How Many Bags of Cellulose Do I Need

Gus Koedding • June 24, 2025

Understanding Cellulose Insulation

Cellulose insulation is made from recycled paper products and is treated to be fire-resistant and pest-repellent. It is often used as blown-in insulation, which means it is applied using a machine that blows the material into the spaces needing insulation. This method helps fill gaps and provides a consistent layer of insulation.

Why Choose Cellulose?

Choosing cellulose insulation offers several benefits:

  • Eco-Friendly: Made from recycled materials.
  • Effective: Provides good R-values, which measure insulation effectiveness.
  • Versatile: Can be used in various parts of the house.

Calculating the Amount of Insulation Needed

To determine how many bags of cellulose insulation you need, follow these steps:

Step 1: Measure the Area

First, measure the square footage of the area you want to insulate. For instance, if you plan to insulate your attic, measure the length and width to get the total square footage.

Example: If your attic is 20 feet by 50 feet, your total square footage is 1,000 square feet.

Step 2: Decide on the R-Value

R-value indicates the insulation's resistance to heat flow. Higher R-values mean better insulation. The recommended R-value depends on your climate zone and the part of the house you're insulating.

  • Attic: R-38 to R-60
  • Walls: R-13 to R-21
  • Floors: R-25 to R-30

Step 3: Check Insulation Bag Coverage

Each bag of cellulose insulation will specify how much coverage it provides at different R-values. This information is crucial for determining how many bags you'll need.

Step 4: Calculate the Number of Bags

Use the following formula to calculate the number of bags required:

[ \text{Number of Bags} = \frac{\text{Total Square Footage} \times \text{Desired R-Value}}{\text{Coverage per Bag at Desired R-Value}} ]

Example: Suppose one bag covers 30 square feet at an R-value of 38. For a 1,000 square foot attic with an R-value of 38, you'd need:

[ \frac{1,000 \times 38}{30} = 1,267 \text{ bags} ]

Using Online Calculators

Many websites offer insulation calculators that can simplify this process. You might come across calculators like the "Tap Insulation Calculator" or "Dense Pack Cellulose Calculator." These tools can provide quick estimates based on inputs like square footage and desired R-values.

Factors Influencing Insulation Needs

Existing Insulation

If you already have some insulation, you may not need as much additional material. Measure the depth of your existing insulation and adjust your calculations accordingly.

Settling

Blown-in insulation can settle over time, reducing its thickness and effectiveness. Consider this when calculating the amount needed, especially for older homes.

Climate

Your local climate will affect the R-value you need. Colder climates require higher R-values for optimal energy efficiency.

Tips for Insulating with Cellulose

  • Hire a Professional: Blown-in insulation requires specialized equipment and skills. Consider hiring a professional to ensure it's done correctly.
  • Safety First: Wear protective gear, like masks and goggles, to avoid inhaling dust during installation.
  • Seal Air Leaks: Before adding insulation, seal any air leaks in the area to enhance energy efficiency.

Conclusion

Determining how many bags of cellulose insulation you need involves measuring your space, deciding on the desired R-value, and calculating based on bag coverage. Use online calculators for convenience and consider hiring a professional for installation. By planning properly, you can ensure your home is well-insulated, energy-efficient, and comfortable.

Remember, proper insulation not only saves energy but also reduces your utility bills, making it a wise investment for your home.

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Washington State Energy Code for Insulation
By Gus Koedding June 19, 2025
🏡 1. Code Overview & Climate Context Washington State mandates adherence to the 2021 WSEC (Residential - WAC 51‑11R‑2021) for homes submitted after July 1, 2023; code enforcement has strengthened since March 15, 2024 rbwarehouse.com+3kingcounty.gov+3insulationspokane.com+3 sbcc.wa.gov . The state falls into Climate Zones 5 and Marine 4 , requiring robust insulation and airtight construction—ceiling air leakage must be ≤ 5 ACH50 insulationinstitute.org+2insulationspokane.com+2insulationinstitute.org+2 . 2. Ceiling & Attic Insulation Requirements Prescriptive R‑Values (Table R402.1.2): Ceilings/Attics : Required R‑60 , equivalent to U‑0.024 en.wikipedia.org+12insulationinstitute.org+12insulationspokane.com+12 . Exception : Installing full coverage of R‑49 loose-fill or batts—uncompressed and extending over wall plates—satisfies the R‑60 requirement energy.wsu.edu+3app.leg.wa.gov+3kitsap.gov+3 . Vaulted Ceilings : Can reduce to R‑38 if insulation runs continuously to the exterior wall line en.wikipedia.org+12klickitatcounty.gov+12kitsap.gov+12 . Loose-fill Rules : Allowed on slopes ≤ 3:12, with ≥ 30″ attic clearance app.leg.wa.gov+1kitsap.gov+1 . Ventilation baffles are mandatory at soffits/eaves to prevent insulation from blocking airflow kitsap.gov . Access hatches must match ceiling R‑value, be weather‑stripped, structurally supported, and insulated to maintain thermal seal app.leg.wa.gov+1kitsap.gov+1 . 3. Wall Insulation Standards Wood‑frame walls have two prescriptive options to meet U‑0.056: R‑20 cavity + R‑5 continuous insulated sheathing , or R‑13 cavity + R‑10 continuous insulation rbwarehouse.com+10insulationinstitute.org+10insulationinstitute.org+10 . Requirements ensure both cavity and continuous components combine to meet wall thermal performance. 4. Basement & Below‑Grade Insulation Adheres to 10/15/21 + 5 TB rules: Acceptable methods include: R‑10 continuous exterior insulation, R‑15 continuous interior insulation, R‑21 cavity with thermal break, or R‑13 cavity + R‑5 ci + R‑5 slab break insulationinstitute.org+2insulationspokane.com+2insulationinstitute.org+2 kitsap.gov+1sbcc.wa.gov+1 insulationinstitute.org en.wikipedia.org+6energy.wsu.edu+6insulationinstitute.org+6 . Interior installations require an R‑5 rigid board thermal break between wall and slab insulationinstitute.org+4energy.wsu.edu+4app.leg.wa.gov+4 . 5. Slab‑on‑Grade Floors Unheated or heated slabs must have R‑10 insulation extending 4 feet from the slab edge, with F‑factor compliance (0.54) insulationspokane.com+2insulationinstitute.org+2energy.wsu.edu+2 . 6. Floors over Unconditioned Spaces Floor cavity insulation must maintain contact with subfloor or sheathing. Use supports ≤ 24″ on center; foundation vents must allow proper airflow, or include baffling if vents are above insulation app.leg.wa.gov+1kitsap.gov+1 . 7. Additional Code Elements Air leakage : Mandatory ≤ 5 ACH50 for whole building tightness en.wikipedia.org+3insulationspokane.com+3insulationinstitute.org+3 . Recessed lighting in ceilings must be IC‑rated or enclosed in a sealed box kitsap.gov . Insulation facings must have flame spread < 25 and smoke density < 450, unless covered properly kitsap.gov . Insulation inspection is required after all cavity insulation is in place and before concealment insulationspokane.com+2sbcc.wa.gov+2kitsap.gov+2 . 8. Compliance & Inspection Builders must schedule wall insulation inspections before walls are closed sbcc.wa.gov . Final inspections confirm performance testing compliance for air-tightness, thermal continuity, and proper installation. Local jurisdictions (e.g. Kitsap, King County) enforce these standards rigorously . âś… Summary Table of WSEC Insulation Requirements ComponentR‑Value / U‑FactorCeiling/AtticR‑60 (or R‑49 looser-fill per exception)Vaulted CeilingsR‑38 (with requirements met)Wood‑Frame WallsR‑20 + R‑5 CI or R‑13 + R‑10 CIBasement Walls10/15/21 + R‑5 thermal breakSlab‑on‑GradeR‑10 extending 4′; F‑factor 0.54Floors over CrawlspaceFull cavity insulation with supportsRecessed LightingIC‑rated or enclosed & sealedAir Leakage≤ 5 ACH50 đź”§ Best Practices for Contractors Plan around current prescriptive paths —ensure R‑values match attic, wall, basement, slab, and floor requirements. Install baffles, hatch-boxes, and venting per spec to maintain insulation and airflow integrity. Document continuous insulation —label thicknesses, materials, and locations for inspection clarity. Seal air barriers early to meet air-tightness and receive smoother final approvals. Schedule inspections before concealment to avoid delays and costly reworks. Stay updated —King County and others may enact local amendments alongside state mandates. Conclusion Washington State's current insulation codes, based on the 2021 WSEC and effective mid‑2023/2024, demand high-performance assemblies: R‑60 ceilings , continuous insulation on walls and slabs, and a tight building envelope . For contractors, compliance ensures energy efficiency, comfort, and code approval. For homeowners, it means lower utility bills and higher indoor comfort. Need help designing or retrofitting to code in Seattle? We’re the straightforward, no‑BS insulation pros—ready to deliver results that pass inspection and last. 🏠 Pacific Insulation & Remediation – “We do what we say, and say what we do.”
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