How to properly insulate a metal garage from the inside: materials, technologies and mistakes

A metal garage without insulation turns into a glacier in winter, and into an oven in summer: the temperature difference between the street and the interior reaches 20–30°C. This leads to condensation on the walls, metal corrosion and damage to stored items. The main mistake is choosing insulation without taking into account thermal conductivity, vapor permeability and risk of fire. For example, cheap foam plastic 50 mm thick will reduce heat loss by only 30%, while sprayed polyurethane foam (PPU) 30 mm thick will give 90% of the effect — but will cost 3–5 times more.

This article contains proven materials for insulating a metal garage from the inside, taking into account the budget, climate zone and type of use (car storage, workshop, warehouse). We will compare 10 options according to 5 criteria: thermal insulation, moisture resistance, installation complexity, price and durability. We'll also look at 3 critical errors, which nullify all efforts: ignoring vapor barrier, incorrect choice of insulation thickness and lack of ventilation.

Why does a metal garage need to be insulated from the inside and not from the outside?

Insulation of a metal garage from the street - a rare solution, as it requires dismantling the casing, protection from precipitation and wind, as well as coordination with neighbors (if the garage is in a cooperative). Internal thermal insulation wins in 4 parameters:

• 🔧 Easy to install: No need to work at heights or in uncomfortable conditions.
• 💰 Savings: cheaper materials and labor (no need for windproof membranes).
• 🛡️ Protection against vandals: the insulation will not be damaged from the outside.
• 🔄 Maintainability: It is easier to replace the damaged area.

However, internal insulation has two risks:

1. Offset dew point inside the wall, which leads to metal corrosion.
2. Reduction of usable area (with an insulation thickness of 100 mm, the garage will narrow by 20 cm on each side).

⚠️ Attention: If the garage is used as a workshop with welding work, exclude flammable insulation (foam, polystyrene foam). Optimal choice - basalt wool with foil layer or expanded clay for the floor.

Top 5 materials for insulation: comparison by thermal conductivity and price

The choice of insulation depends on the climate, budget and purpose of the garage. The table shows the key characteristics of popular materials (data for central Russia, thickness 50 mm):

Material	Thermal conductivity, W/m K	Moisture resistance	Difficulty of installation	Price per m², ₽	Service life, years
Foam plastic (PSB-S-25)	0.038	Low (absorbs moisture)	Just	120–180	10–15
Extruded polystyrene foam (Penoplex)	0.030	High	Just	350–500	25–30
Mineral wool (Rockwool, Isover)	0.035–0.040	Medium (needs vapor barrier)	Average	200–400	20–25
Polyurethane foam (PPU, spraying)	0.025	High	Difficult (requires equipment)	800–1200	30+
Foil penofol	0.037–0.051	High	Just	50–150	10–15

Conclusion: For a garage with heating (for example, a stove) is optimal Penoplex — it is not afraid of moisture and does not support combustion. If your budget is limited, combine penofol (5 mm) + polystyrene foam (30 mm): it is 40% cheaper, and thermal insulation will be improved by 20% due to the foil layer.

Step-by-step instructions: how to insulate the walls of a metal garage

The insulation technology depends on the type of sheathing. Let's consider two options: frame (with lathing) and frameless (adhesive). To work you will need:

• 🔨 Screwdriver, hacksaw or stationery knife.
• 📏 Level, tape measure, marker.
• 🧲 Anti-corrosion primer (for example, Hammerite).
• 🛠️ Vapor barrier film (Izospan B) or foil.
• 🧴 Polyurethane foam or glue (Tytan Professional for foam plastic).

Option 1: Frame method (for mineral wool or polystyrene foam)

1. Preparing the walls: Clean the metal from rust and treat it with a primer. Cover the cracks with foam.
2. Lathing: Attach wooden blocks (50×50 mm) or a metal profile in increments of 60 cm (according to the width of the insulation).
3. Laying insulation: Insert the boards sideways, without gaps. For mineral wool, use a respirator!
4. Vapor barrier: Cover the insulation with film, taping the joints.
5. Sheathing: Sew up the walls with OSB, plywood or corrugated sheets.

Option 2: Frameless method (for foam plastic or Penoplex)

1. Glue the insulation boards to the walls using glue Ceresit CT 83 (for metal).
2. Secure additionally with “fungi” dowels (5 pieces per slab).
3. Seal the joints with reinforced tape.
4. Apply plaster or paint on top (suitable for foam plastic facade paint).

We treated the metal from rust|Checked the ventilation (supply/exhaust)|Calculated the thickness of the insulation according to SNiP|Bought a vapor barrier (if the insulation is hygroscopic)|Prepared a tool for cutting the material-->

⚠️ Attention: Do not use to attach insulation self-tapping screws without thermal washers — they form “cold bridges,” reducing the effectiveness of thermal insulation by 15–20%. For a metal garage, only dowels with a plastic core are suitable (for example, Hilti IBS).

How to insulate gates and floors: features and materials

Goes through the gate to 40% heat — their insulation is mandatory, even if the walls have already been treated. Optimal solutions:

• 🚪 Sandwich panels: ready-made structures with insulation inside (for example, Penoplex + galvanization). The cost is from 2500 ₽/m², but installation takes 1 day.
• 🔩 Self-cladding: foam plastic 50 mm + plywood 10 mm. Important: attach the insulation from the garage side, and not from the outside (otherwise the gate will warp from moisture).
• 🌀 Roll insulation: Penofol or Izolon - suitable if there is little space. Glue in 2 overlapping layers.

For the floor in a metal garage doesn't fit polystyrene - it is pressed under the weight of the machine. Alternatives:

• 🧱 Expanded clay (layer 10–15 cm) + screed. Cheap (150–200 ₽/m²), but requires raising the floor.
• 🏗️ Polyurethane foam (spraying). Expensive (from 1000 ₽/m²), but will last 30+ years without repair.
• 📦 Wood flooring on joists with insulation URSA Geo. Suitable for workshops.

3 critical mistakes that reduce insulation to nothing

Even expensive insulation will not save you from the cold if you make these mistakes:

1. Ignoring vapor barriers
   

   Without a film, moisture from the air penetrates into the insulation, reducing its properties by 50–70%. For example, wet mineral wool loses 60% of its thermal insulation. Solution: use Izospan B or aluminum foil.

2. Incorrect insulation thickness
   

   For Moscow and the region, the minimum thickness of polystyrene foam is 100 mm, Penoplex is 50 mm. If you take it thinner, the walls will “cry” in winter. Calculate the exact value using the formula:
   

   Thickness (m) = R (standard heat transfer resistance) × λ (material thermal conductivity)
   

   For a garage R = 2.8 m² K/W (SNiP 02/23/2003).

3. Lack of ventilation
   

   A closed, insulated garage without air flow accumulates carbon monoxide (if there is a stove) and condensation. Solution: install supply valve (for example, KPV 125) and an exhaust fan (from 1500 ₽).

How to check if there is enough ventilation?

Place a lit match near the vent. If the flame deviates into the garage, the draft is weak. The optimal air exchange rate for a 6x3 m garage is 180 m³/h.

How much does it cost to insulate a metal garage: calculation for 6×3 m

The cost depends on the material and installation method. The table shows estimated prices for a garage with an area of 18 m² (walls + gates + ceiling) for 2026:

Material	Thickness, mm	Price of materials, ₽	Cost of work, ₽	Total, ₽
Foam plastic + lathing	50	3 500	5 000	8 500
Penoplex + glue	50	7 000	6 000	13 000
Mineral wool + vapor barrier	100	6 500	8 000	14 500
PPU (spraying)	30	12 000	10 000	22 000

You can save money if:

• 🛒 Buy insulation in bulk (for example, polystyrene foam PSB-S-25 20% cheaper when ordering from 10 m³).
• 🔨 Do the installation yourself (video instructions are on the channel "Stroyhak").
• ♻️ Use used materials: often on Avito they sell the remains of Penoplex after construction.

Alternative methods: cheap and cheerful

If your budget is limited, consider non-standard solutions:

• 📦 Corrugated cardboard (from boxes): lay in 3-4 layers between the sheathing. Thermal insulation is weak (λ=0.06 W/m K), but better than nothing. Cost - 0 ₽.
• 🧵 Fabric waste: old blankets, batting or denim. Sew the covers and hang them on the walls. Fireproof, but requires treatment with fire retardants.
• 🌿 Sawdust or straw: mix with clay (1:1) and fill cavities in the walls. Cheap (50 ₽/m²), but there is a risk of rodents and mold.
• 🖼️ Foil wallpaper: Apply to walls over thin foam. Effect - +2–3°C due to heat reflection.

These methods are suitable for temporary insulation or garages in warm regions (Krasnodar Territory, Crimea). In Siberia or the Urals they are ineffective - the temperature difference in winter exceeds 40°C.

FAQ: Frequently asked questions about insulating a metal garage

Is it possible to insulate a garage from the inside without sheathing?

Yes, if you use it adhesive insulation (foam plastic, Penoplex) or sprayed (PPU). But for mineral wool and rolled materials, lathing is required - otherwise the insulation will sag.

How to avoid condensation on the walls after insulation?

Condensation occurs due to temperature differences. Solutions:

1. Install vapor barrier (foil or membrane).
2. Organize ventilation: supply valve at the bottom + exhaust at the top.
3. Use insulation with low vapor permeability (Penoplex, PPU).

How to insulate the ceiling in a metal garage?

Suitable for the ceiling:

• 🔥 Foam plastic (lightweight, does not load the structure).
• 💨 Mineral wool (if there is an attic).
• ⚡ Penofol (thickness 10 mm + air gap).

Attach with glue or screws with plastic mushrooms. Don't forget about vapor barrier!

Is it necessary to insulate a garage if it is not heated?

Yes, even without heating insulation:

• Reduces temperature changes, preventing condensate.
• Protects metal from corrosion.
• Preserves tools and spare parts from rust.

Insulation with a thickness of 30–50 mm is sufficient (for example, Penofol).

How to insulate a garage if there is not enough space?

In tight garages (< 2.5m width) use:

• Thin insulation: Penofol (3–10 mm) or Izolon (2 mm).
• Liquid thermal insulation (for example, Asstratek) - applied with a brush, layer thickness 1 mm.
• Foil panels (type Penotherm) - mounted on walls without sheathing.

Effect: +3–5°C with minimal loss of space.