# Spray Foam Fun (DD-750)

> Spray polyurethane foam is mixed on site from an isocyanate side (polymeric MDI) and a polyol blend containing catalysts, surfactants and fire retardants. Open-cell foam is blown mainly by water and expands about 150 times. Closed-cell foam is blown mainly by HFC-type agents and expands about 35 to 50 times, so the 'more than 100 times' claim applies only to open-cell foam. Both types seal air well. Both are combustible and usually need a thermal or ignition barrier, and installers must manage isocyanate exposure.

- Department: Tech (also Science)
- First published: 2026-10-13
- Last updated: 2026-10-13
- Permalink: https://detailenjoyer.com/case/spray-foam-fun/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## Quick facts

- **DD-750-F01**: Open-cell spray foam expands about 150 times; closed-cell about 35 to 50 times. _(Write-up · Exhibit B (Two foams))_
- **DD-750-F02**: Open-cell foam is blown mainly by water; closed-cell mainly by HFC-type agents. _(Write-up · Exhibit B (Two foams))_

## Write-up

Two liquids that swell into insulation. Open-cell foam expands about 150 times; closed-cell about 35 to 50.

1. **Two sides**: SPF is sprayed as two liquids. One side is polymeric MDI, an isocyanate. The other is a blend of polyol resins, surfactants, fire retardants and catalysts. The polyol reacts with the isocyanate to make polyurethane, and the reaction gives off heat.
2. **Two foams**: The main difference between the two types is the blowing agent. Open-cell foam uses mostly water and expands about 150 times its liquid volume. Closed-cell foam uses mostly HFC-type agents and expands about 35 to 50 times.
3. **Lifts and heat**: Closed-cell foam is usually applied in layers ('lifts') of about 2 to 3 inches, or 2 inches or less for medium-density products, because the core can reach 130 to 150 °F. Lifts that are too thick can overheat, scorch or even ignite. Open-cell foam can go on to full depth, often up to 10 inches, in one pass.
4. **Air sealing and fire**: Both types are good at air sealing because the expanding liquid fills gaps and cracks. Both are combustible, so building codes typically require a thermal barrier, such as 1/2-inch gypsum board, or in some attics and crawlspaces an ignition barrier.
5. **Safety**: Installers wear respiratory protection and ventilate the work zone because of isocyanate exposure. EPA says some manufacturers recommend about 24 hours before re-occupancy, but the time varies by product.

## Related case files

- [Smart Plastics (DD-057)](https://detailenjoyer.com/case/smart-plastics/): Another polymer whose properties depend on how it's made: one plastic, hard or soft depending on light.
- [Paint Removers (DD-642)](https://detailenjoyer.com/case/paint-removers/): Another workshop chemical

## Reel vs. record

- **Expansion**: 'more than 100 times' applies only to open-cell foam, at about 150 times. Closed-cell foam expands about 35 to 50 times (US DOE Building America).
- **Missing context**: the two types differ mainly in their blowing agent. Open-cell foam is blown mainly by water and closed-cell foam mainly by HFC-type agents.
- **Missing context**: the reaction is exothermic. Closed-cell foam is sprayed in lifts of about 2 inches because thick lifts can overheat, scorch or ignite.
- **Missing context**: both foams are combustible and usually need a thermal barrier (such as 1/2-inch drywall) or an ignition barrier.
- **Safety**: isocyanates can cause asthma-type sensitization. Occupants should stay out during spraying and curing, and re-entry times vary by product (EPA cites some manufacturers recommending about 24 hours).

## Video: Spray Foam Fun: Two Liquids That Swell Into Insulation

- Duration: 26.2 s · Published: 2026-10-13
- File: https://detailenjoyer.com/assets/videos/Debbdu2RHTW.mp4

### Transcript

DID YOU KNOW? Spray foam insulation can expand to more than 100 times its original liquid volume, filling gaps and sealing buildings against unwanted air flow. It works through a chemical reaction between two liquid components, an isocyanate and a polyol, which rapidly form a lightweight plastic filled with tiny gas-filled cells. There are two main types: open-cell foam, which is softer and more flexible, and closed-cell foam, which is denser, more rigid, and more resistant to moisture vapor. NOW YOU KNOW!

## Questions & answers

**Q: About how much does open-cell spray foam expand?**  
A: Open-cell expands about 150 times; closed-cell about 35 to 50 times. _(Source: Write-up · Exhibit B (Two foams) · US DOE Building America: Which Spray Foam Is Right For You?)_

**Q: What is the main blowing agent in open-cell foam?**  
A: Open-cell foam uses mostly water; closed-cell mostly HFC-type agents. _(Source: Write-up · Exhibit B (Two foams) · US DOE Building America: Which Spray Foam Is Right For You?)_

**Q: Which isocyanate is the A-side of most SPF?**  
A: Polymeric MDI reacts with the polyol blend. _(Source: Write-up · Exhibit A (Two sides) · US DOE Building America: Which Spray Foam Is Right For You?; US EPA: Spray Polyurethane Foam Product Types (Safer Choice archive))_

**Q: Why is closed-cell foam sprayed in thin lifts?**  
A: Core temperatures can reach 130 to 150 °F. _(Source: Write-up · Exhibit C (Lifts and heat) · US DOE Building America: Which Spray Foam Is Right For You?; US EPA: Contractor Checklist, Safer Practices for Installing High-Pressure Two-Component SPF)_

**Q: What typically covers spray foam in living spaces?**  
A: Both foams are combustible, so a thermal barrier is usually required. _(Source: Write-up · Exhibit D (Air sealing and fire) · US DOE Building America: Which Spray Foam Is Right For You?)_

## Sources

- [US DOE Building America: Which Spray Foam Is Right For You?](https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/spray_foam_guide.pdf)
- [US EPA: Spray Polyurethane Foam Product Types (Safer Choice archive)](https://archive.epa.gov/epa/saferchoice/spray-polyurethane-foam-product-types.html)
- [US EPA: Client Checklist, Safer Practices for Installing High-Pressure Two-Component SPF](https://www.epa.gov/sites/default/files/2015-08/documents/checklist_spf_contractor_client_communications.pdf)
- [US EPA: Contractor Checklist, Safer Practices for Installing High-Pressure Two-Component SPF](https://www.epa.gov/sites/default/files/2015-08/documents/checklist_professional_spf_insulation_contractors.pdf)

## Cite this case file

Easterling, Marcus. "Spray Foam Fun." The Detail Department (DetailEnjoyer), case file DD-750, last updated Oct 13, 2026. https://detailenjoyer.com/case/spray-foam-fun/