6 Surprising Project Types You Might Not Think Need a Foundation Vapor Barrier

The Benefits of a Vapor Barrier Beneath Additions, Manufactured Homes, ADUs, and Other Non-Traditional Dwellings and Residential Structures  

For many, building a new home from the ground up can represent many things – a dream realized, a haven for a growing family, a new chapter for empty nesters, etc. At the very least, it will be an investment in time and money to bring to reality. So, it’s no surprise that homeowners, builders, and design professionals pay close attention to getting the details right. Especially when it comes to the building enclosure, product choices made during design and construction often play a critical role delivering long-term peace of mind, lowering home ownership costs, and creating a durable and healthy home for generations to come.  

Isolating the home from the ground with effective vapor protection is an important component in achieving these goals for a traditional site-built home. A high-performance and properly installed vapor barrier at the home’s foundation will impede water vapor and soil-gas (like radon) from the ground, which helps improve indoor air quality and protects flooring and other building materials from moisture-related damage.  

But it is not just full home builds that benefit from taking the ground out of play. In this article we look closer at some of the other applications—less-traditional dwellings and structures—where incorporating foundation vapor protection from the start can deliver long-term value and peace of mind.   

#1 Upgrading Home Improvement or Renovation Projects with Effective Vapor Barriers 

A variety of home improvement and renovation projects offer an opportunity for homeowners to increase usable space, add value, or simply enhance the aesthetics and functionality of their home. In many of these project types, an effective, high-performing vapor barrier may be a useful or critical component to protect that investment.  

Example Projects include: 

• Crawl Space Encapsulation: Vapor barriers can, and should, be installed in existing homes with crawl spaces to cover the ground and be sealed/attached to foundation walls. High-performance and properly installed vapor barriers are critical to crawl space encapsulation, which can have a tremendous impact on home performance, indoor air quality, and energy efficiency.  
  
• Additions: Expanding the square footage of a home with an addition often includes placing a new concrete slab. There may also be renovation situations where an existing concrete slab is demolished and a new one placed. In these examples, there is no substantial (or likely code-enforced) difference to a new home build…meaning the below-slab vapor barrier is important to isolate the new floor, and parts of the home above, from soil threats. This is particularly true when the new slab will have conditioned space above, such as an added bedroom or office.  
  
• New Flooring Over an Existing Concrete Slab: Vapor barrier sheet membranes can be installed on top of existing concrete floor slabs and beneath floating or fastened wood subfloor and/or hardwood systems. This may be common when changing out old, outdated, or failed flooring to something new. Especially in older homes, a vapor barrier on top of the concrete slab before wood-based flooring has benefits (and is a referenced step by the National Wood Flooring Association). This is often because a proper or effective below-slab vapor barrier was omitted during original construction, and the concrete slab may have high moisture levels that could negatively affect a successful wood floor installation.  

In some home renovations, a vapor barrier can potentially be used beneath an unbonded topping slab. You can learn even more about vapor barriers and topping slabs here.   

#2 Do Manufactured and Modular Homes Need Vapor Barriers? 

As opposed to the traditional stick-built home, where building materials and components of the home are constructed on-site, a prefab home (aka prefabricated home) generally has full sections built in a factory and then assembled on the site.  

There is a lot of buzz around prefab homes in the homebuilding industry. Since the home (or sections of it) is built in a controlled factory setting, it can offer benefits related to fast construction (no weather delays), consistent quality (in-house labor), less waste (reduced material damage), and affordability (construction efficiency and labor savings).  

Although prefab homes still make up a relatively small percentage of new home construction (about 3%), it can be a logical and appealing delivery method for many homeowners.  

The two most common types of prefab homes are Manufactured (or Mobile) and Modular. Although similar, here is a snapshot of some key differences: 

• Manufactured Homes: completely built in a factory on a steel frame then transported to a site. They are anchored to a constructed foundation but can be relocated. Manufactured homes were formerly (before 1976) known as “Mobile” homes. The terms are still commonly used interchangeably, but Manufactured is often the preferred terminology today. Manufactured home construction conforms to HUD (Housing and Urban Development) code rather than local jurisdictional building code since they have the ability to be relocated.  
• Modular Homes: generally built to about 80-90% completion in sections off-site in a factory setting then transported and assembled on-site. Modular homes are placed on a permanent foundation, making them essentially identical to a traditionally built home. Thus, Modular homes must conform to the same local building codes. Modular homes generally cost more than a Manufactured home, but they also tend to allow for more homeowner customization akin to a traditional home build.  

One similarity between modular, manufactured, and other types of prefab homes is that they all require a completed foundation before the home can be set down or assembled. A properly designed foundation is critical in supporting the home’s structure and can also play an important role in keeping the home dry, healthy, and comfortable. 

The typical foundation types you would expect to see in a traditional site-built home, are the same options for prefab homes: slab-on-grade, crawl space, and basement. In these cases, effective vapor barriers below-slab or lining the ground of a crawl space will offer the same benefits as constructing the home on-site, including impeding soil vapor risks and helping prevent moisture-related issues to the flooring system.  

Additionally, pier foundations are uniquely common to prefab homes, especially Manufactured homes as they lend themselves to the non-permanent approach. In this case, vertical piers or piles (generally concrete, steel jack stands, or masonry blocks) driven into the ground are used to lift the home off the ground and level. For manufactured homes, the piers will feature an anchoring system to connect to the steel frame on the underside of the home and then a skirting is often added around the perimeter.  

Important Note: Manufactured Home standards from HUD reference the use of minimum 6-mil polyethylene sheeting for ground moisture control, but the drawbacks of generic poly sheeting at the foundation (particularly long-term durability) should be considered.   

In many cases, homeowners work with prefab home manufacturers on their plan selection and design choices of the home, but the foundation may be handled separately with a structural engineer based on the prefab home specifications. So, there can be some autonomy for homeowners to help drive decisions around foundation vapor protection products with their design professional or foundation contractor, based on their risk tolerance, preferences, and budget.  

#3 Should You Use a Vapor Barrier Under an ADU?

This is another category of structures that goes by many different names – Accessory Dwelling Units, Additional Dwelling Units, Auxiliary Dwelling Unit, Backyard Cottage, Granny Flat, etc. By definition, an ADU is a secondary dwelling unit that has kitchen and bath facilities, as well as a dedicated entrance separate from the primary home. An ADU shares the same lot as the primary residence and is therefore a part of the owner’s property in terms of taxes and overall property value. An ADU can be detached (or freestanding) from the main home, but can also be attached – below, above, or on the side.  

From a practical perspective, ADUs have become increasingly common in offering utility for multi-generational families, providing rental income, increasing home value, and offering affordable housing particularly in dense urban areas. As such, more and more jurisdictions are supporting ADU construction in their zoning regulations.  

Code Compliance for ADUs: The design and construction of ADUs must follow the same local jurisdictional building code requirements as traditional single-family homes. Even more, especially in the case of freestanding ADUs, they are typically (or even required) to align with the design of the main home structure on the property. Whether constructed with a concrete slab or crawl space foundation, ADUs will need an effective vapor barrier.  

#4 What About Tiny Homes? 

The lines between ADUs and “tiny homes” can be a bit blurry. ADUs are permanent dwelling units on properties, so tiny homes on wheels (intended to be moved) do not typically qualify as an ADU. Beyond zoning considerations, if a “tiny home” is built on a permanent foundation, its vapor protection needs will be similar to an ADU. 

#5 Vapor Barriers for Workshops, Garages, and Barndominiums  

Garages, or free-standing workshops, are a common addition to homes or properties offering useful space for vehicles, storage, hobbies, etc. A close cousin is the “Barndominium”, a building that combines living space and a workshop (often known as Barndos).  

Given the typical functions, these spaces often use the concrete slab as the finished floor, potentially polished or coated for aesthetics, ease of cleaning, etc.  Workshop and garage structures are often less likely to receive a flooring system, especially ones considered “moisture-sensitive”, such as carpet or hardwood.  

With that said, below-slab vapor protection can still offer benefits for concrete floor slabs which do not receive a moisture-sensitive floor from the start.  

A main consideration is the future use of the garage or workshop – and flexibility or adaptability – over time. As a simple storage or project space, the use of a bare or exposed concrete floor may work well for the initial, practical function. But if these spaces are converted (which may potentially mean adding a moisture-sensitive flooring material or coating) the opportunity to install the cost-effective vapor protection beneath the slab has long passed.   

Even more, it is not just flooring materials that can be vulnerable to unchecked water vapor transmission. Any relatively impermeable material can act like a resilient flooring system, including rubber gym mats and stored goods or boxes. Water vapor accumulation and potential condensation can lead to unwanted mold growth, material damage, or efflorescence – a chalky white alkaline substance left as surface condensation evaporates off the concrete slab.  

There are very few instances where allowing the free movement of water vapor or soil gases from the ground is desirable. For many homeowners, adding a below-slab vapor barrier amounts to cheap insurance and added peace of mind, even in parts of the home historically thought of as less sensitive or critical, like the garage or a freestanding workshop.  

#6 Protecting Your Shed with a Proper Vapor Barrier  

A backyard shed can offer homeowners extra storage, especially for potentially hazardous or dangerous items best kept out of the house and away from kids or pets. In addition, some sheds can give homeowners extra room to work or play and may help increase curb appeal or resale value (especially if constructed well).  

Although sheds could be constructed on-site, the most commonly available options are prefabricated and can be delivered to the site and simply placed on a prepared pad.  

The specifics may vary, but a common approach for a shed’s prepared base is a crushed and compacted stone (or gravel) pad. A gravel pad is relatively affordable (especially compared to placing a concrete slab) and may be right up the alley of a willing Do-It-Yourselfer.  

Most sheds are unconditioned and utilize a wood-based floor system – skids and/or subfloor. A vapor barrier will help isolate the bottom of the shed from any potential soil moisture movement. In areas with subterranean termite concerns (remember termites love wood), there is innovative barrier film technology available for both water vapor and termites.  

Even more, installing a vapor barrier for your backyard shed build is a relatively simple job for any handy homeowner, with product sizes available that won’t break the bank.  

Why Non-Traditional Structures Need Foundation Vapor Barriers Too 

From building a dream home to DIY home improvement (and everywhere in between), taking the ground out of play with a vapor barrier is a smart investment – especially when it is a relatively low upfront cost for long-term performance and peace of mind. But not all barriers are created equal, so it is critical to consider the options.  As the industry-leader in vapor barrier solutions, our team of experts can help evaluate what’s right for any project type – whether you're tackling a traditional build or exploring innovative uses for vapor barriers in non-traditional structures. We're here to make it easier to build healthier, more durable spaces from the ground up.