“How much crawl space vapor barrier should I order?” is undoubtedly the question I get the most from homeowners (and even contractors and homebuilders).
Usually, the homeowners I talk to have been researching what kind of vapor barrier they want to install (or have installed) in their crawl space. So, by the time they reach out to me they are looking to figure out how much they may be spending on material costs for the project or making final price comparisons.
If you’re at that point in your project (or even just preliminary calculations as you research), this article is for you. But, before you can compare and shop, you first need to calculate how much material you actually need.
Below are the simple steps you can take to calculate how much material you will need for your crawl space vapor barrier installation. Once you know that, you can easily estimate the total material cost for your project based on the products you’re looking at. Learn best by watching a video? Click here.
CALCULATION KEY
Before we dive in, here are some frequently used variables we will use to calculate the material. Download and use this helpful Worksheet for your specific project.
P = Perimeter Wall Segment Length
H = Perimeter Wall Height
L = Total Perimeter Wall Length (in Linear Feet)
G = Total Ground Cover
W = Total Wall Cover
S = Size of Column (Linear Feet Around)
C = Total Column Linear Feet
F = Overage Factor (Value Likely Between 10 and 20%)
Z = Total Square Footage of Vapor Barrier to Order
Start by measuring the length of the perimeter foundation walls in your crawl space. A regular old tape measure is fine, but laser distance measuring devices are pretty handy if you have one. If your crawl space is (or fairly close to) a rectangle or square shape, you may only need to do a couple measurements. For example, in the illustration below, a rectangle-shaped crawl space would really only need two adjacent walls measured (P1 and P2).
Of course, some crawl spaces have more complex or odd layouts. If that’s the case for you, you could get a decent estimate by simply taking the longest width and longest length span in the crawl space. But, if you want to get more detailed, it just means you’ll want to work your way around the perimeter of the crawl space measuring the length of each straight wall segment of the entire foundation wall. Not following? Read on, I’ll get to total linear feet at the end of this section.
Pro Tip: Be sure to write down the lengths or note them on a quick sketch of the crawl space layout. Note: if you happen to have a crawl space with an interior wall, make sure to measure its length and note it as well.
Now use the length measurements of each wall segment to calculate the Total Perimeter Wall Length in Linear Feet (L). In the simple rectangle-shaped crawl space example it is as easy as adding up all four sides or: L = (P1 x 2) + (P2 x 2)
If you took the approach of working your way around the crawl space add up all the lengths you measured on your perimeter wall to get L.
While you are still in the crawl space, measure the Perimeter Wall Height (H). Measure from the ground to the top of the wall. Many crawl spaces may have the exposed ground graded to a low point to allow for drainage (sloped), so it is possible to have varying wall heights around the foundation. You could certainly measure in different spots and map it out (this may be a good approach if you have some very noticeable differences in foundation wall heights) or use an overall average. However, I would suggest taking a conservative approach – measuring what looks like the highest wall section and using that number.
Ultimately, this Perimeter Wall Height (H) value will help calculate how much vapor barrier material you need and depends on how high up the wall you extend the vapor barrier. For this article, we’ll assume you are taking the vapor barrier pretty much all the way up the wall. But keep in mind there may be local code requirements (e.g. a termite gap between the top of vapor barrier and top of wall/sill plate) or other considerations that influence how high up the wall the vapor barrier goes. If you need, or want to stop, the vapor barrier at a specific distance up the wall, then simply make that specific distance your Perimeter Wall Height (H).
It is also important to seal the vapor barrier to interior support columns or piers for a complete system. These columns won’t significantly affect the amount of vapor barrier material needed for the job because you typically won’t need to extend the vapor barrier up the column too high (unless you want to for aesthetics). Rather, calculating the total linear feet around the columns will help estimate the amount of accessories you need when sealing to them.
Most crawl space columns, piers, or pier pads are square or cylindrical shaped (see illustration below). For square columns measure the length of each side and add them up to get the total linear feet around the column (S). For cylindrical columns just measure the total circumference, which also gets you (S).
If it were me, I wouldn’t measure every single column, because they’ll most likely be about the same size. Just measure one that’s representative of them all and go with a conservative approach. Then, count and note how many columns you have total (C). Now you know the size of your columns and how many you’ll have to detail around and seal to.
The purpose of the vapor barrier is to encapsulate, or fully line the crawl space, in order to help keep out moisture and soil vapor. If that’s your goal, you need enough material to completely cover the exposed ground of the crawl space and seal it to the perimeter walls. To figure this out you will first want to calculate how much ground and wall area you have to cover.
Ground Cover Calculation
Let’s start with calculating the Total Ground Cover (G), or the total Area of exposed earth (or subbase) you’ll need to line with the vapor barrier. If your crawl space matches the first floor of your home in layout, you may already know that square footage of your living space. If not, you’ll just need your previous measurements and some simple math.
If you’re lucky enough to have a rectangle- or square-shaped crawl space (or close to it), then the calculation for Area becomes easy – multiply the lengths of the two adjacent perpendicular sides you got earlier. Calculating the Area of a rectangle is just Length x Width: G = P1 x P2
If you have a more complex crawl space layout and want to dial in the ground cover, I find it helpful to break the crawl space up into smaller square or rectangular sections (calculating Area of those shapes are easier for me). Then, all you need to do is find the Area of each smaller section and add them all up to get G.
Wall Cover Calculation
You’ve got the ground covered. Now you need to calculate the Total Wall Cover (W), or the total Area of the wall that will be lined with the vapor barrier. All you need to do is take the Wall Height (H) you got earlier in Step #2 and multiply it by the Total Perimeter Wall Linear Feet (L): W = H x L
For the total amount of Area the vapor barrier needs to cover, just add the ground cover and the wall cover. Total Crawl Space Area to Cover with Vapor Barrier = G + W
We aren’t done yet.
Extra Material Calculation
There is one more step that I would suggest to figure out how much vapor barrier material you really need to order: factor in some amount of overage or extra. Why? A few reasons:
I’m the first to admit that I take a conservative approach, but the last thing you want to do is run short on material right as you approach the project completion.
My recommendation is to consider adding 10- 20 percent extra to account for the overlap, waste, and “fudge factor”. A 10 percent overage should cover you with a tight 6’’ overlap and straightforward installation. But, for the DIYers, newbies, play-it-safers, or perfectionists out there, 20 percent overage may be more to your liking.
To calculate this extra amount, just take the Total Area to Cover with Vapor Barrier you got and multiply by the Overage Factor (F) percent you choose. For example, a 15 percent overage would be multiplying that total number by 1.15. A 20 percent overage would be multiplying by 1.2. The approximate Total Square Footage of Vapor Barrier to Order (Z) = (G+W) x F.
Pro Tip: If you took a conservative approach and end up with some extra vapor barrier it can be put to good use. Although high-performance vapor barriers are very durable, some installers do a double layer in areas more likely to see future traffic, like at the access point, around mechanical equipment, or spots designated for storage.
The accessories within the crawl space vapor barrier system are critical to achieving a well-sealed, durable, and low-maintenance solution.
Seam and Sealing Tape
First, you’ll need a good, low-permeance tape to seal the seams of the vapor barrier material that overlap each other, as well as to use for sealing any utility penetrations. I’ll give you a basic rule of thumb on this one: 1 full (180’ long) roll of seaming tape for every 1,000 sq ft of vapor barrier material needed. Take your total vapor barrier square footage needed (including that extra we factored in) and divide by 1,000: Seam Tape Needed = Z / 1,000
That is assuming the tape is about 180’ long. If you have a bunch of utility or other penetrations in your crawl space to detail around, consider an extra roll.
Sealing to the Wall and Columns
For most installations, you’ll also need a means to adhere the vapor barrier to the existing vertical concrete or masonry foundation wall and interior columns to provide a low-permeance seal. A great solution is a double-sided perimeter sealing tape specifically engineered to stick to both concrete and the vapor barrier. Here is where you can refer to your Total Perimeter Wall Length (L). If you have columns, you should also calculate the Total Column Linear Feet (C) you'll need to seal, which is just the Size of Column / Linear Feet Around Column (S) multiplied by the number of columns you have: C = S x # of Columns.
The amount of double-sided perimeter sealing tape needed will be the total linear feet of the wall and columns, so just add those together. Total Linear Feet of Double-Sided Sealing Tape = L + C
Mechanically Fasten to the Wall
Finally, mechanically fastening the vapor barrier to the foundation wall helps ensure it stays in place for a long period of time. Although mechanical fastening is never a bad idea, you may not need it at the columns if the vapor barrier isn’t lapped up too high, since there usually isn’t as much weight exerted compared to the perimeter wall. A termination bar is one option that helps achieve a permanent and consistent seal at the wall. If using a termination bar, you would do the same calculation as you did with the double-sided tape, except possibly exclude the columns: Total Linear Feet of Termination Bar = L
Some of this may seem complicated, but I promise, it’s really not. Once you’ve done all the math for how much square feet of vapor barrier material is needed, as well as linear feet of double-sided tape, termination bar, and rolls of seaming tape, now you can figure out specific unit quantities based on product sizes.
For accessories, I would recommend playing it safe and rounding up to the nearest full unit based on your calculations. Again, it’s not fun to come up a roll short on tape at the end of your project.
With all the calculations done, you can also start to dial in specific costs based on the products you are looking at and their prices per roll or unit. You’ll find some manufacturers and retailers offer size options to maximize efficiency and reduce waste, meet a variety of budgets, and include full system components for an effective and efficient installation. As you start to figure out exactly which products and size rolls will work best for your project, you may start to see what a difference it can make to the overall system cost.
I think what ultimately should help drive your decision will be product quality, safe composition, integrated and effective system components, and other features that fit your specific preferences and budget. After all, installing a crawl space vapor barrier likely represents time, labor, and money that you probably only want to expend once.