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FAQs

What is a “Deep Retrofit” (and how is it different from a standard insulation retrofit)?

A Deep Retrofit is a planned, whole-home upgrade that targets the entire thermal envelope and key comfort systems, so you get a step-change in warmth, dryness and running costs (not just a small improvement from topping up one area).

What typically gets included in a Deep Retrofit package?

Common elements include high-quality insulation upgrades (ceiling, underfloor and - where feasible, walls), draft/air-leakage reduction, improved windows/doors (or targeted upgrades), and a ventilation approach that keeps the home healthy as the envelope improves. The exact mix depends on the home’s construction, moisture risk and renovation scope.

Why do Deep Retrofits talk so much about airtightness and ventilation?

Because once you reduce air leakage and add insulation, you can change how the building keeps dry. Done well, airtightness improves comfort and efficiency - but it must be balanced with appropriate ventilation to control moisture and indoor air quality. (This is one reason “quality of installation” matters as much as product choice.)

When is a Deep Retrofit worth considering instead of piecemeal upgrades?

Deep Retrofit becomes especially relevant when you’re already planning a major renovation, or when a home has persistent comfort/health issues (cold rooms, condensation, dampness) and you want a coordinated solution rather than repeating smaller jobs. NZ research continues to show a large portion of the housing stock still needs meaningful retrofit improvement.

What consents or compliance should I expect for Deep Retrofit work?

Some deep retrofit measures are straightforward, but retrofitting insulation into external walls typically requires building consent (unless your council confirms otherwise). If your project is an alteration/addition, the new work must comply with Building Code, while the rest of the building generally doesn’t have to be upgraded to current H1 unless it’s part of the scope.

Is overheating caused by ‘too much insulation’ in new homes and apartments?

No.

In NZ modelling done for the H1 changes, increased insulation generally reduces daytime overheating risk because it can reduce heat flow and (in some cases) the impact of solar gain through the building envelope.

The real issue is usually too much heat getting in (sun + glazing) and not enough heat getting out (poor ventilation/airflow).

Insulation can slow overnight cool-down, which is why a ventilation strategy matters - but it’s not the root cause.

How do I stage a Deep Retrofit without creating moisture or condensation problems?

Sequence matters. A practical “do no harm” order is:

  1. Fix bulk water first (roof leaks, flashing, cladding defects, plumbing leaks).

  2. Reduce ground/subfloor moisture (often with a ground vapour barrier where appropriate) before adding/repairing underfloor insulation.

  3. Plan ventilation as you tighten the home—better insulation + less leakage can increase indoor humidity unless ventilation is improved (especially kitchens/bathrooms).

  4. Install insulation to a high standard (no gaps, no compression, correct clearances/support) and follow NZ guidance for tricky roof types (e.g., maintaining required clearances to avoid moisture issues).

  5. Treat wall insulation as a higher-risk, consented step—it can change drying rates and increase moisture accumulation risk if the wall system isn’t suitable or the method/materials are wrong, so it must be assessed case-by-case.

If it’s not insulation, what usually causes overheating?

The biggest drivers are typically:

  • High solar gain from large and/or poorly shaded windows 

  • Modern construction general airtightness (which is a good thing when managed correctly) with insufficient natural or mechanical ventilation to purge heat

  • Apartment constraints (single-aspect units, limited opening windows, no crossflow)

  • Design choices (glazing ratios, shading, orientation, low thermal mass, dark external colours in some cases)

NZ guidance notes overheating in many new homes is linked to poor ventilation, larger windows, poor shading, and a warming climate

Don’t bigger windows just mean more ‘light’ - why do buildings overheat?

Large glazing can act like a heat collector. If sun gets through glass and lands on internal surfaces, it becomes heat that’s harder to remove - especially in still air.

The most effective control is stopping sun before it enters the building with external shading (eaves, awnings, louvers, shutters), plus smart glazing selection where appropriate.

(It's not the insulation causing the issue!)

What’s the role of ventilation - what does ‘better airflow’ actually mean?

Better airflow means moving heat out of the building on purpose, not just “having a window somewhere.”

Good strategies include:

  • Cross-ventilation (openings on opposite sides) or stack ventilation (high + low openings)

  • Night purge ventilation (flush heat out overnight when it’s cooler)

  • For apartments: maximising openable area, using secure night-vent positions, and considering mechanical ventilation approaches that can increase fresh air without adding heat

Research and modelling show (lack of) ventilation rate is one of the biggest influences on indoor temperature - often more than insulation levels

How do we fix overheating in practice—without blaming insulation?

Treat it like a design + operation checklist:

  1. Control solar gain first: external shading for west/north-west glazing; review glazing size and specification.

  2. Plan for heat purge: crossflow, high/low vents, stair/atrium stack paths (where safe/appropriate).

  3. Enable safe summer ventilation: secure night-venting options; for apartments, consider mechanical solutions where natural crossflow isn’t possible.

  4. Use insulation correctly: keep it -because it improves winter comfort and can reduce daytime heat transfer; just pair it with a cooling/vent plan so the building can dump heat when needed.

  5. Set expectations: NZ guidance notes there are currently no explicit Building Code requirements that guarantee protection against overheating, so designers need to actively manage overheating risk in the design stage.

Where should I insulate first in an existing home?

Start with the biggest, easiest wins: ceiling/roof space, then underfloor, then walls (walls are typically the hardest/most invasive). Ceiling and underfloor retrofits are usually the most cost-effective comfort upgrade in older NZ homes.

Do I need building consent to retrofit insulation?

Generally no consent is required for adding insulation to ceilings or floors in existing homes, but retrofitting insulation into external walls typically requires building consent (unless your council confirms an exemption).

What R-value should I choose for my retrofit?

Treat minimum R-values as the baseline, not the goal. Aim for the best R-value that fits your home’s constraints (space, access, moisture risk, existing services). Also note: the way insulation is installed (no gaps, no compression, correct support) can matter as much as the label on the bag.  Your recommended IAONZ Installer can provide options that best suit your house and budget.

Can insulation help with damp, mould and condensation?

Insulation forms part of the solution by limiting the energy loss - but only when paired with good moisture management: stop subfloor moisture, improve ventilation where needed, and avoid trapping moisture in the building envelope. Poor drying/ventilation can allow moisture to build up in walls and create conditions for fungal growth.

Should I install an underfloor ground vapour barrier (polythene)?

Often yes for suspended timber floors: it helps reduce moisture vapour rising from the ground into the subfloor space, which supports a drier, healthier home and protects insulation performance. NZ installation guidance is covered in NZS 4246.  IAONZ Installers are trained to install to these requirements.

What does NZ Building Code H1 mean for insulation in a new build?

H1 sets energy-efficiency performance requirements. Compliance may be via calculation or modelling pathways, and it uses construction R-values (the whole assembly), not just the insulation product R-value.  The H1 calculations are prepared by your architect or designer and form part of the NZBC requirements for consent to be granted.  The Insulation R-Value forms part of that calculation. 

What’s the difference between “insulation R-value” and “construction R-value”?

Insulation R-value is the insulation product’s thermal resistance. The higher the R-Value the higher resistance it has meaning it provides better heat loss performance.

Construction R-value is the real-world performance of the entire roof/wall/floor assembly, including framing and linings—this is what H1 calculations require.

How do I know the insulation has been installed properly (quality checklist)?

Ask your installer to confirm installation aligns with NZS 4246 guidance and request evidence/photos where access is limited.

Key quality checks include:

  • Correct thickness (not compressed) and full coverage with no gaps

  • Secure fixing/support (especially underfloor)

  • Safe clearances around heat sources and services (e.g., downlights/vents)

  • Dry materials only—replace any damp or degraded insulation
    NZS 4246 is the NZ reference used for bulk insulation installation quality.

How do climate zones affect what I need to install?

Minimum performance requirements vary by NZ climate zone and by building element (roof, walls, floor, windows/doors). Your designer/specifier should select a compliant system for your zone and build type.

How do I avoid “thermal bridging” and cold spots in a new build?

Thermal bridging (heat loss through timber/steel framing and junctions) can reduce real performance. Common solutions include improved detailing, and in some designs, secondary insulation layers that materially lift wall/roof performance when done correctly.

What documentation should I ask for (quality assurance)?

For high-quality outcomes, request:

  • Product data sheets (R-values, intended uses, limitations) - Select insulation from supporting IAONZ Manufacturers

  • Installation statement confirming alignment with NZS 4246

  • Evidence the design uses construction R-values suitable for H1 (often via BRANZ guidance/tools)
    This makes it easier to prove compliance and protect performance long-term.

Why should I choose an IAONZ Member instead of “any installer” or the cheapest quote?

Insulation performance is won or lost at installation. IAONZ Members commit to quality-first workmanship, correct material selection for the application, and install practices that protect the insulation’s stated R-value (no gaps, no compression, correct support and detailing).

The outcome is a warmer, drier home - and far lower risk of costly rework.

What proof do I get that the job was done properly?

IAONZ Members can provide clear documentation: the products used, where they were installed, and quality checks (often including photos where access is limited).

They also understand compliance expectations for NZ Building Code H1 (new builds/major renovations) and good-practice installation guidance such as NZS 4246—so you have confidence the work will stand up to scrutiny, resale, and time.