Photograph of EIFS synthetic stucco exterior (C) Daniel Friedman Best Practices Guide Exterior Caulks & Sealants for Buildings

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Exterior caulks & sealants guide:

This article discusses the selection and proper application of exterior caulks and sealants on buildings.

This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.

Our page top photo of an open caulk joint over a window demonstrates that even with the best-performing sealants on the market, workmanship remains critical if the sealant (or caulk) is going to actually work to keep water out of the structure.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Best Practices Guide to Caulks & Sealants for Building Exterior Use

Photograph of EIFS synthetic stucco exterior (C) Daniel Friedman

Adapted/paraphrased with permission from Best Practices Guide to Residential Construction. Steven Bliss.

While no residential exteriors should rely solely on caulks and sealants to keep water out, many details require caulk either to mask an expansion joint between materials or as the first line of defense against leakage.

Article Contents

Table of Caulk & Sealant Types & Applications

When choosing a caulk or sealant (another name for a high-performance caulk), look for a product that will bond well to the substrate materials and be sufficiently flexible to tolerate the anticipated movement (Table 1-13).

Table 1-13: Caulk & Sealant Performance below summarizes the properties of different types of caulks and sealants. [Click any table or image to see an enlarged, detailed version.]

[Click to enlarge any image]

Table 1-13 Caulks and Sealants (C) Wiley and Sons, S Bliss

Caulk & Sealant Standards & Specifications

For critical joints where movement is anticipated, choose a caulk that complies with ASTM C920 and is rated for +/- 25% movement.

ASTM C920 indicates that the sealant is highly weather-resistant, durable, and shrinks no more than 10%. For stationary joints, a +/- 12.5% rating for joint movement is acceptable.

As we comment at RADON REMOVAL INDOORS, HOW-TO, when sealing gaps and cracks, you should seal smaller holes with a high-grade elastomeric sealant conforming to ASTM C920-87.

Caulk Joint Design to Avoid Failures: Ideal Joint Shape & Caulk Joint Specifications

Figure 1-37 - correct caulking - Wiley and Sons (C) S Bliss

Just as important as choosing the right caulk or sealant for the job is paying attention to how the caulk bead is applied.

The best quality caulk will fail if applied 1 inch thick and bonded on three sides of the joint. Our photo (above left) of caulk over a building window shows another view of the poor workmanship in our page top photo, and explains that over this window there is a risk of building leaks.

The ideal caulk joint where movement is anticipated is an hourglass shape about twice as wide as it is deep (see Figure 1-37).

This shape allows a caulk bead to stretch without either failing in “adhesion” to the substrate materials or failing in “cohesion” by tearing itself.

A good rule-of-thumb is that a caulk joint should be four times the width of the anticipated movement, limiting the sealant’s stretching to 25%. For most residential building details, this requires at least a 1/4-inch-wide joint.

In general, the sealant should be no more than 1/2-inchdeep. For deep joints, it is best to pack the joint with abacker rod, a flexible foam material that controls the depth of sealant and shapes it into the hourglass profile.

Backer rod is made of either open-cell or closed-cell foam and comes in diameters from 1/4-inch to as much as 2 inches.

In wet locations, such as concrete control joints, use closed-cell foam, since it will not absorb water. Use a backer rod a little bigger than the joint being sealed.

Bond Breakers - Use of Backer Rods in Wide Caulk Joints

Backer rod for caulk joints (C) Wiley and Sons, S Bliss

In addition to controlling the depth and shape of a caulk bead, the backer rod acts as a “bond breaker,” preventing the caulk from sticking to the back side of the joint. A three-sided caulk joint tends to tear when the materials move. Corner joints subject to movement are also prone to fail.

For corner joints, use a small diameter backer rod or any other material that will not bondto the sealant. Plastic and foam tapes sold for weather stripping can work in corners (see Figure 1-38). [Click image for additional details].

Cleaning and Priming Requirements for Successful Caulk & Sealant Use

Since dirt, debris, and loose paint act as bond breakers, sealing to a dirty or flaking joint will fail when the joint moves. Also, the joint should be dry unless using a sealant approved for damp surfaces, such as some polyurethanes and some of the newer synthetic-rubber “Kraton” type sealants.

Do not use compressed air to clean the joints unless a line filter is also used, since the oil from the compressor may coat the joint, interfering with the bond. Although priming is not required for most sealants used in residential construction, some metals may need priming with acid-cure silicones. Consult the sealant manufacturer’s specifications.

Exterior Caulk Choices

Guide to Using Acrylic Latex Caulks & Sealants

The most economical and widely used caulking compound in residential work, acrylic latex caulks come in a wide variety of formulations and prices. To their credit, latex caulks are easy to apply, easy to tool, and can be cleaned before curing with water.

They bond moderately well to a wide variety of materials and have a long tooling time. When cured they are highly paintable, making acrylic latex popular for caulking paintable trim in both the interior and exterior.

Lower-end acrylic latex caulks do not have the same flexibility, temperature range, and long-term durability as butyl, polyurethane, or silicone. Newer premium products, however, promise performance on par with some of the high-performance sealants.

Added plasticizers make the material more flexible and other additives provide better UV and water resistance. For exterior work in joints subject to movement, look for an ASTM C920 rating and a rated joint movement of +/1 25%.

Most latex caulks cannot be applied under 40°F and should not be allowed to freeze in the tube or in place before cured. Also do not apply to wet surfaces or where rain is likely to fall before the caulk has a chance to fully cure.

Guide to Using Butyl Caulks & Sealants on buildings

Butyl is a high-quality, tough, rubber like sealant that is ideal for exterior jobs requiring a durable, watertight seal.

Because of its longevity, temperature range, and high UV resistance, it has long been used as a glazing compound. Notable for its stickiness, butyl bonds very well to a wide range of materials, including wood, concrete, masonry, glass, and metal. Its stickiness, however, can also make its application messy and tooling difficult. Before curing, it can be cleaned with mineral spirits.

Because of its good adhesion and water resistance, butyl is often used to seal metal gutters, metal roofing, and around foundations. It is approved for use below grade. Butyl should not be used, however, in contact with modified-bitumen flashing tapes or roofing membranes, which can degrade it.

Guide to Using Polyurethane Caulks & Sealants on Building Exteriors: Urethane Caulks

Polyurethane is a versatile, water-resistant, high performance sealant and has become the first choice of many contractors for exterior work. Polyurethanes provide excellent adhesion to a wide variety of materials from wood to masonry and remain flexible across a wide temperature range.

Furthermore, they are relatively easy to tool, and some brands accept wet tooling with soapy water. Tooling time is adequate and shrinkage minimal. Polyurethanes are available in only a few colors, but the cured sealant holds paint well.

Although polyurethane is not naturally UV-resistant, UV inhibitors give it good durability in exterior applications. Because of its aggressive bond, polyurethanes are good for sealing between different materials. Polyurethanes are widely used on metal roofs, concrete and masonry control joints, flashing, and exterior trim.

OSI Sealants, (Mentor OH, 800-999-8920) produces urethane caulks.

Guide to Using Silicone Caulks & Sealants on Building Exteriors

Silicone sealant on horizontal trim (C) Daniel FriedmanSilicones bond well to nonporous surfaces, such as glass,tile, and metals, and they are the most flexible sealants made. A good silicone will stretch as much as 50% of its original width before tearing. Silicones are good in cold temperature work and can be applied from well below 0°F to over 100°F.

Once cured, silicone caulks and sealants can also tolerate temperatures from well below 0°F to about 400°F, or higher for special high-temperature formulations.

Unlike most sealants, silicone stays flexible when cold. Silicones are also very resistant to UV radiation and water, making them a good choice for exteriors as well as kitchens and baths.

The main disadvantages of silicone are that it is messy to work with, difficult to tool [notice the irregular bead of silicone caulk in our photo above], and does not hold paint well. Cleanup when wet requires acetone or special-order silicone solvent, and the residue is hard to remove when it is time to reapply.

Because of the residue, once you’ve sealed a joint with silicone, it is best to reseal with silicone as well. Silicone does not bond well to unpainted wood and can stain or degrade porous stone and masonry materials.

Silicones come in two types: acid-cure (acetoxy) and neutral-cure (sometimes called “noncorrosive” silicone). The acid-cure type has a distinctive vinegar like odor. Both types will stick well to glass, ceramics, and other nonporous surfaces. Acid-cure silicone, however, requires primer with most metals to bond well and to avoid corrosion. Neutral-cure silicones are compatible with most metals and metal finishes and bond somewhat better to wood.

-- Above text was adapted with permission from Best Practices Guide to Residential Construction.

Choosing a Caulking Gun for Easy Fast Caulking Work & Good Results

Caulk selections and gun (C) Daniel FriedmanDetails about choosing a caulk gun are at CAULK GUN TYPES, CHOICES. Excerpts are below.

Caulking Problems, Discussion, Possible Solutions

Question: how to get better results using silicone bath tub and tile caulk

(Oct 21, 2011) Ron said:

I have used silicone in the past and have found its performance to be lacking. I have used silicone to seal pvc windows and trim together, to add a preventive measure to my installation. Even though the application is approved on the tube I find that the expansion and contraction ratios of the pvc soon depletes the silicone of its adhesive nature and fails within a year or so.

Also I have read that silicone after curing doesn't allow anything to adhere to it, not even silicone.

So if I have a silicone joint that has failed I cant repair the joint until all of the silicone has been removed, even a thin film. Since not all surfaces will tolerate an acetone cleanup, on occasion I have had to sand the application to insure a good bond.
Indoor in showers it is especially difficult to prep the surface for a silicone repair. I Have tried numerous solvents but I am limited to what will not harm the tub surround or the sealer in the tile. I also find that the silicone based sealers for tile grout don't allow for a strong bond for much of anything to adhere to it.

I am explaining this because I would like to know some of the preferred sealing methods for common repairs, i.e. tub surrounds tile floors pvc windows and exterior pvc trim. Also how to prepare such surfaces in a way that would accept a bond from silicone or a poly.

As I stated before sometimes a solvent cannot be used, like on stucco or wood without compromising the integrity of the substrate.
I am very happy to have discovered your website and will use you often as well as share you with my friends. Keep up the good work!


Thanks Ron. Steve Bliss made some suggestions in the article above.

When I'm using any caulk in a bathroom, because I hate doing it twice, I cut away old caulk with a single edged razor blade or a similar tool, then clean the surface with a clean rag and alcohol before applying the caulk. You could use other cleaners but don't want to leave any surface deposit.

Soap scum can be sneaky - leaving a deposit on what looks-like clean ceramic tile, as can oils used in the bath in body wash, after-bath skin lotions, and some hair treatments.

I've used acetone too but don't recommend it in general as when used in quantity (more than removing fingernail polish) it's a more dangerous chemical and is more easily absorbed through the skin - for dopes like me who used to work with it bare-handed.

The only trouble I've had with sealant failures after this careful preparation was when I used a sanded caulk to match adjacent tile grout at the wall-to-tub joint and where I asked the caulk to bridge too wide a gap. As the sealant dries it shrinks and cracks.

Question: how to seal a sewer pipe to the septic tank

2 Feb 2015 Bill said:
what do you recommend for sealing a plastic sewer drain pipe into the septic tank at the entry to the tank ( ABS to concrete tank ) ?


Concrete for large openings, silicone sealant on clean surfaces for small ones.

Be sure to remove all soil at the tank opening or your bond may leak.

Reader Question: how to seal windows & doors in a Scottish Red Sandstone Cottage

9 Feb 2015 Rick said:

What is the Best exterior caulk sealant to use in the following application. I am fitting sash and case pine framed wooden double glazed windows into a red sandstone cottage. The cottage was built in 1834 and the sandstone is 18" - 24" thick. The Red Sandstone is a local stone to the east coast of Scotland. It can be very porous and has a tendency to crumble against the 'more firm less flexible' caulks. Suggestions and reasons for would be appreciated thanks.


Good question, Rick. If the sandstone is very soft or friable I would not trust any conventional sealant. I'd expect the sealant just to bond with the loose surface and thus not to be durable.

How did people set windows and doors into walls built of this stone a hundred years ago? Perhaps you or I can find a building conservator who knows the answer.

For a modern approach you may need to treat the sandstone first then use a sealant.

See this reference which is my first choice:

For background and additional detail

And if fungal contamination or deterioration is a problem - as it is on some existing buildings built using Scottish sandstone, see

Reader follow-up:

18 Feb 2015 Rick said:

Thanks for the great resources. IIm off to do my research before deciding what the best approach is. Your question about what they used 100 plus yrs ago is a great place to start. Those builders knew a thing or two about the properties of the red sandstone. They knew it was both porous a breathable. It would get wet in the rain but dry out in the wind and sun.

The sandstone itself would regulate humidity and dampness. One of the biggest problems in recent years has been applying cement based mortars to the joints which traps moisture in, which then creates dampness and increased humidity.

The old lime mortars which are porous after application helped to decrease joint crumbling, maybe I should investigate that too. thanks again for the resources I'm looking forward to finding the best solution. - Kind Regards Rick


We agree, Rick. An analagous problem I've seen in both the U.K. and in North America is the selection of modern mortars to re-point old brick walls. The harder modern mortar often leads to brick damage from its different response to temperature and moisture variations, and it also often is such a poor match for the original mortar colour and texture that it's nothing short of a mess.

Here's an example

Brick wall tuck pointing bad job (C) Daniel Friedman


I've also seen some rather horrible attempts at "repairing" spalled brick walls and lost mortar from various masonry walls using caulk.

Where to Buy Exterior Caulks and Sealants

Construction Sealant (Caulks) Suppliers & Products

Polyurethanes and other high-performance sealants

These companies produce urethane caulks used widely on masonry surfaces and in the automotive industry.

Acrylic latex caulks

Exterior Adhesives


Continue reading at CAULKS, NONTOXIC or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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