Overnight Heat Loss

As the airtightness test result is so bad I would suggest that along with bringing fresh air through the vents the MEV unit is also drawing air in through the building fabric.

Ours is B2. On very cold night, if it there was frost outside all the rooms would lose about 3C overnight. These days its about 2C loss overnight.

There may have been confusion regarding the "central extract mechanical ventilation". You have mechanical ventilation for rooms such as the kitchen, bathroom, toilet and Utility, but natural background ventilation (wall vent or trickle vents in windows) for rooms such as the living room, bedrooms etc.

As such, your heat loss is occuring through the wall vents. Air tightness relates to testing the air loss through the fabric/structure of the building (walls, floors, ceilings, joint between wall and windows/doors etc). Wall vents are sealed up for airtightness testing so they're not included in the results. Background ventilation is still required for airflow and air changes to prevent a build up of moisture in the air by providing fresh air unless you have a whole-house mechanical ventilation system.

So yes, your internal temperature will gradually reduce when there's no heating on. If you set your heat pump to 14deg, the heat pump won't kick in until internal temperature drops below 14deg. Insulation and airtightness can only do so much. With background ventilation, you will experience heat loss through same.

If it's a particularly cold night, you could probably close over the wall vent (if there's a sliding cover on it) which will slowdown heat loss (so long as you then open the vents in the morning), but either way you're better off setting the thermostat at a comfortable level and just letting the heat pump do its job to maintain the internal temperature.

I can't judge the fall in temperature overnight because there are too many variables. But I would say it may not be the result of missing insulation.

The airtightness test is really more of an air permeability test. It test air permeability through the actual structure of the house, so any openings the structure such as doors, windows, wall vents, extracts, are all sealed up during the test. It's about testing the air permeability though the structure, as well as joints in the structure (eg. the joint between wall and ceiling, where the window/door meets the wall, the joint between the wall and floor).

Poor air permeability results would result in even more heat loss.

However, there are also ventilation requirements which need to be met, in terms of providing fresh air and removing stagnant air. From the sounds of things, this is provided to your dwelling by wall vents. That does mean you essentially have a hole in the wall to the outside.

Air permeability/tightness and background ventilation are two different things. I understand it sounds a bit bizarre that houses these days are so designed and constructed to be "air-tight", and then there's a big hole in the wall for ventilation, but they're both serving different purposes and complying with different aspects of the regulations that are taking into consideration together when designing a house.

If anything, proper ventilation is more important than ever as houses are now so airtight that without proper ventilation, you don't get as many air changes, and then end up with stagnant air, more moisture in the air, and then conditions which can lead to mould growth.

No, for new houses if you have natural/background ventilation, your result should be between 3 and 5. As such, your result is likely in compliance with the regulations. If your air permeability result was less than 3, you should have mechanical ventilation throughout the house.

Personally, I would suggest a loss of 4deg (from 18deg down to 14deg, over a period of I'm guessing 7-8 hours) with an external temperature of 8deg, is probably reasonable with no heating on. Again, there are too many variables to consider, but the air permeability result and the method of natural ventilation, imo, appears to be sufficent and in accordance with regulations.

There would generally be a plastic sleeve inserted into the core hole in the wall, so no you wouldn't be able to check the insulation.

Again, I'm sorry but there are too many variables to judge. Room size, orientation, window size, external wall area. You could even be losing the heat through faulty seals in windows or doors.

Sliding the cover over on the vent can reduce some of the heat loss or prevent draughts, but it's still a hole in the wall. You are going to lose heat through it. It's like having a window open by two inches. If you pull it over so it's only open by one inch, you might lose heat slower, but not much slower.

If you had a building survey carried out before buying, I would suggest contacting the surveyor/engineer and querying it with them.

A couple of temperature data loggers would do this.

The builder is covered with the att result being below 5 m3/hr/m2@50Pa.

What you need to remember is that, for most house shapes, an att measurement of 5 is very close to a full volume of internal air being exchanged with out door air every 12 minutes (i.e. 5 times per hour) when a 20mph wind is blowing outside. I reckon your heat loss issue is likely not insulation related.

With respect, this is what I mean about there being too many variables. Utility rooms and walk-in wardrobes are going to have less external wall area, less window area (particularly for living rooms which will always have larger windows, and windows will not be as thermally resistant as the rest of the wall), less floor area, possibly a lower level of background ventilation.

Not all A2-rated houses can be judged together on something like this. Different means of construction, different levels of insulation, different amounts of renewable energy, different sizes and orientations of windows.... They cannot be evenly compared on one factor because achieving an A2 rating is based on different balances/results of a lot of different factors.

If its not isolated with a switch, turn if off at the fuse box.

I agree, not all A2 rated houses will perform the same. But with all due respect, this house is acting like a C or D rated house. The op is losing >4 degrees of heat in a room overnight when the outside temp is a warmish 6-8c. What will this be like when its 0c out, I would think the heat pump will absolutely eat electricity trying to keep up in the dead of winter.

Something is wrong here if you look at the other posters posting their heat loss numbers.

I would spend the money on an independent heat loss survey, this will probably answer and identify most of your issues in the house. I hope you find the answers, as this sounds like a very frustrating situation.