The popular siding brand offers clear instructions on this tricky question
We do not recommend caulking Hardie board siding in most cases, but it depends on the placement and project.
The manufacturer’s instructions may indicate adding a permanently flexible sealant in some areas.
Adding too much caulk to Hardie Board can lead to water damage or lack of flexibility.
Debating whether to use caulk or not on Hardie Board siding can send you down a bit of a rabbit hole. The short answer to the caulk debate is: it depends. Hardie Board—the popular brand of fiber cement siding—comes with detailed instructions for installation. While flexible sealant is more common, using caulk highly depends on its placement, so there is no one-size-fits-all answer.
Caulking will typically depend on the type of joint for your Hardie board siding. The manufacturer’s instructions will specify whether to use caulk, sealant, or neither in that particular spot. Installers typically add flashing behind butted joints and avoid caulking to encourage property water flow. Securing Hardie Boards with caulk often goes against the board's natural design to shed water. Adding caulk can inadvertently trap moisture in your siding and lead to rot.
Some instructions include adding a continuous bead of sealant around the outer edge of an area but specify to leave a full side without it. In other cases, such as around wires and pipes, you'll find instructions on how to caulk Hardie Board around specific areas. For the most part, however, caulk itself is rather rare.
Installers should only use "permanently flexible" caulks or sealants on Hardie board siding.
We also recommend using a caulk:
Categorized as an elastomeric joint sealant OR
Categorized as a latex sealant
That has a longevity of 50 years
That is paintable
Bonds easily without primer
Your instruction packet may also discourage repairing Hardie Board with caulk or patches for areas larger than a dime.
Fiber cement siding took off in the 1980s with the James Hardie company at the helm. The popular siding is a composite of water, fly ash, wood pulp, and Portland cement. Consult a local siding contractor to determine whether this type of siding is a good fit for your home based on home climate, age, and size.
The cost of Hardie board falls in the middle of what's on the siding market, with prices between $5 and $14 per square foot. Keep in mind that DIY Hardie Board installation is not ideal, since it's both heavy and difficult to cut without excessive dust. Installing or repairing Hardie boards yourself could void your warranty in some cases.
Hardie board is highly durable, has a Class A fire rating, and is sustainable to produce. Homeowners also enjoy that the siding is very versatile and can mimic other major siding options without the downside of rot or rust. When comparing Hardie board and vinyl siding, Hardie board lasts between 10 and 20 years longer on average.
The confusion about whether to caulk or not is only one of the tricky problems with fiber cement siding. Installation can be complex, leading to higher prices and time frames. Fiber cement has the tendency to absorb moisture over time, leading to potential mold or damage issues. While Hardie board and fiber cement are comparably energy efficient, you will still need to pair the siding with insulation to ensure a high R-value in areas with extreme temperatures.
Professional installers typically recommend caulking between siding boards, but it depends on the material. With Hardie board and fiber cement siding, manufacturers don't recommend adding caulk because it can trap moisture in the joints or discourage flexibility in fluctuating temperatures. Consult your local siding contractor about caulking best practices before adding any to your home’s exterior.
Either ⅛-inch or ¼-inch gaps throughout Hardie board installation allow for expansion throughout the seasons. Metal flashing will typically protect against water damage between the boards, but this highly depends on the board's placement on your building. It's always important to refer to the manufacturer’s specific installation instructions or rely on your siding contractor.
