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Why glass breaks in a wildfire
Glass cracks when the exposed face heats and expands faster than the edge hidden in the frame. PNNL describes that edge-to-center stress. Dual-pane buys a second lite after the outer pane fails. Tempered resists longer than annealed. That mechanism is not Oregon’s R327 glazing menu and not a 20-minute fire door.
You are looking at a cracked outer lite still in the gasket, and an inner lite that is intact. Embers have not come through yet. The frame shaded the edge. The field of the pane ran hot. That is the break.
DET-019 is the glazing menu under R327.3.6. This page is why the lite fails. Do not treat the menu drawing as a stress diagram.
Why it matters
An open hole is an ember door. P-737 Fact Sheet 10 says failed glazing lets embers, hot gases, and radiant heat inside. People buy dual-pane for energy code and think they bought California tempered. People buy tempered and leave the window open. The mechanism is thermal stress at the edge. The code path is a different page. See dual-pane versus tempered.
How the documents split
| Object | What it is | What it is not |
|---|---|---|
| Edge thermal stress (PNNL / UCCE) | Center of surface 1 expands. Frame-shaded edge does not. Cracks start at edge flaws. | A listing |
| Single pane (P-737) | Typical residential glass can fail within 5 minutes (Slack, 2000) | Dual-pane |
| Dual-pane / IGU (P-737) | About 10 minutes for annealed, twice single-pane. Second pane still has to fail. | Unbreakable glass |
| Tempered (PNNL citing Quarles 2010) | More than four times better than single-pane annealed in that citation | Dual-pane |
| R327.3.6 | Tempered or multilayered or glass block or 20-minute rating | CWUIC’s usual both |
The paths
1. Close the window. An open sash is not this mechanism. It is a hole. PNNL tells people to close operable windows before they leave if there is time.
2. Dual-pane is a second lite. FEMA. If the outer pane goes, the inner pane is still a barrier. Oregon R327 treats multilayered glazed panels as a complete option. California CWUIC wants at least one tempered pane in the usual dual-pane path.
3. Tempered is heat-treated glass. Look for the etched bug in the corner. PNNL. It is not laminated hurricane glass, and it is not a fire-rated ceramic pane. P-737 does not recommend annealed glass in wildfire zones unless shutters cover it.
4. Frames can drop the glass first. P-737. Vinyl and wood frames can melt or burn. PNNL notes early vinyl tests where an unreinforced meeting rail deformed and dropped the lite. Metal or metal-clad, or vinyl with the internal reinforcement bar, is the frame conversation. It is not the lite conversation.
5. Vegetation against the glass is a flame-contact problem. PNNL. That is Zone 0, not a glazing listing.
What people mix up
- Dual-pane as tempered.
- Oregon’s multilayered option as California’s tempered-plus-dual-pane path.
- A 20-minute fire door as this window.
- Plastic skylights as tempered glass.
- IBHS Essential as a window rule. Enhanced is the window package. Ask the carrier.
How to check
- Find the bug. Tempered is etched in a corner.
- Count the lites. One or two. Oregon and California do not treat that count the same way.
- Look at the edge. If the crack starts at the frame line, that is this mechanism.
- Open dual-pane versus tempered for the code menu. Do not stop on this page.
- If the window is open, stop talking about glass type.
Limits
This page does not spec a U-factor. It does not reprint Quarles 2010. That PDF did not open here. PNNL quotes it. FireApproved is not PNNL, not FEMA, and not the AHJ. Confirm the adopted glazing sentence.
Related records
Questions
Why does the glass crack at the edge first?
PNNL’s Building America Solution Center, citing UCCE, says temperature differences develop between the glass at the center of the exterior surface (surface 1) and the glass around the edges that the frame shades. If that difference is large and fast, cracks start at edge flaws and run inward. The frame is a heat sink. The field of the pane is not.
Does dual-pane stop the crack?
Dual-pane is two lites with a sealed gap. FEMA P-737 (2008) says a double-paned annealed unit lasts about 10 minutes in a wildfire, about twice a single pane, and that if the first pane fails the second still has to be penetrated. It is a second barrier, not unbreakable glass. Oregon R327 lists multilayered glazed panels as one complete option. California CWUIC wants a tempered pane in the usual dual-pane path. See dual-pane versus tempered.
Is tempered the same as dual-pane?
No. Tempered is heat-treated glass. Dual-pane is two or more lites. PNNL, citing Quarles 2010, says tempered performs more than four times better than single-pane annealed. You can have dual-pane with no tempered lite. You can have one tempered lite.
Will a 20-minute fire door listing save this window?
A 20-minute fire-protection rating is a different object. R327 lists it as one glazing option. A fire door in a garage is not this pane. See WUI doors versus 20-minute fire doors.