

[JohnMo]

5 minutes ago, ProDave said:

struggled to find any time when it is actually advantageous to use bypass mode

When inside is hotter than outside. That is the only time.

[JohnMo]

@Novice what is you normal heat system - ASHP?

[Oz07]

10 hours ago, ProDave said:

Bypass mode, as it's name suggests, bypasses the heat exchanger.

 

In a hot summer, that will result in HOT outside air entering via the mvhr with no cooling effect.

 

Leave it in normal mode.  Then outside air that is hotter than inside, will be cooled by the cooler extracted air.

 

I have struggled to find any time when it is actually advantageous to use bypass mode.

Evenings i used to set mine to come on. 

[Oz07]

11 hours ago, Novice said:

We have been living in our certified Passivhaus since it was completed in August 2017. It's a relatively large house (approx. 400 m²) and overall has proved extremely comfortable and energy efficient.

However, in recent summers we have increasingly experienced overheating in the main living areas. Despite using night purging and running the MVHR in summer/bypass mode, there are periods when we struggle to bring the internal temperature back down.

We are therefore considering installing a couple of air-to-air heat pump/split AC units – probably one serving the main living area and another on the top floor.

My main concern is maintaining the integrity of the Passivhaus airtight layer when installing them.

The wall construction is shown in the attached drawing. The airtight layer is 12 mm SmartPly/OSB on the inside of the 300 mm twin-stud timber frame, with a 38 mm internal service cavity before the plasterboard.

Obviously the installation will require refrigerant pipework, condensate drain and probably cabling to pass from inside to outside. I don't want an installer simply drilling through the wall and filling around the services with expanding foam.

Has anyone installed split AC/air-to-air heat pumps retrospectively in a Passivhaus or similarly airtight timber-frame building?

In particular, I'd be interested to know:

• How did you detail the penetration through the OSB airtight layer?
• Did you use proprietary airtight service grommets/collars or tapes around the pipe bundle?
• Is it better to install a larger airtight sleeve through the wall and pass all the services through that?
• How was the sleeve/penetration dealt with at the breather membrane and external cement board?
• How did you deal with the condensate drain without creating an air path?
• Would you pressure-test the house again afterwards to confirm the airtightness hasn't been compromised?

I'd be very interested to hear from anyone who has done something similar, particularly on an existing Passivhaus.

Didn't Jeremy install aircon with a similar wall make up? You could search his threads i don't even remember his user name. 

[JohnMo]

He did, think he mentioned the insulation was so well packed it came out as a plug.

 

12 hours ago, Novice said:

couple of air-to-air heat pump/split AC units

Why not just one multi split?

 

Remember if its just for cooling you need planning permission, if it's for heating and can also do cooling you do not.

[Nickfromwales]

I’ve done this umpteen times, with all kinds of services; matters not a single jot what service(s) it is / are .

 

For the split AC on my current clients build I simply used offcuts of 92mm MVHR duct as the sleeves, taped these to the interior AT layer, fed the pipes and cables through, and then the duct will get pumped full of Illbruck FM330 AT foam to complete; the foam seals for AT but is also, in a nutshell, liquid PIR insulation to boot.

 

Condensate cannot be terminated internally I assume? If not, just send that through the same oversized duct. 
 

Externally I just use a mastic such as CT1, or similar NON silicone sealant, and bond the duct to the external medivent board, leaving a load of it as excess. Then push the outer membrane into the mastic, smooth with a tool, then go over the lot again until mummified. 
 

I’m home today but back on site tomorrow, so I’ll post some pics for you. 
 

Very easy to do, so don’t be intimidated by this at all   

[Novice]

Many thanks for all your replies.

 

22 hours ago, JohnMo said:

@Novice what is you normal heat system - ASHP?

UFH on ground floor only, from ASHP. 

 

11 hours ago, JohnMo said:

He did, think he mentioned the insulation was so well packed it came out as a plug.

 

Why not just one multi split?

 

Remember if its just for cooling you need planning permission, if it's for heating and can also do cooling you do not.

It will probably end up being one multi split, which will also provide heating.😉

 

11 hours ago, Nickfromwales said:

I’ve done this umpteen times, with all kinds of services; matters not a single jot what service(s) it is / are .

 

For the split AC on my current clients build I simply used offcuts of 92mm MVHR duct as the sleeves, taped these to the interior AT layer, fed the pipes and cables through, and then the duct will get pumped full of Illbruck FM330 AT foam to complete; the foam seals for AT but is also, in a nutshell, liquid PIR insulation to boot.

 

Condensate cannot be terminated internally I assume? If not, just send that through the same oversized duct. 
 

Externally I just use a mastic such as CT1, or similar NON silicone sealant, and bond the duct to the external medivent board, leaving a load of it as excess. Then push the outer membrane into the mastic, smooth with a tool, then go over the lot again until mummified. 
 

I’m home today but back on site tomorrow, so I’ll post some pics for you. 
 

Very easy to do, so don’t be intimidated by this at all   

Thanks Nick, this is very helpful and will probably be the technique I will use. Would appreciate so pics of the detail.

 

12 hours ago, Oz07 said:

Didn't Jeremy install aircon with a similar wall make up? You could search his threads i don't even remember his user name. 

I will trawl my archive and see if I can find the info from Jeremy’s blog

[JohnMo]

10 hours ago, Novice said:

UFH on ground floor only, from ASHP. 

If you have a heat pump already can you not use it for cooling?

 

On 08/09/2026 at 21:42, Novice said:

couple of air-to-air heat pump/split AC units – probably one serving the main living area and another on the top floor

Instead of new A2A just add some fan coils and use the UFH in cooling mode?

[-rick-]

10 minutes ago, JohnMo said:

If you have a heat pump already can you not use it for cooling?

 

Expanding on this. What is your heatpump make/model?

 

Most modern (<10years) heatpumps can be set to cool, though some may require 'enabling' with a dongle, code, or similar. Seems like it would be well worth exploring this before spending money on any new system.

 

You might have to make some adjustments to your existing system but shouldn't be that big to at least get ufh cooling (which is not the same as full A/C but does make things a lot more comfortable in hot weather).

[Novice]

9 hours ago, -rick- said:

 

Expanding on this. What is your heatpump make/model?

 

It is a Panasonic WH-MDC06G3E5.

[Gone West]

23 hours ago, Oz07 said:

Didn't Jeremy install aircon with a similar wall make up?

I don't remember him doing that, but I do remember he had a Genvex MVHR with a built in A2A heat pump which did heating and cooling through the ventilation.

[Nickfromwales]

9 minutes ago, Gone West said:

I don't remember him doing that, but I do remember he had a Genvex MVHR with a built in A2A heat pump which did heating and cooling through the ventilation.

He did retrofit a split AC in the upstairs landing area, 100%, as they were getting uncomfortably hot in the ‘warm’ times of the year. 
 

He also said, even in a sympathetic micro-climate, that not having space heating (a-la the PH ‘way of things……”) would have led to divorce.

[Gone West]

Found it.

 

https://web.archive.org/web/20200923185500/http://www.mayfly.eu/2015/09/part-thirty-eight-heating-and-cooling-controls/

 

 

[JohnMo]

20 minutes ago, Novice said:

 

It is a Panasonic WH-MDC06G3E5.

It should do cooling out the box. Your first stage would be getting UFH in cooling mode.  For that we need to understand a little more about your heating system.  Single zone run from Panasonic controller, or multi zones with thermostats, mixer/pump on UFH manifold or none?