When it comes to the biggest problem with having a basement, most can agree that moisture is probably the worst. There are many things that can be done to reduce moisture in the basement, but some are expensive and difficult. One way to reduce moisture has to do with the structure of the basement’s walls and can be executed right at the start of your building process: gaps between the framing and your basement’s walls.
The purpose of the gap is simple; it prevents moisture in the basement’s masonry wall from coming into contact with the wooden frame. However, there are alternatives to the gap, so why go for the gap? Here, we look at what the building code says (because it’s always the first place one should start), then we will go through the details and pros and cons of the gap, and finally, we end with a comparison of a gap and the main alternative to help you make your decision.
A gap between a basement wall and framing will fulfil its purpose of preventing the wood frame from absorbing moisture from the foundation wall. It is vital if the framing wood is not preservative treated. But, comparing a gap to treated wood with no gap reveals that treated wood is the better option.
Is Gap Between Wall and Frame Compulsory?
The purpose of the gap is to provide physical distance between the foundation walls and the wood framing, so moisture cannot be transferred. This protects the integrity of the framing and ensures that the construction work is long-lasting.
This is a very important part of building in a basement. However, as per the International Residential Code (IRC), a gap between your wall and frame is not explicitly necessary. But why? Well, let’s look at what the IRC says.
Section R317.1 of the IRC talks about where wood must be protected from decay by using naturally durable wood or preservative-treated wood. Point 2 in this list says the following:
“Wood framing members, including columns, that rest directly on concrete or masonry exterior foundation walls and are less than 8 inches (203 mm) from the exposed ground.”
So, while the IRC does not explicitly say that you do or do not need a gap between the masonry walls and the frame, we can deduce from the above section that wooden frames can directly rest on the masonry walls provided they are protected from moisture.
Alternatives to a Gap
Pressure-treating is the process of injecting a preservative into the wood by using high pressure. This preservative protects against moisture as well as insects.
Since moisture and insects are especially injurious to wood, protecting wood against them greatly increases the lifespan and durability of wood. The lifespan of pressure-treated wood can span decades.
Naturally durable wood is brought up by the IRC, but what exactly is it? Natural durability refers to wood’s ability to fend off fungi decay as well as insect attacks. How naturally durable a type of wood is will depend on the location, age, and growing conditions of the wood.
Either pressure-treated wood or naturally durable wood should be used for the framing of your basement wherever it is in contact with the ground or exterior foundation walls, but a gap between your framing and walls is not required by building codes.
Recommended Gap Dimensions
If you’d still like a gap between your framing and wall, the recommended distance is about 4″ between the framing and your basement wall.
Leaving a gap between your framing and walls provides a place for humidity to be trapped so that it does not enter the main part of your basement through the walls.
Direct contact between the framing and the walls can lead to easy movement of moisture to the walls. This moisture can infiltrate your basement. It is one of the causes of basement humidity.
To limit this moisture migration, 4″ is just the right distance.
Too large of a gap (beyond 4″) would waste building materials for a space that isn’t used or needed and can also increase the likeliness of larger animals living in this space since there is room for them.
Are Vapor Barriers Necessary With Gaps?
In connection to where moisture-resistant woods are required, Section R317.1.6 of the IRC states:
“Wood structural members supporting moisture-permeable floors or roofs that are exposed to the weather, such as concrete or masonry slabs, unless separated from such floors or roofs by an impervious moisture barrier.”
This specification is referring to what locations of the home must have naturally durable wood or preservative-treated wood as framing.
The framing of a basement would fall under “wood structural members supporting moisture-permeable floors or roofs that are exposed to the weather.”
Because of this, the framing must be either naturally durable or preservative-treated, or there must be a vapor barrier installed between the framing and the basement wall.
The building code specification is not specific to gaps between framing and walls, though.
The understanding is that a gap is a sort of moisture barrier within itself. It is not foolproof, though. Moisture can still gather in the gap and infiltrate the walls. In this case, a vapor barrier would be very useful.
Benefits of the Gap
Even if a gap is not required by building codes, it can be highly useful in controlling both the humidity and temperature of your basement.
Water can seep into the framing and travel to the walls of your basement when there is direct contact between the framing and walls. A gap of the right size can stop this completely.
Deterring Water Leakage
A large enough gap between framing and walls can also be useful in the event of large amounts of water leaking. The space would be great for trapping said water. Without a gap, the water could easily spread into your basement or other areas of your home.
A gap can serve as an intermediate between the harsh temperatures of the outdoors and your home. For example, in the winter, it will be warmer than directly outdoors but colder than the home.
This is energy-efficient since your basement wouldn’t necessarily be in direct contact with the outdoors. The gap would serve as a barrier, and then the insulation of the basement, as well as the heating/cooling unit, would do the rest of the work.
Drawbacks of the Gap
Loss of Space
When you opt for installing the framing with a gap between the wood and the foundation wall, you are losing floor area. While this may not be problematic when you are just framing perimeter walls, as soon as you start to subdivide the basement, the 4″ gaps add up.
Although 4″ isn’t much space for most creatures, this is plenty enough room to live for some. Small rodents and insects might find the gap between your wall and the framing a quite pleasant place to live.
This gap would provide shelter from extreme temperatures, rain, and wind. It would also be relatively undisturbed by humans since access to it isn’t readily available.
With these factors in consideration, this gap might prove too tempting to live in for invading critters.
Evicting these pests would also prove a problem, since, as said before, the gap is not easy to access. Opening up this space would require repairs down the line and would mean that the invaders could escape into your home.
A benefit of a gap between the framing and the walls of your basement would be that it would protect the rest of the basement and house from flooding, potentially. But there are some cons that go with this as well.
Water that does enter this gap will not be easy to drain. The gap must be breached and drained.
The water stuck in the gap would either leak outside or worse, into your basement. If water stays in the gap too long, it can structurally damage the framing, reducing its integrity as well as its lifespan.
Gap vs Pressure Treated Wood
So, if you can have a gap or preservative-treated wood for interior framing, which should you choose?
In terms of cost, a gap is going to be more affordable. Preservative-treated or naturally durable woods are more expensive to purchase. There is also the added expense of resealing edges where they are cut to size.
As a side note here, I wanted to mention that I recently found out that Home Depot will cut your wood for you, even if it is pressure treated!
However, the masonry wall of a basement is not the only source of moisture in these rooms, so preservative-treated wood would protect your framing from all sources of moisture, while the gap only protects it from the foundation walls themselves.
The preservative injected into the wood heightens the integrity of the wood itself. It is less likely to be damaged by water and insects because of this. It has no effect on limiting humidity, though.
This is mainly for the structural integrity of the wood, though, not to prevent humidity in the basement. It can help a bit, though, since not as much water will be kept in the wood since it isn’t as permeable. This limits the migration of the water in the framing to the basement walls, which not only protects your basement walls but also the house in general as damp basements can affect the upstairs.
Furthermore, the process of treating the wood makes it more resistant to non-moisture-related damage. Thus, in this respect, a preservative-treated or naturally durable wood is going to be the more long-lasting option.
A gap is not going to be resistant to insects, such as termites, while preservative-treated wood is resistant to insects. In fact, the 4″ gap creates a space for any number of pests to occupy within your walls without you being aware until you are overrun.
One final point of comparison is the space that you lose with having a gap. While 4″ does not seem like much, when it is 4″ in 4 walls, it adds up. Basements are usually large spaces, so this might be a moot point, but it is worth mentioning.
Another point worth mentioning is that preservative-treated wood is important for the strength of your framing, but it is not a foolproof method of limiting the amount of moisture entering a basement since the framing is still in direct contact with the walls. A gap or vapor barrier can do this.