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Calculate exact blown-in insulation bags, R-values, and attic requirements. Engineer-reviewed for US, UK, and AU standards.
Blown-In Insulation Calculator
Calculate exact bags of blown-in insulation for your attic. Accounts for R-value, existing insulation, and area.
Drywall Calculator
Calculate drywall sheets and joint compound before installing insulation and finishing your walls.
Construction Cost Calculator
Estimate total project costs including insulation, materials, and labor.
The Ultimate Guide to Insulation Estimation
Insulation is the most cost-effective way to reduce energy bills and improve home comfort. Whether you are blowing fiberglass into an attic or installing batts in a wall, the key metric is the R-Value (thermal resistance). Our calculators help you determine the exact number of bags, rolls, or sheets needed. Key Rule: Never compress insulation, as it reduces its R-Value.
Understanding R-Value and U-Value
Insulation performance is measured by its resistance to heat flow. This is expressed as an R-Value in the US and Australia. The higher the R-Value, the better the insulation.
In the UK and parts of Europe, insulation is measured by its U-Value (thermal transmittance). The lower the U-Value, the better the insulation.
To convert between the two: U-Value = 1 / R-Value (using metric units). Our Blown-In Insulation Calculator allows you to input your target R-Value and calculates the exact bags needed.
Global Insulation Standards
Recommended R-Values vary drastically by climate zone.
- USA: The Department of Energy (DOE) recommends R-30 to R-49 for warm climates (Zone 1-2) and R-49 to R-60 for cold climates (Zone 5-8).
- UK: Building regulations (Approved Document L) require a U-Value of 0.11 to 0.16 W/m²K for roofs and 0.18 to 0.26 for walls.
- Australia: The National Construction Code (NCC) sets minimum R-Values based on climate zone, typically R-3.5 to R-6.0 for roofs.
The Math Behind Blown-In Insulation
Blown-in insulation (fiberglass or cellulose) is sold by weight, but applied by volume. The manufacturer provides a coverage chart that tells you how many square feet one bag will cover at a given depth (which corresponds to an R-Value).
For example, a standard 25lb bag of fiberglass might cover 40 sq ft at R-38, or 30 sq ft at R-49. The deeper you want the insulation, the fewer square feet one bag will cover.
Our Blown-In Insulation Calculator uses these standard industry yields to calculate the exact number of bags required. It also factors in existing insulation depth, so you don't over-order.
The Role of Air Sealing
Insulation slows down heat transfer, but it does not stop air movement. Before blowing insulation, you must seal all gaps around plumbing, wiring, and recessed lights. Warm air leaking into the attic bypasses the insulation entirely, reducing its effectiveness.
In cold climates, a vapor barrier is also essential to prevent moisture from condensing inside the wall or attic. For details on drywall and wall prep, use our Drywall Calculator.
Types of Insulation: Which is Right for You?
There is no single "best" insulation. The right choice depends on where you are installing it and your budget.
1. Fiberglass Batts (Blankets)
The most common and affordable insulation. Batts come in pre-cut rolls or panels that fit between studs and joists. They are easy to install in walls and ceilings. However, they must be installed perfectly without gaps or compression, or they lose R-value.
2. Blown-In Insulation
Blown-in insulation (fiberglass or cellulose) is the best choice for attics and retrofitting existing walls. It can be blown into tight spaces and around obstructions, creating a seamless blanket. Cellulose (recycled paper) is denser and better at blocking airflow than fiberglass. Use our Blown-In Insulation Calculator for exact bag counts.
3. Spray Foam
Spray foam is the most expensive option, but it provides the highest R-value per inch and acts as an air barrier. It is applied as a liquid that expands into a solid foam. It is ideal for rim joists, cathedral ceilings, and sealing difficult cavities.
4. Rigid Foam Boards
Rigid foam boards (EPS, XPS, Polyiso) are used for exterior sheathing, basements, and concrete slabs. They provide excellent R-value and are moisture-resistant. They are often used in combination with batt or blown-in insulation.
The Critical Role of Attic Ventilation
Insulation works hand-in-hand with ventilation. If your attic is not properly ventilated, heat and moisture will build up, reducing the insulation's effectiveness and causing roof damage.
The International Residential Code (IRC) requires a minimum of 1 square foot of net free ventilation for every 150 square feet of attic floor area. This is achieved with a combination of intake vents (soffit vents under the eaves) and exhaust vents (ridge vents at the peak or gable vents).
When blowing insulation into the attic, you must install baffles at the eaves. Baffles are foam or plastic channels that keep the insulation from blocking the airflow from the soffit. Our Blown-In Insulation Calculator assumes you are installing baffles to maintain a 1-inch air gap.
Insulation Guides & Tutorials
In-depth articles on insulation types, R-values, and energy efficiency.
Frequently Asked Questions
Insulation Calculator FAQs
Direct answers to common questions about R-values, blown-in bags, and installation.
01
What R-value do I need for my attic?
02
How many bags of blown-in insulation do I need?
03
What is the difference between R-value and U-value?
04
Can I add blown-in insulation over existing insulation?
05
What is the best type of insulation?
06
Do I need a vapor barrier with insulation?
07
How long does insulation last?
Engineer-Reviewed Accuracy
Our insulation tools are built on verified engineering formulas and aligned with IRC (US), Approved Document L (UK), and NCC (Australia) standards for R-values, U-values, and attic ventilation. We prioritize accurate material estimation to maximize your home's energy savings.