---
title: "Greenhouse Plastic Light Transmission: The Silent PAR Loss Your Eyes Cannot See"
canonical: "https://theyieldgrid.com/greenhouse-plastic-light-transmission/"
model_id: "tyg-769"
model_version: "1.0.0"
last_reviewed: "2026-08-25T01:29:40"
reviewed_by: "Umer Hayiat"
---

# Greenhouse Plastic Light Transmission: The Silent PAR Loss Your Eyes Cannot See

> Canonical calculator: [https://theyieldgrid.com/greenhouse-plastic-light-transmission/](https://theyieldgrid.com/greenhouse-plastic-light-transmission/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Greenhouse Plastic Light Transmission: The Silent PAR Loss Your Eyes Cannot See Visible brightness is a poor proxy for plant-available light. A greenhouse covered with 5-year-old polyethylene film can appear nearly identical in brightness to one covered with new film, yet deliver measurably fewer photons in the 400-700 nm range that drives photosynthesis. That gap is not random — it is predictable, calculable, and directionally tied to the age and type of your glazing material.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Covering Material Type | `ghpar_material` | select |  | — Choose material — = ``; Glass = `glass`; 6-mil Poly = `poly6`; 8mm Polycarbonate = `poly8mm` | No |
| Number of Glazing Layers | `ghpar_layers` | select |  | — Choose layers — = ``; Single Layer = `1`; Double Inflated = `2` | No |
| Age of Covering (Years) | `ghpar_age` | number |  | 0 to 20 | No |
| Outside Solar DLI (mol/m²/d) | `ghpar_dli` | number | m² | 1 to 80 | No |

## Outputs

| Output ID | Default state |
|---|---|
| `ghpar_material_err` |  |
| `ghpar_layers_err` |  |
| `ghpar_age_err` |  |
| `ghpar_dli_err` |  |
| `ghpar_results` | — mol/m²/d Actual Indoor DLI (Crop-Available Light) Internal PAR Transmission — 0% 50% 100% — Initial PAR% — Total Degradation — Internal PAR% — DLI Lost vs New Reference: 6-mil Poly Aging — DLI Impact (Outside DLI = your input) Age (yrs) PAR% Internal DLI DLI Lost Status Upgrade Your Greenhouse Light Efficiency Suntex 4-yr 6-mil Greenhouse Poly Wiggle Wire & Aluminum Channel Lock Greenhouse Repair Tape Apogee DLI Meter |
| `ghpar_results_inner` | — mol/m²/d Actual Indoor DLI (Crop-Available Light) Internal PAR Transmission — 0% 50% 100% — Initial PAR% — Total Degradation — Internal PAR% — DLI Lost vs New Reference: 6-mil Poly Aging — DLI Impact (Outside DLI = your input) Age (yrs) PAR% Internal DLI DLI Lost Status Upgrade Your Greenhouse Light Efficiency Suntex 4-yr 6-mil Greenhouse Poly Wiggle Wire & Aluminum Channel Lock Greenhouse Repair Tape Apogee DLI Meter |
| `ghpar_out_primary` | — |
| `ghpar_out_par_pct` | — |
| `ghpar_out_initial` | — |
| `ghpar_out_degrad` | — |
| `ghpar_out_internal` | — |
| `ghpar_out_loss` | — |
| `ghpar_warnings_box` |  |
| `ghpar_warnings_header` |  |
| `ghpar_warnings_list` |  |

## Formula and method

Our model accounts for cumulative UV micro-yellowing and the thermal barrier light penalty that standard charts ignore. Show the calculation steps Step 1 — Establish Initial PAR Percentage Each material type has a rated transmission for Photosynthetically Active Radiation (400-700 nm) when brand new: Glass = 90%, 6-mil poly = 85%, 8mm polycarbonate = 82%. These represent baseline manufacturer averages, not laboratory-precise values for a specific film brand. Step 2 — Calculate Age-Related Degradation Degradation (%) = Age (years) x 2% A greenhouse film that is 5 years old has degraded by 10 percentage points from its initial rating. This 2%/year rate reflects average field degradation observed across UV-stabilized polyethylene greenhouse films in mid-latitude conditions. Step 3 — Compute Internal PAR Percentage Internal PAR% = Initial PAR% minus Degradation% A physical floor of 30% is applied — degraded film cannot transmit less than this threshold in the model, though real-world degraded material may approach or exceed physical deterioration before this point is reached. Maximum capped at 100% (no scenario produces greater-than-rated transmission from aging). Step 4 — Apply Double-Layer Penalty (if applicable) If Double Inflated: Internal PAR% = Internal PAR% x 0.90 This approximates the roughly 10% additional transmission loss introduced by a second film layer in a pressurized double-poly system. The penalty is applied to the already-degraded internal PAR value, not the original baseline. Step 5 — Calculate Actual Indoor DLI Indoor DLI (mol/m²/d) = Outside DLI x (Internal PAR% / 100) Rounding: Results are displayed to one decimal place. The intermediate Internal PAR% calculation is kept at full floating-point precision before rounding the final DLI output. Step 6 — Crop Threshold Comparison The result is compared to two reference thresholds: – Below 10 mol/m²/d: Critical — most food crops are severely light-limited – Below 17 mol/m²/d: Yield-limiting for fruiting crops (tomatoes, cucumbers, peppers) – At or above 25 mol/m²/d: High-production range for fruiting crops Assumptions and Limits The 2% per year degradation rate is a generalized industry average. Films with stronger UV inhibitor packages (such as 4-year-rated commercial greenhouse poly) may degrade more slowly in the early years. Films in high UV-index climates or under poor storage conditions before installation may degrade faster. Glass is modeled as non-degrading for PAR transmission. While real glass does accumulate surface contamination, that is a maintenance factor rather than a material degradation factor and is excluded from this model. The double-layer penalty of 10% is an approximation. Actual loss in a double-inflated system varies with the air gap distance and the specific film combination used. Manufacturer data for your specific inflated film system should be consulted for precision. Structural shadowing from purlins, bows, gutters, and end walls is not included. In practice, structural components reduce available light by an additional 5 to 20% depending on greenhouse design and orientation. The tool models a single uniform PAR percentage across the entire greenhouse footprint. Canopy-level DLI is not uniform — north-facing benches and areas near end walls receive less than the model output suggests. Outside DLI is taken as a user-supplied constant. Seasonal variation, cloud cover patterns, and altitude effects are not modeled. A single day’s DLI can vary by a factor of two or more from the monthly average. Condensation, dust accumulation, and algae growth on film surfaces are not included. These are additive losses on top of what the calculator produces.

## Verified worked examples

### Scenario 1: New 6-mil Poly, Single Layer, Strong Summer Outside DLI

Material: 6-mil poly (Initial PAR: 85%) Layers: Single Age: 0 years (Degradation: 0%) Outside DLI: 40 mol/m²/d Internal PAR% = 85% minus 0% = 85% Indoor DLI = 40 x (85 / 100) = 34.0 mol/m²/d Result: 34.0 mol/m²/d indoor DLI — above the high-production threshold for most fruiting crops. At installation, 6-mil poly transmits enough PAR to support tomatoes, cucumbers, and peppers even with a modest reduction from the outside environment. This is the performance baseline that ages from this point forward.

### Scenario 2: 5-Year-Old 6-mil Poly, Single Layer, Same Outside DLI

Material: 6-mil poly (Initial PAR: 85%) Layers: Single Age: 5 years (Degradation: 5 x 2% = 10%) Outside DLI: 40 mol/m²/d Internal PAR% = 85% minus 10% = 75% Indoor DLI = 40 x (75 / 100) = 30.0 mol/m²/d Result: 30.0 mol/m²/d indoor DLI — 4.0 mol/m²/d lost compared to new film. The greenhouse interior still appears bright. Crops are not failing visibly. But the operation is now delivering 4 fewer mol/m²/d than it was at year zero — a deficit that accumulates to roughly 1,460 mol/m² over a full growing year, entirely from glazing aging alone.

### Scenario 3: 7-Year-Old 8mm Polycarbonate, Double-Inflated, Winter DLI

Material: 8mm polycarbonate (Initial PAR: 82%) Layers: Double-inflated (apply 10% additional layer penalty) Age: 7 years (Degradation: 7 x 2% = 14%) Outside DLI: 18 mol/m²/d (winter, northern climate) Step 1 — Internal PAR after age: 82% minus 14% = 68% Step 2 — Double-layer penalty: 68% x 0.90 = 61.2% Indoor DLI = 18 x (61.2 / 100) = 11.0 mol/m²/d Result: 11.0 mol/m²/d indoor DLI — technically above the low-light threshold but well below fruiting crop minimums. In winter, this scenario delivers barely enough light for lettuce and herbs but leaves tomatoes and peppers in a yield-limiting state. The combination of aged panels plus double-layer loss plus low winter sun creates a compounding deficit that supplemental lighting or film replacement can address.

## Assumptions

Our model accounts for cumulative UV micro-yellowing and the thermal barrier light penalty that standard charts ignore. Show the calculation steps Step 1 — Establish Initial PAR Percentage Each material type has a rated transmission for Photosynthetically Active Radiation (400-700 nm) when brand new: Glass = 90%, 6-mil poly = 85%, 8mm polycarbonate = 82%. These represent baseline manufacturer averages, not laboratory-precise values for a specific film brand. Step 2 — Calculate Age-Related Degradation Degradation (%) = Age (years) x 2% A greenhouse film that is 5 years old has degraded by 10 percentage points from its initial rating. This 2%/year rate reflects average field degradation observed across UV-stabilized polyethylene greenhouse films in mid-latitude conditions. Step 3 — Compute Internal PAR Percentage Internal PAR% = Initial PAR% minus Degradation% A physical floor of 30% is applied — degraded film cannot transmit less than this threshold in the model, though real-world degraded material may approach or exceed physical deterioration before this point is reached. Maximum capped at 100% (no scenario produces greater-than-rated transmission from aging). Step 4 — Apply Double-Layer Penalty (if applicable) If Double Inflated: Internal PAR% = Internal PAR% x 0.90 This approximates the roughly 10% additional transmission loss introduced by a second film layer in a pressurized double-poly system. The penalty is applied to the already-degraded internal PAR value, not the original baseline. Step 5 — Calculate Actual Indoor DLI Indoor DLI (mol/m²/d) = Outside DLI x (Internal PAR% / 100) Rounding: Results are displayed to one decimal place. The intermediate Internal PAR% calculation is kept at full floating-point precision before rounding the final DLI output. Step 6 — Crop Threshold Comparison The result is compared to two reference thresholds: – Below 10 mol/m²/d: Critical — most food crops are severely light-limited – Below 17 mol/m²/d: Yield-limiting for fruiting crops (tomatoes, cucumbers, peppers) – At or above 25 mol/m²/d: High-production range for fruiting crops Assumptions and Limits The 2% per year degradation rate is a generalized industry average. Films with stronger UV inhibitor packages (such as 4-year-rated commercial greenhouse poly) may degrade more slowly in the early years. Films in high UV-index climates or under poor storage conditions before installation may degrade faster. Glass is modeled as non-degrading for PAR transmission. While real glass does accumulate surface contamination, that is a maintenance factor rather than a material degradation factor and is excluded from this model. The double-layer penalty of 10% is an approximation. Actual loss in a double-inflated system varies with the air gap distance and the specific film combination used. Manufacturer data for your specific inflated film system should be consulted for precision. Structural shadowing from purlins, bows, gutters, and end walls is not included. In practice, structural components reduce available light by an additional 5 to 20% depending on greenhouse design and orientation. The tool models a single uniform PAR percentage across the entire greenhouse footprint. Canopy-level DLI is not uniform — north-facing benches and areas near end walls receive less than the model output suggests. Outside DLI is taken as a user-supplied constant. Seasonal variation, cloud cover patterns, and altitude effects are not modeled. A single day’s DLI can vary by a factor of two or more from the monthly average. Condensation, dust accumulation, and algae growth on film surfaces are not included. These are additive losses on top of what the calculator produces. The 2% per year degradation rate is a generalized industry average. Films with stronger UV inhibitor packages (such as 4-year-rated commercial greenhouse poly) may degrade more slowly in the early years. Films in high UV-index climates or under poor storage conditions before installation may degrade faster. Glass is modeled as non-degrading for PAR transmission. While real glass does accumulate surface contamination, that is a maintenance factor rather than a material degradation factor and is excluded from this model. The double-layer penalty of 10% is an approximation. Actual loss in a double-inflated system varies with the air gap distance and the specific film combination used. Manufacturer data for your specific inflated film system should be consulted for precision. Structural shadowing from purlins, bows, gutters, and end walls is not included. In practice, structural components reduce available light by an additional 5 to 20% depending on greenhouse design and orientation. The tool models a single uniform PAR percentage across the entire greenhouse footprint. Canopy-level DLI is not uniform — north-facing benches and areas near end walls receive less than the model output suggests. Outside DLI is taken as a user-supplied constant. Seasonal variation, cloud cover patterns, and altitude effects are not modeled. A single day’s DLI can vary by a factor of two or more from the monthly average. Condensation, dust accumulation, and algae growth on film surfaces are not included. These are additive losses on top of what the calculator produces. The most important thing this tool surfaces is not a number — it is a mismatch between what you observe with your eyes and what your crops actually receive. Understanding where the model’s warning zones come from helps you evaluate results critically rather than accepting them at face value. Critical Warnings The invisible UV starvation window (years 3 to 5 for standard poly): PAR degradation from UV micro-yellowing of polyethylene film happens in wavelengths that human vision largely compensates for. The interior of a greenhouse with 4-year-old poly can read as fully bright to a person walking through while delivering measurably fewer photons per square meter than the same space did at year one. This tool makes that invisible loss visible as a concrete mol/m²/d number. Below 17 mol/m²/d is yield-limiting, not just suboptimal: At indoor DLI values below 17 mol/m²/d, fruiting crops like tomatoes do not merely produce less — they shift resource allocation in ways that compound negatively across a season. The 17 mol/m²/d figure is a widely cited threshold in controlled environment agriculture literature, not a marketing benchmark. Double-inflated systems trade light for heat retention: The thermal advantage of a double-poly inflated system is real and significant for heating cost reduction. The light cost is also real. In low-outside-DLI climates or during winter months, the combination of aged film and a double-layer system can push indoor DLI below crop minimums even when the outside environment alone would be adequate. Run the numbers before winter, not during it. The 4-year replacement window for standard poly is not a marketing claim: Standard 6-mil greenhouse poly is UV-stabilized for approximately 4 years of service. After year 4, the UV stabilizer package is typically depleted, and degradation rate can accelerate. The calculator models a constant 2%/year rate, but real-world performance after the stabilizer is exhausted may decline faster. Minimum Standards for Fruiting Crop Production Tomatoes, cucumbers, and peppers: 17 mol/m²/d minimum indoor DLI for consistent fruit set and sizing. Leafy greens and herbs: 10 to 14 mol/m²/d indoor DLI is generally sufficient for commercial production. Seedlings and transplants: 12 to 16 mol/m²/d supports vigorous, non-etiolated growth in most species. Competitor Trap: Most articles on greenhouse plastic light transmission present PAR percentages as static material properties and stop there. They list “6-mil poly = 85% transmission” without accounting for age, layer count, or the compounding effect of both. A grower reading those articles walks away thinking their 7-year-old double-poly greenhouse is transmitting 85% of incoming PAR — when the actual figure, after accounting for degradation and layer loss, is closer to 64%. That 21-point gap is not a rounding error; it is a yield decision hiding in plain sight. If you are planning grow light supplementation around a static PAR rating, you are sizing to a number your covering has not achieved in years. The grow light cost calculator can help you determine whether supplemental lighting to close that gap is more economical than film replacement. For growers managing multiple environmental parameters simultaneously, it is worth noting that DLI is only one axis of crop optimization. Crop steering programs use DLI as one input alongside vapor pressure deficit, irrigation timing, and temperature differentials. The crop steering calculator on The Yield Grid integrates several of these variables for a more complete production model.

## Limitations and safety

The 2% per year degradation rate is a generalized industry average. Films with stronger UV inhibitor packages (such as 4-year-rated commercial greenhouse poly) may degrade more slowly in the early years. Films in high UV-index climates or under poor storage conditions before installation may degrade faster. Glass is modeled as non-degrading for PAR transmission. While real glass does accumulate surface contamination, that is a maintenance factor rather than a material degradation factor and is excluded from this model. The double-layer penalty of 10% is an approximation. Actual loss in a double-inflated system varies with the air gap distance and the specific film combination used. Manufacturer data for your specific inflated film system should be consulted for precision. Structural shadowing from purlins, bows, gutters, and end walls is not included. In practice, structural components reduce available light by an additional 5 to 20% depending on greenhouse design and orientation. The tool models a single uniform PAR percentage across the entire greenhouse footprint. Canopy-level DLI is not uniform — north-facing benches and areas near end walls receive less than the model output suggests. Outside DLI is taken as a user-supplied constant. Seasonal variation, cloud cover patterns, and altitude effects are not modeled. A single day’s DLI can vary by a factor of two or more from the monthly average. Condensation, dust accumulation, and algae growth on film surfaces are not included. These are additive losses on top of what the calculator produces. The most important thing this tool surfaces is not a number — it is a mismatch between what you observe with your eyes and what your crops actually receive. Understanding where the model’s warning zones come from helps you evaluate results critically rather than accepting them at face value. Critical Warnings The invisible UV starvation window (years 3 to 5 for standard poly): PAR degradation from UV micro-yellowing of polyethylene film happens in wavelengths that human vision largely compensates for. The interior of a greenhouse with 4-year-old poly can read as fully bright to a person walking through while delivering measurably fewer photons per square meter than the same space did at year one. This tool makes that invisible loss visible as a concrete mol/m²/d number. Below 17 mol/m²/d is yield-limiting, not just suboptimal: At indoor DLI values below 17 mol/m²/d, fruiting crops like tomatoes do not merely produce less — they shift resource allocation in ways that compound negatively across a season. The 17 mol/m²/d figure is a widely cited threshold in controlled environment agriculture literature, not a marketing benchmark. Double-inflated systems trade light for heat retention: The thermal advantage of a double-poly inflated system is real and significant for heating cost reduction. The light cost is also real. In low-outside-DLI climates or during winter months, the combination of aged film and a double-layer system can push indoor DLI below crop minimums even when the outside environment alone would be adequate. Run the numbers before winter, not during it. The 4-year replacement window for standard poly is not a marketing claim: Standard 6-mil greenhouse poly is UV-stabilized for approximately 4 years of service. After year 4, the UV stabilizer package is typically depleted, and degradation rate can accelerate. The calculator models a constant 2%/year rate, but real-world performance after the stabilizer is exhausted may decline faster. Minimum Standards for Fruiting Crop Production Tomatoes, cucumbers, and peppers: 17 mol/m²/d minimum indoor DLI for consistent fruit set and sizing. Leafy greens and herbs: 10 to 14 mol/m²/d indoor DLI is generally sufficient for commercial production. Seedlings and transplants: 12 to 16 mol/m²/d supports vigorous, non-etiolated growth in most species. Competitor Trap: Most articles on greenhouse plastic light transmission present PAR percentages as static material properties and stop there. They list “6-mil poly = 85% transmission” without accounting for age, layer count, or the compounding effect of both. A grower reading those articles walks away thinking their 7-year-old double-poly greenhouse is transmitting 85% of incoming PAR — when the actual figure, after accounting for degradation and layer loss, is closer to 64%. That 21-point gap is not a rounding error; it is a yield decision hiding in plain sight. If you are planning grow light supplementation around a static PAR rating, you are sizing to a number your covering has not achieved in years. The grow light cost calculator can help you determine whether supplemental lighting to close that gap is more economical than film replacement. For growers managing multiple environmental parameters simultaneously, it is worth noting that DLI is only one axis of crop optimization. Crop steering programs use DLI as one input alongside vapor pressure deficit, irrigation timing, and temperature differentials. The crop steering calculator on The Yield Grid integrates several of these variables for a more complete production model.

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## Provenance

- Model ID: `tyg-769`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-08-25T01:29:40
- Runtime SHA-256: `f0ec9444264333480d89d0c917c8c60b59a6a83b2f76f025a7307d01f88637bd`

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