---
title: what a hot block in chelsea depends on
canonical_url: https://ensurance.app/guide/what-a-hot-block-in-chelsea-depends-on
markdown_url: https://ensurance.app/guide/what-a-hot-block-in-chelsea-depends-on.md
subtitle: "flat black roofs, sealed ground and a thin canopy send the heat and the runoff to other people's bills. here is the block, who pays now, and the four levers already on the table"
category: nature-finance
---

# what a hot block in chelsea depends on

*flat black roofs, sealed ground and a thin canopy send the heat and the runoff to other people's bills. here is the block, who pays now, and the four levers already on the table*

Extreme heat in Massachusetts does not land evenly. It lands hardest on blocks like the ones around Bellingham Square and up Broadway in Chelsea: flat black roofs, sidewalk to storefront to sidewalk, and few trees between them. In the summer of 2020, researchers measured a 7°F gap between the hot spots and the cool spots inside this one small city. In a heat wave, the gap ran up to 9°F, from Island End Park at the cool end to downtown at the hot end.

If you run public works or sustainability for a city, plan a feeder for a utility, run a state program, or own a roof on Broadway, the block is not abstract. It is a bill, and it is already on someone's desk.

Chelsea's street trees, the soil under them that takes the rain, and the Mill Creek marsh cool, drain and clean these blocks whether or not anyone buys a certificate. **Ensurance** is how the people who depend on that work can fund it, not what it is.

## extreme heat in massachusetts, on one block

The numbers on Chelsea come from people who went out and measured. GreenRoots, Chelsea's environmental justice organization, ran C-HEAT with BU and stewards the Mill Creek restoration. The study, run with Boston University School of Public Health, placed sensors across Chelsea and East Boston. On hot weeks in 2020 and 2021, the air in both places ran about 6°F warmer than the National Weather Service monitor. An unshaded tennis court reached 115–130°F at the surface while the air sat in the low 90s. In 2017, summer land-surface (ground-surface) readings in parts of the city reached 140°F.

The ground explains most of it. About 80% of Chelsea is impervious: roof, road, lot and walk. Parks and recreation take 3% of the city's land, against a national median of 15%. Public open space totals 53.5 acres. At the 2019 count of 39,992 residents, that is about 748 people for every acre of it.

The health numbers follow. In the latest figures we found, asthma prevalence in Chelsea was about 17% in 2013–2015, against 11.5% statewide. In 2016, emergency visits for an asthma attack ran 91.1 per 10,000 residents, against 61 across Massachusetts. Heat is not the only cause. The same sealed blocks also carry traffic and industry, and these figures do not separate the causes.

One honest gap: we found no current citywide canopy percent for Chelsea. A 2008 trace found about 4.3% canopy in the Chelsea River subwatershed, not the whole city. If every plantable spot in the city grew a tree, cover would top out at 24.71%. That is the room left, not today's cover.

## what the block is made of

A hot block is a set of surfaces, and each one comes due on its own clock. The question at each replacement is what goes back, and who pays the difference.

| surface | replacement cycle | what could go back | cooling | stormwater | who pays now |
|---|---|---|---|---|---|
| City streets | About 5 years | Higher-albedo or pervious asphalt, or concrete | Significant | Yes | The city |
| Residential roofs | 15–50 years, mostly black EPDM | White EPDM or high-albedo shingles | Significant, for that building | No | The owner |
| Commercial roofs | Varies | White EPDM or TPO | Not rated here | No | The business |
| Public roofs | Varies | Mostly TPO already | Not rated here | No | The public owner |
| Street trees | 50–100 years | More of them, kept alive | Substantial | Yes | The city or state plants; the city maintains |
| Rain gardens | Varies | Bioretention in place of pavement | Substantial | Yes | Varies |

Chelsea has already tried cool surfaces on a small scale. The school district put a reflective roof on the Williams School in 2020, and GreenRoots' Cool Block, bounded by Congress Avenue, Willow Street and Maverick Street, added street trees, a lighter street surface and white-painted roofs.

:::johnson
**a white roof cools one building. a tree cools the street, holds the rain and cleans the air.**

Both belong on the block. Only one of them is alive, and the living one is the one a replacement budget rarely carries past year one.

[see the coordinator for urban heat →](/urban-heat.syndicate?from=guide)
:::

## what the living things do

The trees on Chelsea's streets shade pavement that would otherwise store the day's heat and give it back all night. The soil under them is a sponge: rain that soaks into a tree pit or a rain garden is rain that does not race into a storm drain on a block that is four-fifths sealed.

At the edge of the city, Mill Creek does the same work at a larger scale. The city's own report counts about twenty acres of salt marsh left. A 2019 survey found eight marsh species in the lower cells and four in the upper cells, against a healthy benchmark of 20 or more. The land draining to that reach is about 60% impervious. The marsh is still taking the water the streets send it.

How much could the living side change? A design model from the city's Municipal Vulnerability Preparedness work estimated that cutting impervious surface by 25% could lower air temperature by up to 3.0°F on a 95°F day, and raising canopy to 30% by up to 3.5°F. That is a model, not a measurement.

## who pays when nothing changes

When a surface goes back black and sealed, the cost does not vanish. It moves onto ledgers that belong to other people.

| ledger | who carries it | what heat and runoff do to it |
|---|---|---|
| Energy cost | The building owner or tenant | Higher cooling bills under a black roof |
| Stormwater capital and operating | The city | More runoff to pipe, treat and clean up after |
| Electric distribution capital | The utility, and so its ratepayers | Hotter blocks may raise the peak the feeders are built for |
| Health care | Hospitals, insurers, households | Heat illness and asthma visits |
| Economic loss | Employers and workers | Heat costs hours and output |
| Ecosystem benefits | Everyone downstream | A marsh asked to absorb what the streets will not |

The electric meter is one row of six, which is why energy savings alone cannot carry a canopy ([the full argument is here](/guide/energy-savings-from-trees-do-not-repay-the-canopy?from=guide)). We have planned a map of who holds each of these bills, how large it is, and what the living work would avoid. Chelsea is its first case. It has not been built. For who can hold the canopy once the bills are named, see [who pays for the canopy on a hot block](/guide/who-pays-for-the-canopy-on-a-hot-block?from=guide).

## the four levers already on the table

Four levers already exist or have been proposed in public: one is a live program, three are ideas. None of them is ours. Each changes a surface; none, on its own, pays to keep a tree alive for fifty years.

| lever | what it changes | where it stands | the post |
|---|---|---|---|
| A pilot | A mix of cool materials and green infrastructure on real blocks, with a plan for who pays to keep it | Chelsea has run small pilots (a reflective roof at the Williams School, and the Cool Block on Congress Avenue, Willow Street and Maverick Street); a pilot across many blocks, with a plan for who pays to keep it, is an idea | [a white roof is not a tree](/guide/a-white-roof-is-not-a-tree?from=guide) |
| The state revolving fund | Stormwater planning, design and construction, including green infrastructure, through the Massachusetts Clean Water Trust, for municipalities and districts | A live program; heat is not one of its eligibility categories | [what the state revolving fund can already pay for](/guide/what-the-state-revolving-fund-can-already-pay-for?from=guide) |
| A community cooling campaign | Owners on a street change their roofs together, on the model of the Solarize group-buys the state ran for solar | A proposal; no campaign by that name exists. MassCEC no longer funds Solarize. | [cool one street at a time](/guide/cool-one-street-at-a-time?from=guide) |
| Heat-resilient zoning | What a new or replaced roof or lot must be | Tying state funding or Green Communities designation to such codes is an idea, not a rule | [zoning tells the roof what to be](/guide/zoning-tells-the-roof-what-to-be?from=guide) |

We looked for the extra cost of a white roof over a black one at replacement, the number every lever eventually needs. We did not find one.

## one parcel, one policy

A roof, a street segment, a rain garden or a stand of street trees can become a **policy**: a certificate on one parcel, written with whoever holds title, named for that parcel. The heat, the water and the air on that ground travel together inside it. Nobody buys the heat off a tree on its own. In the design, the utility's, the city's and the health system's interests would route inside one bundle.

Policies are bought by a **line**: one buyer's mandate, written as a certificate on an account that funds the work inside it. For heat and stormwater here, we recommend one regional line, named for the buyer's portfolio rather than for Chelsea. If the buyer is the state or a statewide utility, the name is something like Massachusetts heat and stormwater. That line is **not minted**. You can [co-create it](/specific/create?mode=suggest&agent=urban-heat.syndicate&name=massachusetts%20heat%20and%20stormwater&from=guide); a person reviews every suggestion. Why the name follows the buyer: [name the line for the buyer, not the town](/guide/name-the-line-for-the-buyer-not-the-town?from=guide).

Theme lines that are already live can buy the same policies: [urban-heat.syndicate](/urban-heat.syndicate?from=guide), [water-cycle.syndicate](/water-cycle.syndicate?from=guide) and [clean-air.ensurance](/clean-air.ensurance?from=guide).

Where we actually are: no titleholder in Chelsea has signed a policy, and no payor has committed. Our screen for which parcels to start with is designed and has not run. The trees beside the fuel terminals are a buffer for the people who breathe there, not a cleanup of what the terminals release. The flood side of the city, the Island End wall and the marsh behind it, has [its own post](/guide/the-grant-built-the-wall-the-marsh-was-already-there?from=guide).

## frequently asked questions

### is a white roof enough?

It helps the building under it, and replacement is the moment an owner can choose one. We found no price premium for white over black, and a roof choice should account for winter heating. It does not take rain, shade a sidewalk or clean the air. A tree and the soil under it do all three.

### who pays for heat resilience in chelsea today?

The city pays for streets and maintains trees. Owners pay for roofs. The utility, the health system and employers carry costs from heat without owning the surfaces that cause it.

## next steps

If you work for a city or the state, start with the parcel list: the surfaces on the hottest blocks, their replacement dates, and who carries each bill. A regional natural asset analysis builds that list; its price range is on [our services page](/services?from=guide#finance). See how it fits a program on the [governments path](/solutions/governments?from=guide&topic=chelsea-heat), a feeder on the [utilities path](/solutions/utilities?from=guide&topic=chelsea-heat), or several cities on the [regional collaboratives path](/solutions/regional-collaboratives?from=guide&topic=chelsea-heat). Or [talk to us about Chelsea](/contact?from=guide&topic=chelsea-heat).

Island End Park already shows the cool end of this city. The question this series asks is who pays to bring that end up Broadway, and to keep it standing once it gets there.

## sources

- [MAPC, C-HEAT: Chelsea and East Boston Heat Project](https://www.mapc.org/wp-content/uploads/2025/06/C-HEAT-Chelsea-East-Boston-Heat-Project.pdf): the 7°F gap inside Chelsea in 2020
- [Scammell et al., C-HEAT abstract, Health Effects Institute annual conference 2023](https://www.healtheffects.org/system/files/scammell-abstract-ac2023.pdf): about 6°F above the weather-service monitor; up to 9°F from Island End Park to downtown; 115–130°F court surfaces
- [Boston University, "BU researchers study Chelsea heat island," 2022](https://www.bu.edu/articles/2022/bu-researchers-study-chelsea-heat-island-climate-change): land-surface temperatures up to 140°F
- [High-Profile, "Mitigating the urban heat island effect in Chelsea"](https://www.high-profile.com/mitigating-the-urban-heat-island-effect-in-chelsea/): 80% impervious, 3% parks, 53.5 acres of open space, and the design model for impervious and canopy changes
- [GreenRoots, COVID-19 in Chelsea, 2021](https://greenrootsej.org/sites/default/files/2023-01/greenroots_covid19_report_2021.pdf): asthma prevalence and emergency visits, citing the state health department
- [Healy, Massachusetts urban canopy dissertation](https://www.maurbancanopy.org/wp-content/uploads/2023/09/Dissertation_MHealy_Submission.pdf): the 24.71% canopy ceiling
- [City of Chelsea, Mill Creek water quality improvement project, Volume II](https://cms5.revize.com/revize/chelseama/Document_Center/Departments/Housing%20&%20Community%20Development/Environment%20and%20Climate%20Resilience/volume_ii_-_mill_creek_water_quality_improvement_project.pdf): about twenty acres of salt marsh
- [Horsley Witten, Mill Creek restoration assessments, July 2019](https://cms5.revize.com/revize/chelseama/Document_Center/Departments/Housing%20&%20Community%20Development/Environment%20and%20Climate%20Resilience/mill_creek_restoration_assessments_-_horsley_witten_-_20190715.pdf): marsh species counts and the 60% impervious subwatershed
- [Mass.gov, available funding for stormwater projects in Massachusetts](https://www.mass.gov/service-details/available-funding-for-stormwater-projects-in-massachusetts): clean water state revolving fund eligible stormwater uses and applicants
- [MassCEC, Solarize Mass](https://www.masscec.com/solarize-mass): the community group-buy model and its current status
- [NPR, "A block in Massachusetts is the test site for ways to cool cities in the summer," 24 May 2022](https://www.npr.org/2022/05/24/1101040221/a-block-in-massachusetts-is-the-test-site-for-ways-to-cool-cities-in-the-summer): the Cool Block pilot and its boundaries
- [MAPC, successful implementation of cool roofs](https://www.mapc.org/planning101/successful-implementation-cool-roofs/): the Williams School reflective roof

## the series

1. **what a hot block in chelsea depends on** (you are here)
2. [a white roof is not a tree](/guide/a-white-roof-is-not-a-tree?from=guide)
3. [what the state revolving fund can already pay for](/guide/what-the-state-revolving-fund-can-already-pay-for?from=guide)
4. [cool one street at a time](/guide/cool-one-street-at-a-time?from=guide)
5. [zoning tells the roof what to be](/guide/zoning-tells-the-roof-what-to-be?from=guide)
6. [who pays for heat resilience, by the parcel](/guide/who-pays-for-heat-resilience-by-the-parcel?from=guide)