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Insulation Calculator: Upgrade Your R-Value & Save Money

admin_ShockTrail 2 weeks ago

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Insulation Calculator: Upgrade Your R-Value & Save Money

Heat naturally moves from a warm place to a cold place. In winter, the expensive heat in your living room rises and escapes through your thin attic insulation. In summer, the 140°F heat in your attic pushes down into your bedroom. This is called the “Stack Effect.”

The only defense is R-Value (Thermal Resistance). Use this engineering-grade tool by ShockTrail to calculate the material needed to seal your thermal envelope.

The R-Value Estimator

Calculate required Bags (Blow-In) or Rolls (Batt) to reach Department of Energy standards.

Siding & Trim Estimator

Squares • Cartons • Accessories

Total Wall Area (Sq Ft)

Perimeter Length (Ft) For Starter Strip

Number of Windows

Number of Doors

Siding Type

Waste Factor

Main Material

Net Area (after deductions) 0 sq ft

Total Squares Needed 0

Est. Cartons (Vinyl) 0

Trim Accessories (Est.)

Starter Strips (10ft pieces) 0

J-Channel (12ft pieces) 0

Outside Corners (10ft posts) 4

*Accessory counts are estimates based on standard opening sizes.

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PART 1: The Physics of R-Value

Insulation is measured by its resistance to heat flow. The higher the “R,” the better it insulates.

R-Value is Cumulative: If you have R-10 existing and add R-30, you now have R-40.
Material Density: Not all fluff is equal. 10 inches of Fiberglass is R-25. 10 inches of Cellulose is R-35.

Department of Energy (DOE) Recommendations

Where you live dictates how much you need. [Image of insulation r-value zone map usa]

Zone 1-3 (South/Hot): Aim for R-30 to R-49.
Zone 4-7 (North/Cold): Aim for R-49 to R-60.

PART 2: Real-World Case Studies

Is it worth topping up your attic? Let’s look at the math.

Case Study 1: The “Top-Up” (Cellulose Blow-In)

The Home: 1,000 sq ft attic in Chicago (Zone 5). Currently has 3 inches of old fiberglass (approx R-10). Target is R-49.

The Math:

Needed: R-49 (Target) – R-10 (Current) = R-39 Needed.
Material: Cellulose (R-3.7 per inch).
Depth Needed: 39 / 3.7 = 10.5 inches.
Volume: 1,000 sq ft × (10.5/12) = 875 cubic feet.
Bags: A standard bag covers ~40 sq ft at R-30. For R-39, coverage drops to ~30 sq ft.
Order: 33 Bags. Cost: ~$500.

Case Study 2: The New Build (Fiberglass Batts)

The Home: An open attic with 2×10 floor joists. 1,500 sq ft.

The Math:

Joist Depth: 9.25 inches.
Material: R-30 Fiberglass Batts (Unfaced) fit perfectly in 2x10s.
Total R-Value: R-30. (Note: This is below DOE recommendation for cold zones).
Solution: Install R-30 batts between joists, then run a layer of R-19 batts perpendicular (crosswise) on top. Total R-49.
Order: 1,500 sq ft of R-30 + 1,500 sq ft of R-19.

Case Study 3: The Knee Wall (Spray Foam)

The Project: A finished attic room with vertical “knee walls” that are freezing.

The Solution: Fiberglass batts often fall down in vertical spaces. Closed Cell Spray Foam is best here.

The Math:

R-Value: Spray foam is R-6.5 per inch.
To get R-21 (Wall standard), you only need 3.5 inches of foam.
This seals air gaps that fiberglass misses.

PART 3: Material Types (Pink vs. Grey)

The two kings of DIY insulation are Fiberglass and Cellulose.

Material	R-Value per Inch	Pros	Cons
Fiberglass (Loose)	2.5 – 2.7	Fireproof, doesn’t settle much.	Itchy, lower R-value per inch.
Cellulose (Paper)	3.2 – 3.8	High R-value, blocks sound, eco-friendly.	Dusty installation, settles over time (loses depth).
Fiberglass (Batt)	3.0 – 3.2	Easy DIY (no machine needed).	Gaps reduce effectiveness drastically.
Mineral Wool	4.0 – 4.2	Fireproof, water resistant.	Expensive.

PART 4: Before You Blow (Prep Work)

Do not just start blowing insulation. You will rot your roof.

Air Sealing: Use “Great Stuff” foam to seal holes where wires and pipes come up from the house. This stops air leakage (which is more important than insulation).
Baffles (Rafter Vents): You must staple pink foam baffles between the rafters at the eaves. This ensures the insulation doesn’t block the soffit vents. If you block the vents, moisture gets trapped, and mold eats your roof deck.
Can Lights: Build boxes around old recessed lights (unless they are IC-Rated) to prevent fires.

Frequently Asked Questions (FAQ)

Should I remove old insulation?

Only if it is wet, moldy, or full of rodent droppings. Otherwise, save money by just blowing new insulation directly on top of the old stuff.

Faced vs. Unfaced Batts?

“Faced” (Paper) has a vapor barrier. It goes against the warm side (down towards the drywall). If adding a second layer on top, ALWAYS use “Unfaced” to prevent trapping moisture between layers.