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Furnace Flue Pipe Insulation [A Complete Guideline]

Have you noticed that the furnace is heating up your attic? This unplanned situation is causing your home to heat up more than necessary. Maybe you need to add insulation to your furnace flue pipes.

How to install furnace flue pipe insulation?

Measure the flue pipe’s diameter first. Take some flat aluminum flashing and cut it in a proper measurement. Make metal notches for the pipes. Once you’re done shaping them, apply high-temperature caulk on the flue pipe. Add the flashing onto the caulk. Lastly, secure the junction where the flue pipe enters the furnace.

That was the preview of the whole process. To know the process in detail read our piece till the end. We’ve prepared a step-by-step guideline for you.

Let’s begin!

Why Insulating Furnace Flue Pipe Is Important?

The flue pipe carries carbon monoxide gas from the furnace to the home exterior. It carries the exhaust from burning propane or natural gas. 

When the gas runs through the flue pipe, it heats up the whole pipe. This can heat up your attic too. 

By insulating the flue pipe you can contain the heat inside it. This helps to keep your attic cooler.

Insulating flue pipe is mainly done to contain the heat inside the pipe. But that’s not the only reason to use it. This will give you the additional work of detecting ceiling leaks

Heated flue pipes can cause water leakage when snow piles up on the attic. The heat from the pipe melts the snow. This melted water can leak through the ceiling. But an Insulated flue pipe can prevent this problem.

Types Of Furnace Flue Pipes

Furnace flue pipes are constructed in three types. Let’s go through a brief introduction about them.


The first one we’re going to talk about is the single-walled flue pipe. It’s also known as type C vent. Single-walled flue pipes are made from 1 layer of galvanized metal. They require a 6-inch clearance in between pipe and insulation.


There’s a double-walled furnace flue pipe as well. It’s also known as type B vent. Similar to type B, double-walled flue pipes consist of 2 layers of galvanized metal. It also requires a 1-inch clearance between pipe and insulation.


Lastly, the tripled-wall furnace flue pipe. Same as the other two types, it consists of three layers of galvanized metal. It doesn’t require any clearance between the pipe and the insulation.

If you don’t know which type of flue you’re using, check the manufacturer’s specs. Most of the furnace flue pipes are double-walled these days.

Insulating Furnace Flue Pipe: Step by Step Guide

To insulate your furnace flue pipe you’re gonna need some tools and components. Here’s a list of the things you’re gonna need-

  • Flat sheet of aluminum flashing
  • Dremel
  • Measurement scale
  • High-temperature caulk
  • Protective gloves

Got all of them? Let’s move on to the process of insulating the furnace flue pipe.

Step-1: Take Measurements

Turn off your furnace first and wait till it cools down enough to touch. Take the circumference measurement of the flue pipe. Wear protective gloves before cutting metals for insulation.

Step-2: Cut Metal Flashing

Take two flat sheets of aluminum flashing now and cut them in 14” length. Use a dremel that cuts ductwork for cutting aluminum sheets. The sheets have to be narrow enough to fit between the ceiling joists.

Step-3: Cut The Metal Notch

Cut a semicircular-shaped notch into one end for each sheet now. The notches should be wide enough to fit the flue pipe. The edges also have to be long enough to overlap each other by 3”.

Step-4: Apply High-Temperature Caulk

You’ll need about 18” open space from each side of the flue pipe. That’s why detach the insulation between the flue pipe and the joist. Apply a thick coat of high-temperature caulk about 1” away from the flue now. Apply it to one side at a time.

Press one of the notched sheets of flashing aluminum between the joist afterward. Press it firmly against the caulk. The notched edge should be upward against the flue pipe.

Step-5: Secure The Junction

Repeat step-4 on the other side as well. Apply high-temperature caulk on the other side. Install the notched flashing sheet against it. Remember to press it tightly against the high-temperature caulk.

Apply some high-temperature caulk around the junction where flue lodges into the attic.

Don’t have this type of caulk? Here are some suggestions from us-

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Get them and start caulking!

Step-6: Cutting Slits On The Flashing

Cut the flashing aluminum to the size of the flue’s circumference taking an extra 6”. Cut a 1” deep slit on the top edge of the metal flashing now. You can use a jigsaw that cuts ductwork to do the job. 

Cut the sheet in every 2” gap until you reach the bottom edge. But the last slit near the bottom edge should be cut 2” deeper.

Step-7: Placing The Insulation Properly

Bend the notches along with the aluminum flashing at a 90-degree angle. Do these for both top and bottom edges. Wrap the metal flashing circle around the flue pipe. 

Once done, nail the flashing inside the ceiling joist through the notches at the bottom. These will prevent the insulation from touching the flue directly. While your working with the furnace, don’t forget to check furnace flame issues as these are pretty common. You can fix it easily.

With these, you’re done with installing furnace flue pipe insulation. Not that complicated right? The process of insulating basement ductwork is also similar to this.


Question: At what temperature does insulation catch on fire?

Answer: For insulation that is made out of PVC, catches fire at 100 degrees celsius. Metal and fiberglass insulations might burn after the temperature exceeds 212 degrees celsius.

Question: Can I put insulation around the heat ducts?

Answer: Yes, you can put insulation around heat ducts. Insulate the ducts with special-purpose duct insulation or standard insulation batts. Insulating your heat ductwork will prevent 10-30% of energy losses and save the bills.

Question: Do return ducts need to be insulated?

Answer: Insulation is used on ductwork to improve its thermal performance. Return ducts are to be insulated only if they affect the air temperature. If it passes hot air into the environment, it’s best to insulate it.


We’ve come to the end of our discussion about the furnace flue pipe insulation. Now, you know how to install insulation for furnace flue pipes. This will definitely save you some extra money.

Good luck with your renovation!