U-factor and SHGC, and what they have to clear here

The two numbers

U-factor — heat leaving. How readily heat passes out through the whole window, frame included. Lower is better. It is the number that matters on a January night.

SHGC — sun coming in. Solar heat gain coefficient: the fraction of the sun's heat that gets through the glass. Lower means less of it. It is the number that matters in July, and it is the one people overlook because heat loss is easier to imagine than heat gain.

They pull against each other. Glass that lets in plenty of solar warmth in winter also lets it in during a Maryland August. Which way to lean is a real question and depends on which elevation the window is on.

What the code requires

Maryland has adopted a statewide energy code, and for a replacement window in this part of the state it sets:

• U-factor of 0.30 or lower
• SHGC of 0.40 or lower

A county may not weaken those numbers. Local jurisdictions can amend the building code to suit local conditions, but not in a direction that loosens the energy requirements. So the figures above are the floor everywhere in Prince George's County, whatever anyone says about local practice.

That makes them a useful thing to hold a quote against. A product that does not clear both is not a specification choice — it is below what the code allows.

Where to find the numbers

On a new window the numbers are on the NFRC label stuck to the glass, tested and certified by an accredited laboratory rather than asserted by the manufacturer. U-factor, SHGC and visible transmittance are all printed there.

Do not let the label be removed before you have read it. Once it is off, the window is just a window.

An unlabelled product is assigned a default by the code, and the default is deliberately poor. A window that arrives without certified numbers is treated as the worst case rather than given the benefit of the doubt — so the absence of a label is itself a finding.

ENERGY STAR against the code

Two different bars, often confused.

The code is the legal minimum. Clearing it is not optional.

ENERGY STAR is a program threshold, set tighter than code, and it varies by climate zone. Lake Arbor falls in the North-Central zone. A product certified for one zone is not automatically certified for another, so "ENERGY STAR rated" is worth pinning to which zone.

Neither is a warranty. They describe the product as tested, not the installation. A window that clears both numbers and is fitted badly will underperform both.

A thing that changed for 2026

The federal tax credit that covered part of the cost of efficient windows no longer applies. It ran through the end of 2025 and was terminated for property placed in service after 31 December 2025.

Much of the advice online still assumes it is available, because it was written while it was. If a credit comes up in a sales conversation, it is worth asking exactly which one is meant.

What to ask

• The U-factor and SHGC of the product being quoted, as numbers rather than as a claim.
• Whether those numbers are NFRC-certified for the whole window or for the glass alone.
• Which climate zone any ENERGY STAR certification refers to.
• Whether the same specification is going in on every elevation, or whether the south and west sides are being treated differently.

Common questions

Is triple glazing worth it here? It lowers U-factor and adds weight and cost. In this climate zone the code can be met comfortably without it, so it is a preference rather than a requirement — worth pricing both ways.

Does a lower SHGC make rooms darker? Not directly. Visible transmittance is a separate number on the same label, and the two can be traded independently.

Do the numbers apply to a patio door? Yes, doors with glass carry the same label and their own limits.