Furbnow - Make your home cosy without the carbon footprint

Your home, your priorities

Where things stand

Your home is a 1930s semi-detached house in Harrogate, built with solid brick walls (225mm) and a timber ground floor over a ventilated void. The loft has some insulation but only around 100mm — below current standards, with gaps at the eaves. The property is heated by a gas combi boiler installed in 2018. Total floor area is approximately 120 m².

The main issues you have raised are high energy bills, cold rooms during winter — particularly the ground floor and north-facing bedrooms — and draughts from the original windows and front door. There is no mechanical ventilation; the building relies on natural infiltration through gaps in the fabric, which contributes to the heat loss.

The priority for this phase is the thermal envelope: internal wall insulation, window and door replacement, and loft top-up. As the envelope tightens, mechanical ventilation with heat recovery (MVHR) is needed to maintain air quality and manage condensation risk. These four measures are interdependent and must be sequenced correctly — this Build Pack sets out that sequence.

Property type

1930s semi-detached

Floor area

~120 m²

Wall construction

Solid brick, 225mm

Current heating

Gas combi boiler (2018) — to be replaced

Current EPC

E (42)

Assessment type

Build Pack

Property floor plans

Survey floor plans

Floor plans recorded at survey, alongside annotated versions showing the proposed measure positions.

Survey floor plan

Annotated survey plan

The whole job

Measures at a glance

Every measure in one view, grouped by phase.

Measure	Area / size / quantity	Specification	Target start
Windows & Doors	—	Minimum whole-window U-value of 1.4 W/m²K. Triple glazing recommended for north-facing elevations. All frames to achieve at least Window Energy Rating Band B. Product selection confirmed by the appointed installer to these performance standards.	—
Internal Wall Insulation	—	Minimum U-value of 0.30 W/m²K post-installation. BBA-certified system required. All penetrations sealed to maintain airtightness. Junctions designed to avoid thermal bridging. We specify either the <strong>IWI Pro mineral wool system</strong> or the <strong>K Systems M mineral wool system</strong> (BBA-certified, A1-rated). System selection confirmed at specification stage.	—
Loft Insulation Top-Up	—	Minimum total depth of 300mm. We specify <strong>Knauf Earthwool Loft Roll 44</strong> (glass mineral wool, 0.044 W/mK, Euroclass A1, BS EN 13162). Second layer laid perpendicular to joists to eliminate thermal bridging. Eaves vents maintained where required for roof ventilation.	—
MVHR — Mechanical Ventilation with Heat Recovery	—	System designed by an independent MVHR specialist before the installer is appointed. Minimum 85% heat recovery efficiency. Units specified from <strong>Zehnder, Paul, or Brink</strong>; ductwork <strong>Lindab</strong>. Semi-rigid or rigid duct throughout; no flexible duct run longer than 1.5m.	—
Air Source Heat Pump	—	Monobloc system from <strong>Vaillant, Mitsubishi Ecodan, or Daikin Altherma</strong>. EN 14825 SCOP ≥ 3.5 at flow temperature 45°C against the post-retrofit design heat load. R290 refrigerant preferred where available. Cylinder minimum 180 L with ErP class B or better. Weather compensation enabled at commissioning.	—
Solar PV & Battery	—	Modules: tier-1 monocrystalline, <strong>≥ 410 W per panel, 25-year product warranty</strong>. Inverter and optimisers: <strong>SolarEdge</strong> per the attached system design. Battery: minimum 10-year warranty, ≥ 6,000 full cycles. Roof fixings rated to local wind load; no fixings through any insulation layer added under the IWI or loft works.	—

Specification by measure

Measures in detail

Each measure is specified on its own — dimensions, target performance, and what we will and will not accept.

Windows detail — elevations and schedule for the replacement units.

Doors — elevations and schedule for the replacement door sets.

What is included

Full window replacement throughout
Front and rear door replacement with insulated door sets
All frames installed before IWI commences

Furbnow specification

Minimum whole-window U-value of 1.4 W/m²K. Triple glazing recommended for north-facing elevations. All frames to achieve at least Window Energy Rating Band B. Product selection confirmed by the appointed installer to these performance standards.

We will specify

Minimum U-value 1.4 W/m²K whole window, triple glazing on north elevations
Frames installed before IWI so insulation dresses around the outer frame

We will not accept

Windows installed after IWI — this creates a cold bridge at the reveal
Trickle vents specified as the primary ventilation strategy where MVHR is planned

Installer requirements

FENSA or CERTASS registered installer required for all glazing replacement. This is a legal requirement for replacing windows in England and Wales. The installer will issue a certificate of compliance on completion.

Annotated plan — wall insulation positions called out across the ground floor, with junction notes and key dimensions.

What is included

Internal mineral wool insulation board, 120mm minimum
Reinforced base coat and plaster finish
All window reveals, sills, and eaves detailing

Furbnow specification

Minimum U-value of 0.30 W/m²K post-installation. BBA-certified system required. All penetrations sealed to maintain airtightness. Junctions designed to avoid thermal bridging. We specify either the IWI Pro mineral wool system or the K Systems M mineral wool system (BBA-certified, A1-rated). System selection confirmed at specification stage.

We will specify

BBA-certified mineral wool board system (IWI Pro or K Systems M)
Continuous insulation at all junctions including around window openings

We will not accept

EPS as the primary insulation layer where mineral wool is achievable
Bridged or discontinuous insulation at eaves or window reveals

Installer requirements

Installer must hold relevant accreditation for internally insulated systems and carry appropriate liability insurance. System manufacturer approval and training certification required.

Loft top-up — 300mm mineral wool laid between and over joists, with eaves ventilation maintained and access walkway preserved.

What is included

Top-up from 100mm to 300mm total depth
Eaves sealing to prevent cold air infiltration
Tank and pipework lagging in the loft if present

Furbnow specification

Minimum total depth of 300mm. We specify Knauf Earthwool Loft Roll 44 (glass mineral wool, 0.044 W/mK, Euroclass A1, BS EN 13162). Second layer laid perpendicular to joists to eliminate thermal bridging. Eaves vents maintained where required for roof ventilation.

We will specify

300mm total depth, second layer perpendicular to joists
Eaves sealed against cold air infiltration

We will not accept

Top-up layer running parallel to existing without a perpendicular cross-layer
Blocking of roof ventilation vents

Installer requirements

Installer must be registered with the relevant accreditation schemes. Our vetted specialist installers carry appropriate insurance and references for this type of work.

What is included

MVHR unit, ductwork, and installation throughout
Independent specialist design (duct layout, airflow calculations)
Supply to all habitable rooms; extract from kitchen, bathrooms, and utility

Furbnow specification

System designed by an independent MVHR specialist before the installer is appointed. Minimum 85% heat recovery efficiency. Units specified from Zehnder, Paul, or Brink; ductwork Lindab. Semi-rigid or rigid duct throughout; no flexible duct run longer than 1.5m.

We will specify

Independent specialist design commissioned before installer tendering
Units: Zehnder, Paul, or Brink; ductwork: Lindab

We will not accept

Installer designing their own system without independent review
Flexible duct runs exceeding 1.5m

Installer requirements

BPEC-accredited installer required. The MVHR system design must be produced by an independent specialist before the installer is appointed. This is not the installer's responsibility to design.

What is included

MCS heat-pump design package against post-retrofit heat load
Monobloc outdoor unit, indoor hydraulic kit, and hot water cylinder sized to household demand
Emitter survey and upgrade of undersized radiators for low-flow operation
Commissioning, weather compensation setup, and BUS grant administration

Furbnow specification

Monobloc system from Vaillant, Mitsubishi Ecodan, or Daikin Altherma. EN 14825 SCOP ≥ 3.5 at flow temperature 45°C against the post-retrofit design heat load. R290 refrigerant preferred where available. Cylinder minimum 180 L with ErP class B or better. Weather compensation enabled at commissioning.

We will specify

Flow temperature ≤ 45°C at design outdoor temperature; emitters upsized where needed to achieve this
Heat-loss recalculation against the post-IWI fabric, not the existing baseline
Gas supply capped at meter once commissioning is complete

We will not accept

Hybrid bivalent systems that retain the gas boiler as supplementary heat
Sizing taken from a rule-of-thumb floor-area calculation rather than a full room-by-room MCS heat-loss survey
Flow temperatures above 50°C at design outdoor temperature

Installer requirements

MCS-certified heat pump installer required to release the Boiler Upgrade Scheme grant (currently £7,500 toward the install). The installer must produce a full MCS-compliant design package; this is not a quote-from-photos job. The Heat Loss Estimator Report is provided as background only — the formal MCS heat-loss calculation is part of the installer's design scope.

What is included

~4 kWp panel array on south- and east-facing roof pitches per attached design
SolarEdge inverter with per-module optimisers
~5 kWh AC-coupled battery storage, EV-ready
DNO G98/G99 notification, MCS certificate, and SEG registration

Furbnow specification

Modules: tier-1 monocrystalline, ≥ 410 W per panel, 25-year product warranty. Inverter and optimisers: SolarEdge per the attached system design. Battery: minimum 10-year warranty, ≥ 6,000 full cycles. Roof fixings rated to local wind load; no fixings through any insulation layer added under the IWI or loft works.

We will specify

Per-module optimisers (not a plain string inverter) to manage the morning shading on the east pitch
Battery sized against the post-ASHP evening load, not generic household averages
Installer handles all DNO and SEG paperwork end-to-end

We will not accept

String inverters without per-panel optimisation, given partial shading on this roof
Fixings penetrating IWI or new loft insulation without proper detailing
Batteries with warranties under 10 years or below 6,000 cycles

Installer requirements

MCS-certified PV installer required for both the array and battery to qualify for 0% VAT and SEG export payments. The installer handles G98/G99 notification with the Distribution Network Operator before commissioning. Confirmation of available grid capacity is required given the ASHP load also added under the heat-pump measure.

Recommended sequence

Sequencing & timescales

These measures are sequenced to work together. Windows and doors are installed first so the IWI system can be dressed around the frames correctly, bringing them within the insulation layer. The loft top-up completes the envelope. MVHR follows once airtightness is confirmed. Heating is then switched from the gas combi to an air source heat pump, and a roof-mounted solar PV array with battery storage is installed last.

1

Windows & Doors

Installed first, before IWI, so the external insulation system can be dressed around the frames and the window reveals brought within the insulation layer. The correct construction sequence for a solid-wall IWI project.

Before IWI · ~£14,000-£19,000

2

Internal Wall Insulation

Applied after windows are in, so the system wraps correctly around the frames. Addresses the largest source of heat loss in this property — solid brick walls with no cavity offer no existing insulation value.

Fabric first · ~£12,000-£16,000

3

Loft Insulation Top-Up

Topping up from 100mm to 300mm completes the thermal envelope. Can be carried out alongside IWI. Quick and low-cost for the thermal benefit it delivers.

Completes the envelope · ~£700-£1,000

4

MVHR

Once the envelope is sealed, airtightness must be tested before MVHR is designed and installed. An independent specialist produces the duct layout and airflow calculations from the confirmed test result.

After airtightness test · ~£11,550

5

Air Source Heat Pump

With the fabric envelope complete, the property's heat demand is recalculated and the gas combi is replaced with an MCS-designed air source heat pump. Emitters are upsized where needed for low-flow operation, and a hot water cylinder is installed.

After fabric works · ~£10,000-£14,000 (before BUS grant)

6

Solar PV & Battery

A ~4 kWp roof-mounted PV array with per-module optimisers and a ~5 kWh battery is added last so scaffolding, loft top-up, and roof penetrations are coordinated. Sized against the post-ASHP electrical load profile.

Final stage · ~£10,000-£13,000

Sequencing of works

How this comes together

Indicative durations based on similar projects. Works can be phased to suit your schedule and budget.

Stage 1

Windows & Doors

Installed before IWI commences. Lead time of 6-10 weeks from order. Full-house replacement typically over 2-4 days depending on number of openings and access.

2-4 days

Stage 2

Internal Wall Insulation

Scaffold required. Site needs to be clear of obstructions at the external perimeter. No internal access required during main works. IWI dressed around the installed window frames.

2-3 weeks

Stage 3

Loft Insulation Top-Up

Can be carried out alongside IWI. Loft access required; typically completed in one day.

1 day

Stage 4

MVHR

Designed following the airtightness re-test result. Ductwork installation typically 3-4 days. Some ceiling void access required in rooms receiving supply ducts.

3-4 days

Stage 5

Air Source Heat Pump

Post-retrofit MCS heat-loss design (1-2 weeks) followed by install. Outdoor unit, cylinder, emitter upgrades and commissioning typically 2-3 days. Gas supply capped at the meter on completion.

2-3 days

Stage 6

Solar PV & Battery

Array, inverter, and battery install typically 1-2 days. DNO G98/G99 paperwork submitted by the installer; commissioning once approval is received.

1-2 days

Estimated project costs

What to budget

These are indicative estimates based on our experience of similar projects and current market data. We will firm up all costs with our vetted specialist installers before you commit to anything — these figures are here to help you plan and prioritise, not to replace a formal quotation.

£14,000-£19,000

Windows & Doors
~11 windows x £1,100 + 2 doors x £1,650 (indicative)

£12,000-£16,000

Internal Wall Insulation
~120m² wall area x £110/m² mineral wool system

£700-£1,000

Loft Insulation Top-Up
~50m² x £23/m² top-up rate (100mm to 300mm)

~£11,550

MVHR — Mechanical Ventilation with Heat Recovery
Small property rate — unit, ductwork, and installation

£10,000-£14,000

Air Source Heat Pump
MCS-designed monobloc system, cylinder and emitter upgrades; before £7,500 BUS grant

£10,000-£13,000

Solar PV & Battery
~4 kWp array with SolarEdge optimisers and ~5 kWh battery, fully installed

Estimated total project cost£58,000-£75,000

VAT: Most energy efficiency measures are 0% rated for VAT under HMRC VAT Notice 708/6. Where any element of work falls outside the relief — for example, certain structural or decorative works — this will be clearly identified in formal quotations. We do not apply a blanket 20% assumption.

BUS grant: The £7,500 Boiler Upgrade Scheme grant toward the ASHP is not netted off the figures above — the installer applies on your behalf and the grant is paid directly to them, reducing your bill at the point of install.

Outcomes after all measures

The difference this makes

Projections are based on current energy tariffs and SAP/RdSAP modelling. Actual savings depend on occupancy and how the heating is used.

Energy savings

~£1,900/yr

estimated annual saving

Heating & hot water (gas displaced)~£1,300
PV self-consumption & SEG export~£600
Solar generation~3,600 kWh/yr
CO₂ reduction~4.3 tonnes/yr

EPC improvement

E → A

from 42 to approximately 92 points

Current EPCE (42)
Projected EPCA (92)

Savings projections are indicative. Final figures will be confirmed once formal quotations are received and a full SAP calculation is completed. Energy prices used: October 2024 Ofgem price cap (gas 5.48p/kWh, electricity 24.50p/kWh). Once the ASHP is in, heating costs sit on the electricity tariff only; the gas standing charge falls away.

What still needs confirming

A few things to verify

These items need further investigation before specifications and quotations are finalised. Tick each one off as it is resolved. None block you from moving forward.

1
External wall area — measured survey required
The IWI cost estimate is based on an approximate wall area. A surveyor will measure the property precisely before quotations are issued. Gables, bay windows, and any existing attached structures will affect the final area.
Action: Furbnow to arrange at survey
2
Planning permission — IWI on a semi-detached property
IWI on a semi-detached house in England does not typically require planning permission under permitted development, but the local authority should be consulted. Conservation area status and any Article 4 directions need to be confirmed for this address.
Action: Furbnow to check with North Yorkshire Council prior to specification
3
Current airtightness — baseline test recommended
No airtightness test result is available for this property. A baseline test before works begin is recommended to confirm the current infiltration rate and inform MVHR sizing. A re-test after windows and IWI are complete is required before MVHR is designed.
Action: Furbnow to arrange baseline blower door test
4
Loft access and obstructions
Access hatch size and any obstructions (tanks, pipework, stored items) need confirming at survey. If the hatch needs enlarging, this will be noted in the quotation.
Action: Confirmed at survey
5
Heat-loss recalculation against post-IWI fabric
The attached Heat Loss Estimator Report is a pre-retrofit baseline (external walls 1.64 W/m²K). The MCS heat pump design must be redone against the post-IWI U-values once the fabric specification is firm, so the ASHP is not oversized.
Action: MCS installer to produce design after IWI specification is signed off
6
DNO capacity for ASHP plus PV
Combined ASHP load and PV export need Distribution Network Operator confirmation. Notification under G98 or G99 (depending on system size) is handled by the PV installer, but capacity should be confirmed before formal quotations.
Action: PV installer to confirm with Northern Powergrid

Supporting documents

Design files & technical documents

Available now

Expected in due course

MVHR Feasibility ConfirmationWritten confirmation from MVHR specialist — required before windows are ordered.
Pending
MVHR System DesignIndependent duct layout, airflow calculations and equipment schedule.
Pending
ASHP MCS Design PackageFull MCS heat-loss recalculation, system schematic and emitter schedule, produced once IWI U-values are confirmed.
Pending
Boiler Upgrade Scheme application£7,500 BUS grant application submitted by the MCS installer on your behalf.
Pending
DNO G98/G99 confirmationConfirmation from Northern Powergrid that the property has capacity for the proposed ASHP load and PV export.
Pending
Solar PV MCS & SEG certificatesMCS certificate, commissioning report, and SEG export registration for the PV array and battery.
Pending
Post-Installation EPCUpdated EPC to be lodged once all measures are complete.
Pending

What happens now

Ready to move forward?

Here is what to expect from here. No surprises, no pressure — you stay in control at every stage.

How it works

A clear process from Build Pack acceptance through to installation.

Confirm and accept this Build Pack
We arrange any initial surveys required
We finalise quotations and designs with our system designers and contractors
Some quotations require a site visit from our contractors — they will be in touch directly to arrange
We present the final prices for your review
Our vetted specialist installers carry out the works, following this Build Pack throughout

Questions?

Not ready to commit yet, or want to talk something through first?

Questions about any of the measures or the sequencing
Want to prioritise one or two measures first
Not sure about timing or budget
Want to book a call with your Furbnow contact

Email hello@furbnow.com

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