Bit misleading to claim there'll be a five year payback.
In order to have mvhr you need to start looking at having airtightness membrane around the house so there are no leakages affecting the system . Before all of this you should be considering insulation.
How could you possibly
1 insulate
2 install membrane
3 plaster surfaces
4 not to mention ducting and heat recovery unit for 3 grand.
Vincenzo,
Look at the big picture for a minute, Even if you just use hole in the wall vents 10 air changes at 50 pascals is equivalent leakage area to leaving a door open. You dont need membrane. Hardwall plaster and OSB do the job better. The membrane is not technically an airtight barrier, its a vapour control layer. It is essential to have vapour control no matter what build up you select. This generally means lining the dormer in OSB and taping as a vapour control. This creates a services cavity with battens between osb and plasterboard. I usually put softboard on the rafters, as wind acting on the insulation reduces its performance.
In Germany and Austria they design houses to last 60 years, in Ireland every weekend I'm testing houses often no older than 2 years old, which are impossible to heat, have condensation problems and are drafty. The way we build houses is 'shocking bad'.
Solar is now mandatory to meet regulations. The new ventilation regulations say you need to increase the size of the hole in the wall vents by 40% if under 7 air changes, and boilers have to have efficiencies of over 86%. This is the extra cost in building, its coming from regulation, so that extra cost is just for good practice. In this type of house having to heat the air twice an hour is an expensive task. The MHRV retains about 90% of the heat. The extra over good practice building for adding MHRV is about €3k. Ya sure if you compare a bog standard one off and leave out the mandatory renewables and stick kingspan in the cavity and drill big holes for services and cables it will cost about €15k to add MHRV.
The fact is to build celtic tiger type houses now is a stupid idea. Bolting heat pumps or renewables onto a minimum standards build is ridiculous unless your a speculator who can sell it fast and leave the homeowner with an underperforming asset. The way we build now has changed dramatically but many builders are left behind because of the low levels of knowledge and skills in the industry. Anyone still building bog standard will probably see me visiting their house in a couple of years to show them the uninsulated knee walls, or the wind blowing through downlighters and dormer crawl spaces, uninsulated boiler pipes, blocked wall vents and closed trickle vents, the underfloor leaking heat through the rising walls, the open fireplaces which get used just twice a year, quilt insulation on the slope, I could go on.
All I'm saying is, think about where your wasting energy in a home and think about what comfort is. You'll soon arive at the inevitable conclusion that you need to pay attention to maintaining the integrity of the insulation using thermally storing insulats like cellulose and softboard in the roof. You'll need to remove the rising wall, eaves and windoe cill/head/reveal cold bridge and you'll need to prioritise minimising heat loss over buying a huge boiler. In the end of the day you'll have a future proofed home that will be comfortable and healthy and pretty close to passive. So good luck with that