Hi All, So glad I have found this forum, I have found so much useful discussion. We are just about to start a renovation and large extensions to a 50 year old bungalow. We have planning but just starting building control details. Current property has poor ventilation and is damp but we are pretty sure that is just from cooking and bathrooms as there is no extraction and not many opening windows.

We want to make the property as efficient as possible so have a big wish list of technologies we hope to use but struggling to find anyone to help select the best selection on choices we have.

The current property has 2 bedrooms and 1 bathroom. brick construction with foam filled cavities. The loft space has been badly insulated with >300mm of glass on on the floor but also glass inside the tiles (never seen that before). Ventilation has been blocked and I have condensation.

We will be adding a 9m x 7.5m open plan living space and converting the current property to 3 beds and 3 baths. In addition there will be a very large garage, office, laundry room and hallway.

The new build sections have been specified with thermal blocks, 150mm of Kingspan and a 10mm cavity. Only a small amount of the existing walls will remain external so we would like to consider external insulation for these walls.

We would ideally like wet UFH, solar PV, battery storage and possibly ASHP. The current flooring is a suspended wooden floor with around 600mm cavity, below this is a concrete pad which is clean and dry. The new build areas will be beam and block so wet UFH is not an issue but we are not sure what to do in the original footprint. Should we fill the void? Should we insulate between the current joists and fit one of the lower profile UFH solutions?

I can find people who will design the UFH install for me but they don't seem to be able to advise the best construction choices for the floors or how we consider the full building choices for insulation etc. We also plan to include MVHR in all rooms, again we can find MVHR people but can't build a full project view of where best to spend the money.

I appreciate I have asked many different questions, I would really appreciate any input on specific questions but more so on how to take a full project view to get the best balance for the overall design.

Thanks in advance.

In the roof insulate at ceiling level and ventilate loft

For your partial fill cavity, make sure your brickies fully understand the installation process and why they have to do things in a certain way. If they get it wrong youll be stuck with a thermally leaky extension!

Following on from Peters and others ideas
Walls, single skin 150mm medium dense recycled aggregate blocks with >200mm Graphite EPS EWI , thin coat mesh render.
same EWI can be added to existing walls.
Floor , Ground bearing slab throughout, consolidated back fill, sand blinding, >200mm EPS, >100mm concrete slab with wet UFH tied to light reinforcing mesh set mid way in slab, floated to good finish ,self level thin top coat if required
Roof >350mm cold vented loft, good detailing to tie in wall and loft insulation
Windows Triple glazed 44mm units.
MHRV , system using loft space under insulation to run pipes. fan sited in utility room cupboard.
Solar PV inroof system of new roof section if possible to save on new roof finish materials.
Consider 6+kWp of panels and 8kWh batteries , plus immersion export dump
High level of air tightness is important.

With these levels of insulation and detail it may be possible to have very little heating demand.
this would allow you to save on UFH and ASHP and just rely on a cheap plugin heater on the odd cold day (depending on location and other factors, so a risk that needs to be thought through and considered)
Perhaps a wall mounted Air to Air HP might be a good back up (significantly cheaper than a wet ASHP system)
HW would then have to be via an electric immersion , 3/4 of the
year PV could cover this.

A rough plan of the current situation and of the proposed finished design would be helpful, I think.

Breathable is ok air leaky is not. Breathable me and open to water vapour not air movement

Also can often mean lower levels of insulation - I prefer highly insulated