MVHR Recommendations for a Farmhouse Refurb

[Bemak]

As posted elsewhere, I have started renovation works to an old stone farmhouse (150sqm) which involves the addition of a two-storey extension. We've decided to go down the route of UFH on both floors which will be run with a traditional oil boiler for now. We've allowed provisions for a HP in the future. The heating will be supplemented by a back boiler in a solid fuel stove.

 

As the existing stone walls will be rendered internally with an insulating lime render to allow moisture to escape - I want to install a good ventilation system that will help to extract moisture from the house. The preference is for the unit to go in a warm attic, from which I can duct all the rooms. As the layout of the house is quite simple, the routes are relatively straightforward. 

 

In researching MVHR units, I stumbled upon Nilans Compact P2 unit which combines the ventilation system with the HW Tank which has it's own heat pump to generate hot water. On paper it sounds like a nice system - but it might be overkill for my situation. I suspect installing separate systems would be more cost effective. 

 

I would be curious to hear peoples thoughts on it, particularly if you installed a MVHR unit in a similar refurb scenario. At this point I wouldn't know which manufacturers would be considered 'premium' and which would be 'budget' so even to get recommendations in that regard would be great. 

[BotusBuild]

Sometimes referred to as Exhaust Air Heat Pumps, these were originally designed for studio or 1 bed flats, in which they worked well.

 

For you property be very careful about whether it can cope with the volume of the building. I suspect it may not.

 

Do your due diligence

[Bemak]

oh absolutely. I'd likely oversize it to be safe

[saveasteading]

22 minutes ago, Bemak said:

insulating lime render to allow moisture to escape

There is a fair amount of published information on stone walls.

My understanding is that a traditional stone wall will breathe and transmit moisture quite well, always from in to out. 

The insulation is not good, but not zero, so ufh seems sensible.

The outer face must be in good condition, and that should be a priority and a skill worth learning.

Are you putting lots of underfloor insulation in?  Beware head heights at existing  doors.

[Bemak]

yes we're putting in a new insulated GF slab which should make a big difference. the external walls are sand and cement render unfortunately. we looked into the possibility to taking it all off and replacing it with lime but it's a non-runner because of a) the prohibitive cost and b) as we're in an exposed area in the west of Ireland, lime render doesn't particularly suit the driving rain that we can experience for weeks on end. so the only direction for the walls to breathe will be inwards. we've stripped off the internal render however and thankfully the walls are quite dry as is. I'll install a french drain around the perimeter of the the house as well which should help with moving ground water away from the base of the walls. it's all we can do really

[JohnMo]

39 minutes ago, Bemak said:

The heating will be supplemented by a back boiler in a solid fuel stove.

Doesn't that drive you to a thermal store? Especially when coupled with an oil boiler? The thermal store provides mixed down water to the UFH and a plate exchanger the DHW.

 

I would miss out the Nilans unit altogether, bucket load of cost, that really adds no value.

 

So airtightness - are you aiming sub 3 m³/m² @ 50Pa score, if not don't waste your money on MVHR it will actually add to the heating costs.

 

A MEV or dMEV setup to be humidity controlled with matching humidity activated trickle vents would be away better option.

 

Just keep the heating/DHW simple and do not combine with a ventilation unit.

[Bemak]

Thanks John. That's interesting. Hoping to achieve that level of airtightness. I guess once the envelope is sealed we could carry out a test to see where it's at before deciding on MVHR. As you say, I'd probably be happy with a MEV setup. dMEV isn't really an option as I don't want to go coring 600mm thick rubble stone walls. I'm reroofing the house so I'll be able to add vents to the roof for a MEV system

[JohnMo]

DucoBox for MEV looks good so does Passivent https://www.passivent.com/product/intelligent-mechanical-extract-ventilation-system-imev/

[Bemak]

Just thinking. A MEV system utilising trickle vents in windows would be a cheap to run solution that would do what I want which is to extract moisture. I'm used to living in older houses to the heat recovery aspect is probably lost on me. Can MVHR units be run as extract only if required?

[Bemak]

48 minutes ago, JohnMo said:

DucoBox for MEV looks good so does Passivent https://www.passivent.com/product/intelligent-mechanical-extract-ventilation-system-imev/

Thanks for this link. Think this is probably the better solution for us. The reason I ask if MVHR can be run as extract only is because I don't want to run the risk of being sorry after the fact for not installing it. I assume I wouldn't be 

[saveasteading]

4 hours ago, Bemak said:

taking it all off and replacing it with lime

I'd plan on the render falling off over time and you can lime point it in stages.

I'm assuming the stone is impermeable and well built, but you may not know!

Granite does not soak up and hold the rain. Render does. 

I gather that research on trad farm buildings is all pretty recent. The 3 layer walls work well....high quality outer skin, medium internal, and a rubble and lime core. 

 

Are you insulating the roof?

 

What insulation level is the new render quoted to provide?

 

On our steading we fixed internal stud with a dpm facing the gap to the wall. Then insulation between studs, then a services void and plasterboard.

We first thought that we couldn't afford the loss of width but pleased with the cosy result, up to newbuild standards.

[Bemak]

I hacked a bit of render off a few years ago to see how labour intensive it would be. Found 3 layers of render had been added over time and it was very difficult to remove, often pulling stone with it. So I repaired the patch I removed. The intention is to render the house completely in the end.

 

I had originally looked at dry lining internally but we've opted to go for 40mm of insulating lime render instead. I was told that over insulating internally could be a bad thing as the walls would sweat behind the gutex board.