Spray Foam vs. Blown-In Insulation:
Which Is Better?
Both work. But they don't work for the same situations. A Maryland BPI-certified contractor breaks down the real differences — cost, performance, and when each one belongs in your home.
The Short Version
- Neither is universally better. Spray foam and blown-in solve different problems — the right answer depends on where in your home you're insulating.
- Spray foam wins where air sealing matters. Rim joists, attic-floor penetrations, knee walls, crawl space walls, and roof decks (especially when HVAC is in the attic).
- Blown-in wins on open attic floors. Cellulose or fiberglass delivers excellent R-value at a fraction of the cost — once air sealing is done first.
- The best Maryland attic jobs combine both. Foam the leaks, then blow in for depth. This combination also maximizes BGE & PEPCO rebate eligibility.
- If HVAC lives in your attic, encapsulate the roof deck with open-cell foam. It pulls the equipment into conditioned space and stops it from fighting a 130 °F attic.
- Start with a blower door test. A $100 energy audit tells you exactly where your home leaks before you spend a dollar on materials.
It's Not One vs. the Other — It's About Where You Use Them
Every week we get homeowners asking some version of the same question: "Should I go with spray foam or blown-in insulation?" And almost every time, the honest answer is: it depends on where in your home you're insulating, and what you're trying to achieve.
Spray foam and blown-in insulation solve different problems. Spray foam excels at air sealing, moisture control, and tight spaces where coverage matters as much as R-value. Blown-in cellulose or fiberglass is the workhorse of attic insulation — cost-effective, easy to install at depth, and perfectly suited for open attic floors.
In fact, the most effective attic jobs we do at Leonard Home Performance combine both: spray foam seals the penetrations and rim joists where air leakage concentrates, then blown-in covers the attic floor for thermal resistance. Think of it as using the right tool for each part of the job, not picking a winner.
What drives heat loss in most Maryland homes isn't just missing insulation — it's air leakage. Warm indoor air rises and escapes through attic penetrations, pulling cold outside air in through your basement and lower floors — a phenomenon called the stack effect, and it's a big part of why insulation alone often disappoints. A blower door test can quantify exactly where your home is leaking before you spend a dollar on insulation. Our $100 energy audit includes this diagnostic step.
Quick Comparison Table
| Factor | Spray Foam | Blown-In |
|---|---|---|
| R-Value Per Inch | R-3.7 (open-cell) / R-6.5 (closed-cell) | R-2.2–R-3.8 (cellulose / fiberglass) |
| Air Sealing | ✓ Yes — seals as it insulates | ✗ No — requires separate air sealing |
| Durability / Settling | Does not settle; lifetime material | Settles 15–20% over time |
| Moisture Resistance | Excellent (closed-cell acts as vapor barrier) | Moderate; cellulose vulnerable to saturation |
| Best Application | Rim joists, roof deck (HVAC in attic), crawl spaces, knee walls | Vented attic floors, open cavities |
| BGE/PEPCO Rebate Eligible? | ✓ Yes — eligible | ✓ Yes, when upgrading to R-49+ |
| Reversibility | Permanent — difficult to remove | Easy to remove or top off |
| DIY-Friendly? | No — requires trained installer | Rentable machines, but best left to pros |
Where Each Type Belongs in Your Home
The application is everything. Here's how we think about which material to reach for in each part of a home.
Best Uses for Spray Foam
Rim Joists & Band Joists
The single most effective place to use spray foam in most Maryland homes. Rim joists are a primary air leakage point and difficult to seal with any other material.
Attic Floor Penetrations
Before blown-in insulation goes down, we spray foam around every pipe, wire, and wall top plate. This is where air sealing wins are biggest.
Roof Deck / Encapsulation
When HVAC equipment lives in the attic, spray foam on the underside of the roof deck brings the attic into conditioned space — dramatically reducing duct losses. See our attic insulation page for details.
Crawl Space Walls
For homes with HVAC running through the crawl space, insulating the walls with spray foam (rather than the subfloor) is almost always the better approach. Learn more about crawl space insulation →
Knee Walls
The short vertical walls behind finished attic space are a notoriously difficult air barrier. Spray foam on the back of the knee wall drywall stops both heat loss and air infiltration. What is a knee wall? →
Cathedral Ceilings
When there's no attic access above a vaulted ceiling, closed-cell spray foam applied to the roof deck — from the exterior or via drill-and-fill — may be the only viable option to reach adequate R-value.
Best Uses for Blown-In Insulation
Vented Attic Floors
The bread and butter of blown-in insulation. After air sealing is complete, blown-in cellulose or fiberglass brings the attic floor to the R-49 Maryland code requirement quickly and cost-effectively — and keeps the attic cold enough in winter to help prevent ice dams from forming at the eaves.
Topping Off Existing Insulation
If your attic already has R-19 or R-25, blown-in is the easiest and cheapest way to close the gap. No need to remove the old insulation — just blow on top.
Finished Walls (Dense-Pack)
Dense-pack cellulose can be blown into wall cavities through small holes drilled from the exterior or interior — a minimally invasive way to insulate older walls without gutting them.
Open Garage Ceilings
The floor of a room above a garage is often under-insulated and uncomfortable. Blown-in into the cavity between floors is an effective solution for rooms above garages that run hot or cold.
When HVAC Lives in the Attic, Spray Foam Changes Everything
If there's one scenario where spray foam is almost always the right answer, it's this: your air handler and ductwork are installed in the attic.
With a traditional setup — fiberglass or cellulose on the attic floor, a vented attic above — that HVAC equipment is sitting outside your home's thermal envelope. A Maryland attic can easily hit 130°F in July and fall below freezing in January. Your air conditioner is trying to push cool air through ducts that are baking, and your furnace is losing heat through those same ducts before the air ever reaches your vents. The system ends up fighting the attic before it even gets to condition your living space. This is one of the most common reasons your second floor stays hot in summer no matter how low you set the thermostat.
The fix is to move the thermal boundary from the attic floor up to the roofline. We spray foam the underside of the roof deck, which pulls the entire attic — including all the HVAC equipment inside it — into the conditioned envelope. The attic now runs close to the temperature of the rest of your home instead of the outside.
The result: your HVAC stops working against a 130°F attic in summer and a freezing attic in winter. The system runs shorter cycles, uses less energy, and typically lasts longer because it isn't being constantly overworked. For homes with attic-mounted HVAC, this is the single highest-impact upgrade we install — and it makes the whole house more efficient, not just the equipment.
Why We Always Use Open-Cell Spray Foam at the Roofline
When we encapsulate an attic by foaming the roof deck, we use open-cell spray foam — not closed-cell. This is a deliberate choice, and it comes down to how each type handles moisture.
Open-cell foam is vapor permeable, meaning water can pass through it. If you ever develop a roof leak, the moisture comes through the foam at the point of the leak. You can see exactly where the water is coming in and have it repaired before damage spreads.
Closed-cell foam is a vapor barrier. If you have a roof leak behind closed-cell foam, the water has nowhere to go at the source — so it travels down along the inside of the roof deck, following the path of the foam, and eventually shows up somewhere far from where the leak actually is. Often it migrates down the wall and shows up as staining, warped drywall, or baseboard damage on a floor below. The homeowner has no idea where it's coming from, and by the time they track it down, the damage has been spreading unseen for months.
Open-cell at the roofline keeps your home diagnosable. A roof leak remains a visible, locatable problem instead of a hidden one — and that's worth more than the small performance difference between the two foam types.
Not sure whether your attic has HVAC in it? If you hear your air handler running from somewhere above you, or if you see flexible ductwork when you open the attic hatch, you have HVAC in the attic — and this upgrade is probably the highest-leverage improvement available for your home. A $100 energy audit will confirm it and lay out the options.
Frequently Asked Questions
Authoritative References on Insulation
The recommendations on this page align with guidance from federal agencies, industry research bodies, and certifying organizations. If you'd like to verify or dig deeper, these are the sources we trust:
Learn More on These Related Pages
Insulation is only one piece of the picture. These pages dig deeper into the topics that come up most often alongside the spray foam vs. blown-in decision.
Not Sure What Your Home Needs?
A $100 energy audit gives you a blower door test, thermal imaging, and a written recommendation — so you know exactly which approach makes sense for your home before spending a dollar on materials.
Schedule Your $100 Energy AuditRelated Pages
Explore more guides from Leonard Home Performance on making your Maryland home more comfortable and efficient.