Fog that stays between the panes of a sealed double-glazed window usually points to a failed glazing seal. But that does not automatically mean you need a whole new window. First establish which surface is wet. Then ask whether your particular window can accept a replacement insulated glass unit, needs a replacement sash, or has damage that justifies replacing the frame too.

This guide helps you organise those decisions before requesting quotes. It covers sealed double glazing, not the separate gap between an original window and an added storm window or secondary glazing panel.

1. Is the fog inside, outside or between the glass?

The word “inside” causes confusion. It can mean the surface facing your room or the inaccessible space inside a sealed glass unit. These are different problems.

Pella distinguishes these three locations: room-side condensation can occur when humid indoor air meets cold glass, exterior dew can be weather-related, and between-pane condensation is primarily associated with seal failure. Exterior dew alone is not evidence that you need a replacement. Source: Pella’s condensation guide.

A simple observation routine: photograph the whole window and a close-up, gently wipe a small room-side patch with a clean cloth, and inspect the outdoor face only if you can reach it safely from the ground. Never lean out of a window to complete this check. If the location remains uncertain, send the photographs to the installer instead of guessing.

Repeat your photographs when the appearance changes. Label each with the time, room and whether the glass was wiped. Record what you actually observed: “room-side surface wiped; haze remained” is more useful than “window definitely broken.” A photograph cannot establish every hidden defect.

2. What is the sealed glass unit?

An insulated glass unit (IGU) is the glass assembly: two or more panes separated by a spacer and sealed around their edges. The surrounding sash holds that assembly; the frame is the structure fitted into the building opening. Understanding those three components makes a quotation much easier to read.

The US Department of Energy describes insulated glazing as spaced, sealed panes with an insulating gap. Some designs use gas fills, low-emissivity coatings and specialised spacers. The replacement specification therefore involves more than matching the visible width and height. Source: DOE, Window Types and Technologies.

Ask the supplier to identify the existing assembly before proposing an upgrade. “Double glazing” is too broad a specification for a purchase order. Keep the product label, original invoice and any available installation paperwork together so that each bidder works from the same information.

3. Compare glass-only, sash and full-window replacement

The following is a decision framework, not a promise that every option is available for your window. Ask each bidder to explain why its proposed scope suits the actual product.

Product design matters. Andersen’s support guidance says many of its windows require sash or panel replacement rather than separate glass replacement; it identifies exceptions for some older direct-set picture windows. That is a manufacturer-specific example, not a rule for every brand. Source: Andersen’s glass replacement guidance.

Request two clearly scoped alternatives where feasible. A broader quote may include work missing from the smaller one, so compare the inclusions before comparing the totals. If only one scope is offered, ask for the product-specific reason in writing.

4. Does defogging repair a failed seal?

InterNACHI, a home-inspection association, distinguishes removing visible moisture from repairing the failed seal. Its guidance warns that a defogged window may look clearer without recovering its insulating performance. It also describes glazing-assembly replacement as a possible repair route. Source: InterNACHI’s double-pane condensation guidance.

If a company offers defogging, separate the promises into three questions: will it improve appearance, will it restore a sealed assembly, and what performance is guaranteed? Ask for the method, warranty and exclusions before agreeing. “Clear glass” and “restored insulation” should not be treated as interchangeable descriptions.

Avoid experimenting with drilling, heating or dismantling the glass while investigating a claim. First let the original supplier explain its inspection process. Keeping the unit unaltered also preserves the evidence you want the supplier to examine.

5. A checklist to send with every quotation request

The most useful comparison starts with identical questions. Copy this list into your enquiry and ask bidders to respond against each item:

1. Identification: manufacturer, product series, approximate installation date and any readable label or spacer marking.
2. Diagnosis: where the moisture is located, what was inspected, and any uncertainty that needs an on-site visit.
3. Scope: glass assembly, sash or complete window; list retained and replaced parts.
4. Specification: pane construction, overall unit thickness, coating, gas fill if specified, spacer and appearance.
5. Compatibility: who takes final measurements and confirms that the replacement suits the existing product?
6. Performance: what documented ratings apply to the proposed product, and are they for the glass or the whole window?
7. Installation: access arrangements, removal, protection of the room, waste disposal and making good.
8. Commercial terms: itemised total, taxes, deposit, delivery estimate and conditions that could change the price.
9. Warranty: separate coverage for the replacement component and installation labour; explain exclusions and the claims process.
10. Handover: invoice, product specification, care instructions and a contact for problems after installation.

For products carrying an NFRC label, the council’s Certified Products Directory provides a way to verify label data. It is not a substitute for checking compatibility with your window.

If energy performance is part of the proposal, remember that a lower U-factor means better insulation, while a lower solar heat gain coefficient (SHGC) admits less solar heat. The appropriate combination depends on climate and design; do not simply choose the lowest number in every column. Source: DOE’s guide to energy-efficient windows.

6. A worked example: compare scopes before prices

This is a hypothetical decision example, not a real inspection or a quotation. Imagine a homeowner with one hazy bedroom window. They have photographs and a product label, but no confirmed diagnosis. Installer A proposes new glass. Installer B proposes a replacement sash. Installer C proposes a complete window.

The homeowner first asks all three to identify the product and explain the proposed scope. Suppose A has not checked whether separate glass replacement is supported, B supplies a matching component reference, and C gives no reason for removing the existing frame. At this stage the cheapest or most extensive proposal is not automatically the best supported one.

The next step is to ask A for compatibility confirmation and C for an explanation of the frame work. Then compare the corrected proposals: what is replaced, what is retained, what is warranted, and what remains uncertain? B may still be the appropriate route, but the decision now rests on evidence rather than the phrase “new window.”

Use a short decision record: “I chose this scope because ___; the supplier confirmed compatibility by ___; the warranty covers ___; the remaining exclusions are ___.” If you cannot complete those blanks, ask another question before paying a deposit.

7. Check the warranty before commissioning work

Do not assume that a remembered warranty period settles the claim. Retrieve the document for the particular product and purchase, then ask the supplier to confirm what applies.

Send your invoice or installation record, product identification and dated photographs. Ask whether inspection is required, whether the proposed remedy includes labour and access, and whether work by another contractor would affect the claim. Keep the reply alongside the quote.

For tenants or apartment residents, start with the landlord or building manager before commissioning alterations. Identify who is responsible for arranging the inspection and authorising the work. This is an administrative step, not a statement about tenancy law in any particular country.

Common questions

Can a dehumidifier clear fog between sealed panes?

Indoor humidity control addresses room-side conditions; it does not directly repair a compromised sealed glass assembly. Confirm the moisture location before buying equipment. Pella’s guide addresses humidity management separately from between-pane seal failure.

Must every foggy window be replaced completely?

No. The available scope depends on the product and its condition. Ask specifically about glass-assembly and sash replacement before accepting a full-window proposal.

What if the fog disappears later?

Keep the photographs and describe the pattern to the supplier. A clear moment is not a complete assessment of the assembly. InterNACHI notes that failed units are not necessarily visibly foggy under every condition.

How much will replacement cost in India or elsewhere?

This guide does not give a universal price. Request local, itemised quotes for the same specification and scope. Record the quote date, currency, taxes and access costs so that a comparison remains meaningful.

What should I check at handover?

Match the invoice and product documents to the agreed scope. Review appearance and operation with the installer, record any outstanding work, and keep the warranty contact. Ask how to report a problem rather than relying on a verbal assurance that “everything is covered.”

Research notes

Prepared by Rvasus on 9 October 2026 using the primary manufacturer, government and professional-association guidance linked above. The comparison framework, enquiry checklist and worked example are editorial synthesis. No window inspection, product testing or installer price survey was performed for this article. The image is an original AI-generated illustration, not evidence of a diagnosed defect. Manufacturers cited are sources, not endorsements.

For corrections or questions, contact Rvasus. Read our editorial policy for the site's approach to sources and corrections.