MVHR into a cold roof ?

[Post and beam]

My plan always was, and is, to install the unit in the loft. Too late really to change that now.

 

I read only yesterday that the miminum temperature should be 10 degrees C at the installation location. Clearly this is not possible in a standalone cold roof space. 

So do i

#A build a little box room around it

#B not worry about it

#C have a word with myself and think of a better solution.

 

 

What have those of you that are ahead of me on this topic  done please?

 

 

[Dreadnaught]

It poses the question as to why 10º minimum exists.

 

I have my MVHR unit within the thermal envelope of my house. I notice that condensation is huge in the winter. Warm moist air from the house hits the heat exchanger cooled by incoming air at say 0º. Huge amounts of water vapour condenses.

 

Is the risk if the unit is below 10º that that condensate, if it were to freeze elsewhere within the unit away from the heat exchanger, and not drain, could cause problems?

 

Just a guess as to why the 10º limit exists.

Edited 22 hours ago by Dreadnaught

[jayc89]

I built an insulated "room" in our cold loft - boarded it out, 3x2 stud walls, rock wool in the studs and 50mm PIR adhered to 12mm ply screwed to the underside of the rafters - wasn't aiming for massive amounts of thermal efficiency up there, given there's 400mm of insulation beneath it, just enough to keep the frost at bay. I did plan to stick a small electric heater up there, with a stat to come on at under 5-10c but haven't got around to that yet. So far so good. 

 

Where the "room" is, I laid some 6x2 joists across the existing trusses (we have some beefy king posts), and used loft legs everywhere else, then ran some Caberfloor across the lot. 

[Marko]

Mines in the loft. No where else to put it really. Might end up ‘cladding’ the sides with PIR, at some point. 
 

On the coldest day we had last week, the temperature difference between the room extracts and supply valves was hovering around 0.3-0.5 degrees. There’s no insulation in the loft yet either, but ducts are lagged with 80mm ISO sleeve. 

[Post and beam]

13 hours ago, jayc89 said:

insulated "room" in our cold loft

Thats my #A option. Seems like the best resolution since i discovered this little gotcha. I think i will build up a little stepped platform above the loft deck 100mm high, sit the MVHR unit on that on some isolation mounts and then box it in. 

The 100mm gap under the platform is so that i can deal with the need to accommodate crossing ductwork.

[Mike]

On 12/01/2025 at 11:12, Post and beam said:

My plan always was, and is, to install the unit in the loft. Too late really to change that now.

Yes, option A. However will it fit through the loft hatch for installation & future replacement?

[Marvin]

The MVHR units can be quite big. Ours fits through the loft hatch. We rolled 400mm of loft insulation over it and around it. 

[MJNewton]

Ours is in a cold loft and we've never had any issues as far as I am aware. The unit is insulated internally and only the control boards sit outside of this and likely warm themselves up enough not to be a problem.