A basement knee wall is a poured concrete wall with a height of 2 to 3 feet, built to protect damaged basement walls. They are most commonly built in older houses where the existing basement wall is starting to crack or bow due to water seepage or lateral load from the exterior of the basement.
What is a Basement Knee Wall?
A basement knee wall or an engineered knee wall is a short-height poured concrete wall installed on the inner side of the basement foundation wall.
The height of a knee wall ranges from 2 to 3 feet. This, in effect, doubles the thickness of the foundation wall.
A knee wall supports an existing damaged basement wall and serves to protect it from further structural damage.
They reinforce the foundation wall by filling existing cracks and fissures and offering additional support to the basement wall.
Normally, new constructions do not require a basement knee wall, and they are commonly constructed in older homes.
This is because the basements in older houses are made of bricks, concrete blocks, and sometimes even stones.
Such foundations have hundreds of joints, and these deteriorate over time.
To maintain structural integrity, a basement knee wall can be built inside the existing basement wall.
This strengthens the old walls and improves their structural integrity.
They are called knee walls because their height is generally around the height of a human knee.
Generally, the height of knee walls ranges from 2 to 3 feet.
However, it is not absolutely necessary that knee walls are made at that height. They can be taller or shorter depending on your needs.
A permit from the local building department is also necessary to add a knee wall to the basement.
Need for Knee Walls in a Basement
A leaking basement and cracked foundation is a risky situation.
If you have a leaky or flooded basement or a cracked foundation, then a knee wall can be a good solution.
Additionally, cracked or bowed basement walls are a sign of damaged basement walls and need immediate action to avoid any further structural damage.
If you have a poor drainage system around your home, then this situation can get aggravated.
Seeing zig-zag cracking patterns in your basement wall is a strong indication that water is likely the issue.
When soil absorbs water, it expands and creates lateral pressure on basement walls. Most old basement walls are not designed to withstand lateral pressure, and this causes deterioration.
Whether you have finished or semi-finished basements, knee walls offer a good structural option to keep the basement dry.
Additionally, basement knee walls are important for walkout basements which are more prone to flooding.
Also, the soil around a home can be super absorbent. It absorbs water heavily and expands abnormally.
Another risky situation is when the roots of nearby trees encroach through your basement walls. As these roots expand, they create cracks in basement walls.
A basement knee wall is an economical solution for all these issues.
Another vital step in renovating your basement for structural reasons is reinforcing or replacing your basement beams.
How to Build a Basement Knee Wall
Building a knee wall can be a bit tricky, but it's possible to construct it perfectly if you understand the process.
Here's the step-by-step process of constructing a basement knee wall.
Step 1: Remove any Obstructions
As the first step, clear any hindrances near your basement wall.
These obstructions may be plumbing pipes, water pumps, or any other utilities.
Plumbing lines have to be reoriented if they are active.
Take the help of a plumber if necessary.
If you have a jack post or lally column near your basement wall, then that should also be either removed or reoriented.
Make sure there's no obstruction within 12 inches of your existing basement wall.
Step 2: Determine the Height of the Knee Wall
This step is crucial. You have to observe up to what height your basement wall is damaged.
After determining the damaged height of the basement wall, plan your basement knee wall’s height.
Normally, the height of a basement knee wall ranges from 2 feet to 3 feet.
Step 3: Installing Plywood Formwork
Next, you need to install formwork near your basement wall for pouring the concrete. Wooden ply boards of sufficient strength should be used.
The thickness of the plywood should not be less than ¾ inches.
However, if you are using dimensional lumber boards, the thickness should not be less than 1 inch.
Orient these boards in a straight direction and support them with vertical wooden posts at every 4 feet.
This will provide sufficient strength to boards and prevent them from collapsing after pouring concrete.
In addition, you'll need proper reinforcement of these boards. Inclined wooden posts should be installed at regular intervals.
These inclined wooden posts should rest on previously installed vertical posts.
Step 4: Pour Concrete
Opt for either ready-mix concrete or prepare the concrete mixture on your own by adding sand, gravel, cement, and water in the right ratio.
Transport the concrete to the basement and pour it into the gap between the wall and the forms.
Compact the concrete with a concrete vibrator. These vibrators will help you achieve better compaction and eliminate the voids or entrapped air from the concrete.
After filling the concrete up to the required height, smooth the top surface of the concrete.
Step 5: Remove the Forms and Cure your Concrete
The curing of the concrete is very important for achieving good strength.
Remove the forms 7 days after pouring the concrete.
If you want, keep them in position for a longer duration, but a minimum of 7 days is required for concrete to set.
Then, moisturize the concrete with water for the next 28 days.
Step 6: Framing of Your Basement Knee Wall
This step is optional. If you want to frame your basement knee walls with a wooden frame, then you can follow this step.
First, you have to insulate your knee wall with proper insulating material.
Next, prepare a wooden skeleton according to your knee wall and stiffen it on the top and bottom.
Knee Wall vs Pony Wall
Knee walls and pony walls serve distinct purposes.
Pony walls, which are 2 to 3 feet high walls separate different areas of your house without blocking the passage of air.
Moreover,unlike knee walls,they are common in balconies and rooms and are less critical members in a structure.
Alternatives to a Basement Knee Wall
While a basement knee wall provides structural support, there are alternatives that you can consider to achieve similar results.
| Alternatives to a Basement Knee Wall | Description |
|---|---|
| Liquid Polyurethane | Protect basement from water damage |
| Carbon Fiber Straps | Provide support to basement walls |
| Wall Anchors | Strengthen basement walls |
Liquid Polyurethane
Liquid polyurethane, when injected into the cracks, expands and hardens, sealing the cracks and preventing water leakage into the basement wall.
Moreover, it's a cost-effective and quicker solution but seals only small cracks.
Carbon Fiber Straps
Carbon fiber straps are strong,space-efficient solution for weak basement walls and require no excavation.
Place these straps on the wall, seal them with epoxy, and secure the top and bottom edges with the help of anchors.
Typically, the spacing between two straps is 4 feet.
After completion of installation, paint them with a desired color.
Wall Anchors
Wall anchors are another alternative to strengthen a deteriorated basement wall.
However,they are more expensive than carbon straps and require 10 feet of usable space outside the basement for perfect installation.
Mount a steel plate on your basement wall and bury another in your yard and connect them through a steel rod.
This rod, when tightened, creates a pull to hold both the plates ,providing additional support to your basement wall.
Usually, the spacing between two wall anchors is 5 feet.
While these are great alternatives to building a basement knee wall, you should also try to improve your home's exterior.
A better exterior can be a great way to prevent water from seeping into your home, which will make your basement knee wall much more effective.
Plus, it's also a great way to improve the value of your home!
Frequently Asked Questions
Can I Remove Basement Knee Walls?
It is possible to remove basement knee walls.
However,it is important to consult a professional and adhere to the local building codes to ensure safety.
What is the R-Value of Basement Knee Walls?
R-value indicates the effectiveness of the insulation in preventing heat flow through the walls.
The recommended value is R-13 to R 20.
How to Maintain Basement Knee Walls?
Regularly inspect knee walls for cracks,water damage and mold growth.
If any,consult a professional and address the issue promptly.
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