Example of How a Box Sill is Constructed using Standard Building Techniques

Box Sill-What is it? And How to Insulate the Area 

In this Article I'm going to explain how to Seal and Insulate the Box Sill of your home and why it is important to add Insulation in this area of your Building. Adding Insulation will: Reduce the Energy Needs of your Property, while Increasing Your Personal Comfort level. 
  • Its a Win-Win option for any Home-Owner.

The Box Sill of your home is located underneath the flooring materials on the First Floor of your Home and consists of: Header Joist, Sill, Floor Joists, Sill Plate, Outside Joist. (If your home is built with a slab on grade this information does not apply to your Building.) Drawing 1 is the Typical or Standard Technique Home Builders use in constructing homes. It does not matter if your home has a Basement or a Crawl Space. It will be constructed similar if not identical to Drawing 1.

Why is it important to insulate this area?

  1. During the Winter months in cold climates this area is key to stopping the cold weather from seeping into your building. If the exterior flooring areas around your building are noticeably cooler during the winter months- I can almost guarantee that you have infiltration of outside air and cold seeping into the home through this area.
  2. During the Summer Months: Warm Air will Infiltrate this area of your Home and continue upwards and into your Home. This will cause additional cooling loads on your AC System and your Electric Bills will reflect the needs for the added use. It will cost more money for you to cool your home to a comfortable temperature.
  • There are a few ways to Insulate and Seal this area against the unconditioned outside air. All of which may require you to roll-up your sleeves and get a little dirty. Don't let that stop you. You Can Do It and the $ Money $ you save will come back to you in savings for both “heating and cooling” costs associated with your home. There are a few recommended procedures for Sealing and Insulating this area. I'm going to cover my preferred ways of sealing and insulating this area of your Building.
My preferred way to insulate and seal this area is by using Closed Cell Foam Insulation. Applied to a finish Depth 1- 6in. This material both seals the Air and Insulates at the same time- Utilizing Closed Cell Spray Foam Insulation is a time saver and reduces the labor costs.
In Illustration 1007 shows a before picture.

Illustration 1008 Completely Sealed and Insulated Box Sill with Closed Cell Spray Foam Insulation.

Completely Sealed Box Sill with Closed Cell Spray Foam Insulation
Illustration 2 1008- Notice how the Closed Cell Spay Foam Insulation seals the Entire Area to an average depth of 6-7 Inches. This provides an estimated R30-42 Value. The Closed Cell Spray Foam not only Insulates but seals the area from the UN-wanted air from infiltrating your Building and requiring your Furnace or HVAC from having to work harder to cool or heat your Building. In other words it saves you $ Money $ by reducing your Electricity Costs and Gas Needs. The Closed Cell Spray Foam Insulation also acts as a guard against unwanted bugs and insects that may try to take up residence in the Box Sill Area.
Here are some handy DIY tips from Foam It Green
All Foam It Green® Standard Spray Foam Insulation Kits are:
  • Designed for Quick and Easy Use and Professional Results
  • R-6.7 at 1 Inch, CLOSED CELL foam
  • ASTM E-84 Fire Rated Insulation
  • 100% Free of CFC's, VOC's, Penta-BDE's, Urea Formaldehyde  
  • Designed for use with no outside source of pressure or power.
  • Bulk Pricing Available for 6 or More Kits
  • A Fit-Tested Respirator Mask is Required when Applying Spray Foam Insulation.
  • A Mask with Organic Vapor Filters and a Prefilter for Particulates is Required.

The Second and more labor intensive way to insulate the Box Sill of your home is by:

  1. Sealing All the places where the: Header Joist, Sill Plate, Floor Joists, Rim Joist, and Foundation with Caulk and then
  2. Insulate the Area with 8in Batt Insulation. The estimated Rvalue of Batt Insulation is R19. Or
  3. Insulate with ISO Foam Board Insulation. Depending upon the Thickness Chosen the ISO insulation has an estimated Rvalue of R6 per inch and comes in thickness ranging from 1in to 4 inches. ISO insulation can also be layered to create Higher Rvalues.
See Drawing 2 for an example of Insulating with the above mentioned types of Insulation.

If you decide that this DIY Box Sill Insulation Project for your Home is a bigger project than you can handle Scotts Contracting is available to assist you in your Green Building Home Improvement Project Needs. Scotty supplies firm estimates for every job. What this means is that-
  1. He will inspect the areas to be repaired or improved on.
  2. Take Digital Photos of the Areas to be worked on- Pointing out any areas in Dis-repair, Damaged, or Areas to be Improved upon. With Special Attention on Areas that are causing excessive heat loss or gain.
  3. Scotts Contracting will then go over the entire project with you while showing you the photos of the Areas to be improved upon. Explaining-
  • Why the area needs Improvement
  • How it is costing you $ Money $ on your heating and cooling bills
  • Best Solution(s) to Repair the Problem Area
  • While answering any questions and addressing your Concerns
  • Scotty will then determine all the needed materials to fix the areas in question, and
  • Present you with a Firm Proposal of both the: Labor and Material Costs with payment schedule and then schedule a time to perform the repairs that is as least intrusive to your schedule as possible.
  • Upon completion and Final Payment- Scotts Contracting will provide you with any needed forms to file for the Tax Benefits/Credits that you may be entitled to for adding Green Building Up-Grades on your Property. (For Web Resources on the Applicable Programs see the Reference Section Below)
  • (A) Because of Scotty's prior Experience from working in the Construction Industry and Training in Residential and Commercial Drafting / Design. He is able to for-see many of the issues that the other contractors miss when they are performing their visual inspections and making out their material lists. (See About Me Web Link for Training and Experience)
  • (B) It is for these same reasons that Scotts Contracting can- Determine: Material Costs with-out having to buy UN-needed materials that add to your Projects Material Costs. This eliminates
  • UN-Needed Trips- to and from the Lumber Yard. Leaving him to spend the time on your project.
  • Cuts down on Waste of UN-Needed Materials and Job Site Left-Overs

Prior Home Weatherization Articles Can Be Found on the Web Links

  1. St Louis Renewable Energy Old Man Winter
  2. Closed Cell Foam It Green Spray Foam Insulation-Foam It Green®

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Tags: Contracting, Efficiency, Energy, Home, Insulation, Louis, Scotts, St, Weatherization


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Comment by Pat Dundon on September 13, 2011 at 8:01am
Also, are you aware of the requirements for thermal barriers on foam in houses?  In a rim joist, closed cell class one (flame spread 25, smoke <450) is allowed uncovered at thicknesses up to 3 inches. over 3 inches you must cover it. if the foam is class 2, (flame spread 75 ) you have to cover it in all cases.  some portable sets are class 2, flame spread 75 foams.  the intumescent paints used as thermal barriers are all tested with specific brands of foam, and must only be used as thermal barriers at the correct thickness on those specific brands of foam. 
Comment by scottscontracting on September 13, 2011 at 6:36am
Mr Dundon,  I agree with your latest comment; but, cannot recall a time when I have had not enough space to add insulation in this area.  Generally the homes that I work on were built using standard building techniques of proper layout / design and didn't use this extra floor joist in the building process.   Now that I've shot my mouth off I'm sure I'll run into a situation like you mentioned.  Thanks for the tip.  Scotty
Comment by Pat Dundon on September 13, 2011 at 3:35am

You are missing a very important thing here.  where the floor joists run paralle to the exterior wall, there frequently are situations where the nearest parallel joist to the exterior is within 3 inches of the inner surface of the foundation wall or actually is directly over the foundation wall.  this creates a void space that is usually 10 inches or more tall, and between 6 and 13 inches wide.  this space cannot be insulated with closed cell foam becasue the only way to do that is to inject away until it is full, which results in foam that is impropelry installed and will degrade over time.  it the worst case, it can start a fire because of spontaneous combustion. 

there are 3 remedies: 

two are design changes. 

1 place all joists so they intersect the rim joist perpendicularly (use cripples 2 ft or so long to replace the last parallel joist

2 use a 2 inch narrower joist as a replacement for the joist that is close to the foundation and hoild that joist up against the floor deck.  that gives foamers an opportunity to acces the space. 


the other is substitution:  use densepack cellulose or open cell foam in this area where it occurrs. 



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