holes in the walls and holes in the pockets

Since I didn’t expect our “ engineers “ to deliver any usable information about the issue what it costs to have draughty homes compared to a proper ventilated home I did some research myself. My source, from which I cite in the following is the
“Timber Construction Manual by Herzog, Natterer, Schweitzer,Volz, Winter “
Publisher is Birkhaeuser Edition Detail , ISBN 3-7643-7025-4 , first edition 2004.
Chapter “Thermal performance of buildings (Gerd Hauser)” page 64 –67
At page 65, 3rd column
“…….. Airtightness :
The airtightness of a building is generally specified with the help of the n50 value, which relates to the air change rate at 50 Pa pressure difference. The requirements for the airtightness of the building envelope are given in DIN 4108 part 7: - building with natural ventilation : n50 =< 3.0 h-1 - building with mechanical ventilation: n50=< 1.5 h-1………..[here a description follows about how blower door tests are done and their merits] “
At page 66, 3rd column
“Airtighness
The inclusion of more and more insulation in the external increases the proportion of ventilation losses in the annual heating requirement. Ventilation concepts based on airflows based on joints or manual surge ventilation (i.e. opening windows) are unsuitable for low-energy buildings owing to the uncontrolled and wide variability of the air change rate. Furthermore, the necessary average air change rate of about 0.5h-1 cannot be guaranteed in less favourable areas. A central ventilation system is therefore included in the standard house. It should be remembered as the leakage rate (infiltration) of the building increases, so the effectiveness of the ventilation system quickly decreases. In comparison with the standard case with its ni-value of 0.10 h-1 , the heating requirement reduces by 4 Kwh/m2a for ni = 0.05 h-1 . With a less airtight envelope where ni = 0.20 h-1 , the heating requirement rises by 9 Kwh/m2a. High airtightness requires special care during planning and construction of the airtight barrier and its connection to other components.”

NOW IN MORE SIMPLE WORDS:
So with an increased air change of 0.1 changes per hour the annual heating requirement increases by 9Kwh/ m2 annum . 9 Kwh per square meter per year compares to 1liter of oil per square meter per year burned in a central heating system with a 90% efficiency . That would be only achieved with a high performance condensing boiler at an efficiency rate of over 91% , electric energy for the pump included. But the average dirty boiler has an efficiency rate of only about 50%. So an increased air exchange of 0.1 exchanges per hour will result in an increase of fuel demand of about 2liters per square meter per year. Replacing the lost energy blown through holes. IF the exchange rate is at an rate of 0.1. But with a felt drought, in a room/house where there are several holes in the walls which are supported in their efficiency by open chimney stacks the air exchange can be set at a much higher level . Exact numbers would have to be calculated at the individual building , with the aid of a blower test.
Just the extra 0.1 exchanges per hour, which are unnecessary, will ad to the necessary heating oil demand at the average 120m2 house a staggering 240liters per year .
240 LITERS PER YEAR !!!! With only 0.1 extra air exchanges per hour.
If I have erred in my calculations than let me know. Better ask your engineer !

I've living in a recently built house, and there are no vents in the rooms. The builder said they were old hat. If your calculations are correct, I can see where he was coming from.

However, I have noticed mould spore build-up on a couple of window frames, especially in the bedroom and bathroom. How would I go about getting rid of this without losing heat?

Wipe it off (smiley). No, more seriously.The mould is a result from bacteria growth.Bacterias grow on anything but they need one thing for sure, moisture i.e. water. So there seems to be a condensation problem. Condensation starts at a temperature difference of about 5 or 6 degrees Celsius. So that means that the frame is at least 5 degrees colder than the sorounding air.
2 ways can be gone. Either you reduce the room/air temperature or you increase the frame temperature. Since it is a room you want to use its the second way to choose.
Check if the sealant (silicone or acryl) around the frames at these cold points is intact, present.At both sides , inside and outside. Cold air flowing through gaps and frames have a cooling effect.
After that is hooked off you should check if the sealant strips are realy sealing, use a playing card, the smoke of an incense stick or a cigarette. Better the playing card. If that is done and nothing found than the window frames are badly insulating.
Stop, one more point:Check also the window sill -inside and outside- if there are any gaps. And meassure the temperature of the window sill inside by laying a thermometer onto it , well covered , for a few hours or overnight. compare the temperature at this thermometer with the room temperature. As long as the difference is less than 5 degrees (4 degrees maximum) then it should be fine. If the difference is larger than you have find out why. When the house has a cavity wall the situation seems almost clear. You would have to fill the cavity with non rottening insulation material, propably builders foam ( PU foam) , use a good quality that allowes for long term flexibility, cheaper ones will get to hard and the stickyness, the surface contact, will come off . And you have an airgap.
Last possibility: The frames are of bad insulation quality. Either exchange them or reinforce them with PVC ducting or strips of timber on the outside. PVC ducting including corner pieces and end pieces is available from B&Q in the electric department , 16mm , 25 mmm and 40 mmm.
Make sure that the ducting is sealed with acryl to keep rainwater out, it could loosen the sticky strip at the backside after a while.

Hi Heinbloed

When you seal up the holes in the house you then need a mechanical controlled ventilation system or else the house becomes deplete of oxygen or have you some other solution.
Do you have the name of the company who can supply heat exchange units for 1k.

Have a look under the outside sill, umop3p!sdn. Recently built? Hmm, bet there is a gaping crevice between the bottom of the sill and the wall.

But the fact the the building does not subscribe to the antiquated guidance is encouraging. Active over passive, I always say. Stuffy? Open a window. Damp? Switch on the extractor. Beats have some reactionary builder usurp your right to self-determination.

And FWIW, never a mention was made in any posts about sealing attic or roof areas. Some intelligently applied batts of rockwool in the attic mitigate heat losses.

Ventilation via the windows is endeed possible , I didn't know that this news to some readers. It is the common form of ventilation in most countrys where people can afford windows that can be opened.
To Touchwood:
I did some research in the www (just for you, smiley) and took the first that sprung up , see [broken link removed]
Sorry , it's in German, so some details here in short:
CVS (stands for controled Ventilation System, a so called heat recovery unit with 90% recovery rate) "standard":€ 470.- with automatic defrosting (for wet rooms I suppose),
or €290.- without automatic defrosting, technical specs:2-22m3/h, 12-18m2 room size, 1-2.5 persons/room, 12v, 1-8W, 34-48 dBa , in a silenced room 27-38 dBa.
CVS "robusto":€342.-, 12-18m2,1-2.5 persons, model A :8-14-20m3/h depending on setting, 26-39dBa, 12V, 2-6W. Model B 6-10-14m3/h, 20-31dBa, infiltration noise reduction>45db, 12V, 1-3.5W.
CVS "Jumbo":€638.-, 50-80m2,10-15 persons,50-10-150m3/h, 12v, 4-11W, 26-32-40dBa,infiltration noise reduction >40dB.
As well you need a transformer,230V-12v, here for €44 (what I consider expensive) if freely pluged in or for €79.- if to be fitted in a box in the plaster (the Continentals have standard electric boxes within the wallplaster that would take anything from lightswitch to socket to transformer, so things are tidy and invisible).
For absolute quietness there is an extra soundinsulation box available for €90.- that fits all models.This would be for the bedroom.Or the caravan or tent or the open plan room , noise reduction of minus 40something dBa.
All prices incl. VAT at 16%.
I have nothing to do with this company, I prefer the rapid ventilation system that comes for free, see Max Hopper's post above.

Thanks for the info Heinbloed. Much appreciated.
Do they have a supplier in Ireland that you know of?

No.I just fund them in google.But there are many manufacturers on the Irish market present I supose, think about the containers/caravans which are used for living. Any winter proof caravan/container must have these things I would say. Boat cabins as well I would say. Check google under "controlled ventilation+room" or a similar headword.