How to build heat shields for wood stoves

Heat shields for Clearance Reduction

Adequate clearances are one of the two most important safety features of any wood stove installation.  (The other is proper materials.)  A properly installed and maintained wood stove can be one of the safest appliances in your home.  But cheating on clearances can create a very dangerous situation.

Don't Trust Pictures

Glossy tile hearth and Dwarf 4kW

We get a lot of questions about photos of stoves that appear to have much closer clearances than allowed.  "How are they able to do that?"

Sometimes it can be hard to tell how close a stove is to a wall from a photo.  Clearances may be OK, but appear in a photo to be closer than they actually are.

People do frequently violate clearances and post the results online.  Just because someone did it doesn't mean it's safe.

In some cases, stoves are "staged" in a way that they could not be installed, and then corrected later.  For instance, on Tiny House Nation S5E15, they didn't have time to install the flue system before filming the episode, so the set dressers just put the stove where they thought it looked best.  Photos on the episode show the stove way too close to the wall, but if you look closely, you'll see that the stove was not hooked up yet.

Pyrolysis - Why Clearances are So Large

Clearances are important because wood stoves get very hot while in operation.  Combustible materials that are too close to the stove can heat up past their autoignition temperature, and catch fire.  A spark is not required to start a fire, just heat, fuel, and oxygen.

In most cases, clearance violations will not cause a fire immediately.  As material is repeatedly heated, it deteriorates on a molecular level and its autoignition temperature begins to drop in a process is called pyrolysis.  After months or years of repeated heating, a surface that "hadn't had a problem yet" can spontaneously burst into flames.

To be safe, exposed combustible materials around the stove should never exceed 117 degrees F over ambient temperature, and unexposed areas (under the hearth, for example) should never exceed 90 degrees F over ambient.  If you observe potentially combustible materials around a stove discoloring, that can be an indication that pyrolysis is occurring.  But you won't always be able to see it.

What Happens When you Cheat

Aftermath of Sauna Fire

Clearance violations are an especially dangerous problem because they often don't cause a fire immediately.  Sometimes pyrolysis is visible as discoloration or charring on the surface of combustibles, but sometimes the pyrolysis can occur inside a wall.  You could use your wood stove with no problems for months or years until one day, your wall catches fire from the inside without warning.

Cheating clearances can also be caused by using improper materials.  For instance, if you use pellet pipe instead of proper Class A chimney pipe to penetrate your roof, and you follow the manufacturer's suggested clearance to combustibles, you'll create a dangerous clearance violation.  Pellet pipe is rated for much lower temperatures that wood stoves produce, so the clearances are calculated assuming those lower temperatures.  Connect pellet pipe to a wood stove, and the manufacturer's rated clearances are no longer adequate.

How to Safely Reduce Wood Stove Clearances

Wood stove clearances cannot be eliminated entirely, but they can be reduced significantly by using a properly constructed heat shield.  It's critical that heat shields are designed properly, and that rated clearance reductions for the type of shield are followed.

NFPA-211, the industry standard for wood stove installations, describes three different types of heat shields.  (Note: NFPA-302 is the standard for boats, which is a bit different from the rules outlined in NFPA-211 and this article.  The rules in NFPA-302 are less detailed than NFPA-211, so we suggest following NFPA-211 standards unless you're installing on a boat.)

How to Construct Air-Cooled Heat Shields

Dwarf Stove with Currogated Metal Shield

By far the most effective heat shields are the air cooled type.  These shields are constructed with a sheet of 24 gauge or thicker sheet metal, or 1/2" or thicker cement board, with 1" of air space behind the shield and around the perimeter to allow free air flow.  1" heat shield spacers are available online or sometimes at your local hardware store for this purpose.

Try to avoid placing a spacer directly between the center of the stove and the wall, since spacers can conduct heat through the shield to the combustible wall.  It's usually best to place the spacers around the perimeter of the shield.

The airflow behind and around the perimeter of the shield is critical for its success.  As the shield is heated, the natural convection of air around the shield will keep the shield and the material behind it cool.

A proper air cooled heat shield reduces clearances by up to 2/3 (18" becomes 6") when used as a wall protector, or up to 1/2 (18" becomes 9") when used as a ceiling protector.

How to Calculate Clearances with a Heat Shield

Clearances are calculated from the heat source to the combustible surface.  So, 18" clearance reduced to 6" with a shield that stands 1" away from the wall would need at least 5" from the shield to the heat source.

How Big Should Heat Shields Be?

Sheet Metal Shield on Bench

Clearances are calculated in all directions in a straight line.  The easiest way to figure out what surfaces a shield needs to cover is to cut a dowel to the rated clearance you're testing.

For example, from the back of the Dwarf stove, the rated clearance is 18" to combustibles.  With an 18" dowel, measure all points on the wall that you can touch with one end of the dowel, while the other end is touching the back of the stove.  All those points must be covered by the heat shield.

Dwarf Stove with Tile Shield

Do the same for the stovepipe and the sides of the stove at their respective clearances.

You'll note a couple of things on this exercise.  First, the heat shields will tend to be significantly larger than the profile of the stove or stovepipe.  Second, the further away the stove or stovepipe is from the combustible material, the smaller the heat shield needs to be.

Attaching Shields Directly to the Stove or Stovepipe

Laser Cut Wall Shield and Pipe Shield

The industry standard NFPA-211 guidelines for wall and ceiling shields does not cover attaching shields directly to the stove or pipe, but it is often possible to reduce clearances with attached shields in some situations.

Attaching air-cooled heat shields directly to the body of Dwarf stoves using our factory heat shield kit cuts required clearances in half, so 16" from the sides becomes 8", and 18" from the back becomes 9".  Similarly, attaching air-cooled heat shields to our single-wall pipe creates a double-wall pipe, which requires 9" clearance to combustibles with the heat shield in between instead of the standard 18".

The easiest way to make air-cooled heat shields for stovepipe is to cut up a piece of single-wall stovepipe.  Then, screw the shield to the pipe using 1-1/4" heat-proof screws and 1" ceramic or metal heat shield spacers.

Check out Nick's 5er installation video for an example of using shields attached directly to the stove and to the pipe to reduce clearances with minimal visual impact.

If your stove is produced by a different manufacturer, attaching a DIY shield directly to the stove or stovepipe may not be permitted, and factory heat shields may or may not be available.  So be sure to check with the manufacturer.

Insulated Shields, Masonry Shields, and Other Shield Types

Dwarf Stove with Masonry Shield

There are two other types of shields described in NFPA-211, but they're rarely used for tiny living applications.  These two types of shields are no easier to construct than air-cooled shields, and they are less effective.

Insulated shields are identical to air-cooled shields, but instead of 1" of air space behind the shield, you use 1" of fireproof insulation like rockwool or ceramic fiber.  A shield of this type can reduce wall clearances by 1/2 or ceiling clearances by 1/3.

Masonry shields are 3-1/2" thick masonry blocks with no air space, i.e. a standard brick wall.  Shields of this type can reduce wall clearances by up to 1/3, and are not generally used for a ceiling.

Any shield type other than the three types mentioned in NFPA-211 should generally not be used for clearance reduction unless that configuration has been tested.  While shields with an amount of air space or insulation other than 1", or shields with a double-layer of sheet metal would likely provide some benefit, the amount of safe clearance reduction they would provide is unknown.

Shields That Are Not Shields

Tile attached directly to the wall with no insulation or air space is not a heat shield.  Tile will conduct heat directly through to the combustible surface behind it.  Unless it's at least 3-1/2" thick, a layer of tile it provides no rated clearance reduction.

Sheet metal attached directly to the wall with no air space will also conduct heat straight through it, and provides no rated clearance reduction.

Note for Residential Spaces

Spaces that are subject to building codes generally have specific requirements for clearances.  If a stove is UL listed for residential spaces, you follow the manufacturer's instructions in the manual.  However, most tiny stoves are not UL listed, so you may need to follow the local building code's guidelines for unlisted stoves, which typically require 36" clearances to combustibles in all directions.

If you are using a heat shield to reduce clearances in a project subject to building codes, the shield cannot reduce clearances to less than 12" in any direction unless the shield and stove are specifically tested and listed for that purpose.

31 thoughts on “How to build heat shields for wood stoves”

    1. Keith-

      Good question. NFPA-211 only specifies “24 gauge sheet metal” for wall protectors. It doesn’t specify the type of metal. I’m using 0.032″ 2024-T3 alclad aluminum for my wall heat shield in my Airstream, and the shield hardly gets warm when the stove is burning. I don’t expect 24 gauge or thicker aluminum sheet would be a problem for air-cooled heat shields attached to the wall or ceiling. Aluminum certainly has the advantage of being rust-resistant in a sauna environment.

      For heat shields attached directly to the stove body or stovepipe, I’d suggest opting for stainless steel or carbon steel instead. Aluminum has a melting point of around 1,200 degrees F. That’s roughly the temperature you’d expect from a chimney fire, so there’s a chance it could fail and allow the fire to spread to the structure. Steel, with a melting point in the 2,500 to 2,800 degree F range, would be less likely to fail during a chimney fire than aluminum.

  1. What is the size and thread pitch of the bolts used to mount a heat shield directly to the Stove with the factory bolt holes?

    1. Hey Mark-

      The vast majority of the bolts on the Dwarf Stove, including the accessory mounting points, are threaded M6 1.0. The factory screws are pretty short since they’re mostly there to plug up the holes when you’re not using them. If you’re making a DIY heat shield, I suggest using 1-1/4″ long stainless machine screws with 1″ ceramic or steel heat shield spacers between the sheet metal and the stove. If you want to paint the screw heads and the shield to match the stove, use Stove Bright Flat Black.

  2. Hi,

    Are there any specification requirements for the Rockwool for the no airspace alternative? Does Rockwool 60, 1 inch thick, work?

    Thanks.

  3. For clearance above the stove, can any metal with 1″ air space work?
    My design puts the stove 5″ closer to the ceiling than recommended and I’m trying to figure out the best way to deal with it.

    Thank you!

    1. Jamaica-

      NFPA-211 specifies 24 gauge or thicker sheet metal. It doesn’t specify the type of sheet metal. You want something solid (no perforated sheets), and non-combustible (not plastic coated or paint other than high-temp paint). For an air-cooled shield attached to the ceiling, stainless or carbon steel, or even copper, aluminum or galvanized steel should be fine. I wouldn’t use aluminum or galvanized steel in contact with the stove or flue system (due to the lower melting points of aluminum and zinc), but attached to a wall or ceiling shouldn’t be a problem.

  4. Thanks for this information. It’s super helpful. I know that there needs to be an air gap of at least an inch below the heatshields (as well as behind and around). Quick question though, can it be higher than an inch off the ground? My stove has 6″ legs, and I’m curious if I could raise the bottom of my heatshield up to 3″. Thanks!

    1. Elizabeth Paashaus

      That shouldn’t be a problem as long as it doesn’t expose any combustible material to within 18″ of the stove. Heat rises so not as much heat will be radiating below the level of the stove body.

  5. i get the 18 ” clearance from all points of the stove itself, but i dont see any clearances mentioned for the stove pipe. do i have to run the heat shield all the way up?

    1. Chip-

      Yes, you need to take into account the clearances for both the stove and the stovepipe when you’re designing your installation. Different stoves and different pipes require different clearances, so you’ll need to consult the manufacturer’s specs. The Dwarf, for instance, requires 18″ clearance from the back and 16″ from the sides. Our single-wall stovepipe requires 18″ clearance to combustibles in all directions, and our insulated pipe requires 2″ clearance.

      Whether you need to run the shield all the way up the wall or not will depend on the details of your specific installation. Typically, if you need shielding around the stove, you’ll probably also need similar shielding around any single-wall pipe. If you’re not sure about exactly what you need to do or what your options are, shoot us an email with some pictures or drawings to support@tinywoodstove.com and we’ll be happy to help.

  6. Can you layer heat shields, as long as there’s at least an inch of space around each successive layer? For instance, use the heat shield that is sold for the dwarf 3k, then 2″ away set up another heat shield with 2 inches of space behind it? This is for a tiny home/mobile application, and I would prefer to avoid the weight of a masonry surround, if I can help it.

    1. Marc-

      That’s a great question. Can you install a second heat shield around your wood stove to reduce clearances even further?

      Intuitively, it seems like it would work. If one shield provides 1/2 or 2/3 reduction, wouldn’t a second additional shield provide an additional 1/2 or 2/3 reduction? Unfortunately, the reality is more complex than that. A second shield would likely provide some benefit, but it’s impossible to know what the actual minimum safe distance from combustibles would be without testing the assembly.

      Since the clearances around your wood stove are what keep your home and family safe, we recommend sticking to industry-standard clearance reduction methods. If you were to use air-cooled wall shields (2/3 reduction) along with the Dwarf Heat Shield Kit (1/2 reduction), our clearance recommendation would be no more than the 2/3 reduction allowed by the more effective of the two shields.

      Since you mentioned a masonry surround, it’s worth noting that covering the walls around your stove in masonry is not required, and does not provide any inherent benefit. Outside of the required clearances to combustibles, your surround can be wood, drywall, or whatever material you’re using for the rest of the room. Simply covering the surrounding material with tile does not provide any rated clearance reduction unless the tile is at least 3-1/2″ thick (a standard brick wall), and then only 1/3 reduction.

      Many professionally installed tile surrounds are actually part of an air-cooled or insulated heat shield, built according to the guidelines of NFPA-211. They’re often installed seamlessly enough to provide the appearance of a simple tiled wall, but they either incorporate 1″ of fireproof insulation or 1″ of air space between the backer board and the wall.

  7. What about floor protection? I’ve built a hearth recently that is stainless steel sheeting, over top of 2 layered 1/2 inch sheets of micore 300 (mineral fiber board) which is then mounted on wood. Is that sufficient to protect the wood?

    1. Jessica- Good question. Floor protection (hearth design) is a bit different from wall/ceiling protection. The stove tends to be closer to the floor than the walls/ceiling, and the amount of heat radiating or conducting through the bottom of the stove can vary quite a bit depending on the design.

      It would be best to check your wood stove’s manual to find the manufacturer’s instructions for building a hearth for that particular model. The hearth design you mentioned would be acceptable for our Dwarf Stove, but might not be acceptable for another model wood stove.

      Absent guidance from the manufacturer (or for unlisted stoves), NFPA-211 has some general guidelines for safe installation. This guidance is probably overly conservative for most wood stove models, so seeking out the manufacturer’s requirements is worthwhile if they’re available. The guidance varies depending on how much ventilated air space is below the stove. Stoves with taller legs and ventilated stands require less protection than those with shorter legs/stands or no legs.

      See NFPA-211 section 12.5.1.2 or consult with your installer. The requirements vary from 2″ solid masonry with sheet metal on top for stoves with 6″ legs or ventilated stands, to 4″ hollow masonry with internal air flow and sheet metal on top for stoves with 2″ legs or ventilated stands, to requiring a fully non-combustible floor (gravel, concrete, or earth with nothing combustible on the underside) for stoves with legs shorter than 2″.

  8. Hello, I’ve read your recommendations for clearances, including the installation of heat shields. But, I don’t see your recommendations for the floor under your Dwarf 3kw. I do see the 18″ dimension, but I don’t see your recommendations for the floor itself. I would like to place your heater over the steel floor of a bus.
    Thank you. R.B.

    1. Roger-

      Check out the Dwarf Stove Manual for installation requirements. Hearth design starts on page 12 of the current version. If you have a combustible surface like plywood subfloor under the stove with the standard legs, you’ll need a minimum 1/2″ thick non-combustible material covering the floor, extending 12″ in front of the stove, and matching the clearances to the sides and back. That can be 1/2″ cement board plus sheet metal on top, or 1/4″ cement board with 1/4″ tile on top. Thicker is always acceptable. If you’re using the wood storage stand or tall legs, you just need ember protection under the stove.

      If your floor is solid steel and there’s nothing combustible anywhere under the stove, including on the other side of the steel, then you could put your stove directly on the bare floor if you wanted. Most people would build some sort of hearth for aesthetics, but there are no strict requirements for hearth thickness if it’s not protecting anything combustible.

  9. Since we are including the “wood storage stand” and 4” legs. Does that change how we treat the floor regarding a combustible surface. Is concrete board/tile still needed?

    1. Aaron-

      Yes, if you’re using the Dwarf wood storage stand and/or the tall legs, that reduces the thickness required for the hearth. In that case, you don’t need the 1/2″ thick noncombustible material, only ember protection. 24 gauge sheet metal or similar material under and in front of the stove would be sufficient. See the Hearth section of the Dwarf Manual for info.

  10. Hi Dan! First off, thanks for being such a wonderful resource! I just bought a home with a WETT certified wood stove in Canada, which reassures me it has been installed safely. It has a large heat shield silver coloured metal heat shield which although functional, it quite an eyesore as it looks pretty industrial! I am wondering if it would be safe to either paint the heat shield black with high heat paint, or clad it with decorative tin ceiling tiles (white or black). Thoughts on whether this would compromise the safety/protection provided by the heat shield? Thank you!

    1. Meg-

      While I’d defer to the manufacturer’s instructions, local codes, and the judgment of your WETT inspector for a final decision, in general, there’s nothing in the NFPA-211 guidelines that would prevent you from painting a sheet metal shield with high-temp paint or making the sheet metal thicker by adding decorative tin to the face of the shield. As long as you’re not adding anything combustible to the shield or blocking the air space around/behind the shield, you wouldn’t be changing the type or performance of the shield by doing so.

      1. Awesome thanks Dan! And what about adding metal ceiling tiles as a “cladding” to the heat shield? That’d be my preference, aesthetically, and I’ve seen it online BUT per your post above.. you can’t trust the pictures!

        1. Meg-

          Adding another layer of sheet metal on top of the existing metal shield material should simply have the effect of making the shield metal layer thicker. NFPA-211 requires the metal to be 24 gauge or thicker, so adding additional thickness to the face of the shield shouldn’t negatively impact the shield’s performance. Keep in mind that the tiles need to be entirely non-combustible, so make sure the tiles have no combustible coating and make sure they’re actually metal and not plastic or something else. If the existing shield has gaps around the edges, don’t obstruct them since the gaps are needed for airflow behind the shield.

          If in doubt, you can always get a quick service call from your WETT inspector to verify your work is safe. The inspector will be familiar with local codes, which vary from place to place.

  11. My manufacturer says combustibles should be 400mm We have them at 225mm. If I placed a cementfire board 12mm onto that timber would this timber then become non combustable? Or would I need an air gap?

    1. Jeremy-

      Good question. This is a very common point of confusion when calculating clearances to combustibles.

      Per the NFPA-211 guidelines, simply covering the surface of a combustible material with a thin layer of non-combustible material provides no rated clearance reduction. The heat will conduct through the noncombustible material, and may heat the combustible material behind it to an unsafe temperature. The minimum thickness of the noncombustible layer needs to be at least 3-1/2″ thick (i.e. a standard brick wall) to provide a rated reduction with no air space or insulation in between.

      In your case, you need at least a 44% reduction to get from 400mm to 225mm. So, the acceptable methods would be either (1) to use a 1″ layer of fireproof insulation behind either a 1/2″ (12.7 mm) layer of cement board or 24 gauge (0.61 mm) or thicker sheet metal for a 50% reduction, or (2) to use the same 1/2″ cement board or 24 gauge sheet metal with 1″ of air space behind and around the perimeter for a 66% reduction.

      Note that both of the above clearance reductions are for a wall-mounted shield. Ceiling protectors are less effective than wall protectors, so the only option in this case would be the air-cooled shield for a 50% reduction to the ceiling.

      Keep in mind that the National Fire Protection Association which publishes the NFPA-211 standard is a US-based organization. Since we’re talking metric, I’m guessing you’re outside of the US. While physics tends to work the same across the pond, building codes might not, so be sure to check with your local permit office before finalizing your plans.

  12. when putting a heat shield in a corner, should there be a 1″ gap between the two side pieces or can they butt up to each other in the corner

    1. Chip-

      Good question. The side pieces should butt against one another in the corner so that there’s no line of sight past the shield to the corner.

  13. Nathan-

    Yes, you will still need to build a proper hearth underneath the stove. The Dwarf Heat Shield Kit reduces the required clearances to combustible walls around the stove, but does not affect the thickness of the hearth required to protect the floor. See the Dwarf Manual for detailed hearth requirements.

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