Some questions to ask.

Internal Wall Insulation and Room-in-Roof Considerations.
Internal Wall Insulation (IWI) can be highly disruptive, requiring the removal of elements such as skirting boards, window sills, coving, door frames, electrical sockets, light fixings and radiators. Ensure that your installers will reinstate all removed items and clarify whether the originals will be restored or if new replacements will be provided. Confirm that qualified professionals, such as joiners for woodwork and plumbers for wet works, will be handling these tasks—get this in writing.
Ask what system will be used and how areas that may be tricky (coving, cupboards, meters etc) will be insulated. Will all hatches in room in roof be professionally installed or reinstalled and not just boarded over. If you use storage space within room in the roof, will the installers make this accessible and suitable for storage, as they should be insulating this area if required. Will all alarms, TV, satellite, internet etc be routed through insulation and reinstated.

Ventilation will be installed, so obtain a clear specification of what will be used and ensure all new ventilation meets a high standard of finish. If your home has existing fireplaces, inquire about spillage tests and combustion ventilation to maintain safety. 

Finally, inspect external walls, damp course and rainwater systems, including gutters and drainpipes. Any existing issues must be addressed by the installers or agreed with the occupiers before work begins, as they could interfere with the insulation process and lead to future problems. Ensure all debris will be removed and clarify the timeline for this. Get all contact details in writing including office managers, installers names (may use subcontractors) and if installed under a UK grant scheme the retrofit coordinator in charge of the project.

Cavity wall insulation. Ensure that all possible areas are insulated, which may require internal drilling. Clarify in advance where the drilling will take place and what materials will be used to fill and finish the holes.

For attached, unheated garages, discuss insulation options, especially if shelves or cupboards need to be removed. If drilling into external walls, ask what colour mortar will be used to fill the holes—this is particularly important for rendered or painted properties. Will the installers repaint to match?

Additionally, confirm the type of insulating material being used and conduct your own research to understand its properties. Finally, establish whether the installers will be responsible for cleaning up any spills or debris—ensure this commitment is documented. Inform the installers of any outside livestock features (ponds etc) that may need to be secured in case of overspill.

Ventilation should be installed, so obtain a clear specification of what will be used and ensure all new ventilation meets a high standard of finish. If your home has existing fireplaces, inquire about spillage tests and combustion ventilation to maintain safety.

Finally, inspect external walls, damp course and rainwater systems, including gutters and drainpipes. Any existing issues must be addressed before work begins, as they could interfere with the insulation process and lead to future problems. Ensure all debris will be removed and clarify the timeline for this, especially on a remove and refill.

Under floor insulation: It can be difficult to assess what’s happening beneath the floor unless you have a basement. Ensure you know the type and depth of insulation being used, as well as the support method. Confirm that all possible areas will be insulated. If floorboards need to be lifted, be aware that this could cause damage—ask whether replacements will match the originals rather than being a mismatched patchwork. (take pre-install photographs)

Cross ventilation is crucial. Make sure that any ornate grilles or vents are not replaced with visually unappealing alternatives. Get written confirmation of how cross-flow ventilation will be upgraded and what units will be used, and make sure the cross flow ventilation is high enough to stop water ingress.
If a basement is to be insulated, inquire about fire regulations and conduct your own research to ensure compliance.

Ventilation should be installed, so obtain a clear specification of the system and ensure it meets a high standard of finish. If your home has fireplaces, ask about spillage tests and combustion ventilation for safety.

Lastly, inspect external walls, damp-proof courses, cross flow ventilation and rainwater systems, including gutters and drainpipes. Any existing issues must be addressed before work begins, as they could compromise the insulation process and create long-term problems. Ensure all debris will be removed and clarify the timeline for this.

Loft Insulation: Ask what products are being used, spray foam, mineral wool, PIR, etc. Make sure you understand limitations and how the insulation will affect conditions of the property (insurance, mortgage etc). Ask about storage and access to services in the loft space (boiler, solar inverters etc). If you have down lights in the ceilings, then all these need to be protected from air infiltration (caps) and possibly heat (fire caps). If you have an electric shower and cables run within loft space, you want to see proof that the cables are not buried within the insulation. All water pipework should be insulated. The loft hatch should be insulated and draft proofed, this should be done correctly and not just a piece of poorly fitted insulation stapled onto the hatch. Airflow in the loft is very important to protect timbers, so ask if they will be maintaining or improving airflow.

Ventilation should be installed, so obtain a clear specification of the system and ensure it meets a high standard of finish. If your home has fireplaces, ask about spillage tests and combustion ventilation for safety.

Gas Boilers: Most combination boilers require a larger diameter gas feed, which may need to be upgraded from the meter. Before installation, confirm the gas pipe route to avoid unnecessary external runs when an internal route is possible. If runs outside, it should be clipped and routed to regulations. Discuss heating controls and the thermostat placement—it should be in a room without a radiator thermostat (TRV) or secondary heat source (fire, cooker etc).
All condensing boilers require a drainage point for condensation, preferably indoors. If positioned outside, correct insulation (class 0) is necessary. Inquire about the type of lagging used, as well as how it will be supported and secured.

Ensure all debris will be removed and clarify the timeline for this. Most manufacturers require a system flush before installation—this is crucial—so verify that it’s done and observe the process. The use of chemicals is essential to maintain system health, and their application should be recorded in the benchmark (boiler installation and service history), with a copy provided for your records. Ask about warranty length and installation guarantee in case of leaks and workmanship issues. Always ring the boiler manufacturer post install (after a few weeks) to give peace of mind that a warranty is in place.

Solar. Coming soon.

Heat Pumps. Coming soon.

Get Everything in Writing: Once you’ve decided an installation company, make sure to get all agreements, including costs, timelines, and scope of work, in writing. This helps protect both parties and ensures clarity and accountability throughout the project.,