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Basements and Crawl Spaces FAQ's

Please email your questions and we will post them here.

Question:  Should I insulate the floor or the walls?
Answer:  Insulate the floor if the building is up on stilts and adding walls to close in below is impossible. This requires that you protect pipes and drains below the floor level.  If you can close in the area under the building, insulating the walls will have the following advantages:

  1. More space for living, storage, equipment, etc.

  2. Protects the water pipes, drains, ducts, etc.

  3. Protects the footings from frost action.

  4. Costs less to do - usually fewer sq. feet, easier to install, and eliminates having to insulate the mechanical systems.

Question: Should I have it sprayed or use rigid foam board?
Answer: Either approach will work well if it makes a good seal and is continuous. This includes wrapping up to the underside of the floor around the perimeter if your insulation is on the bottom of the floor framing. Spray applied foam has the following advantages:

  1. It is continuous, air-tight, and vapor-tight.

  2. Spray foam is fast, adheres completely, and won't fall off.

  3. No cutting, fitting, or sealing.

Question: Should I use open-cell or closed-cell foam?
Answer: Closed-cell foam won't absorb water. If there is any ground water or the potential for flooding, stay with closed-cell foam.

Question: How far down the wall should I insulate?
Answer:  Our recommendation is to insulate walls that may be susceptible to frost action (ground water, roof run-off, poor drainage, etc.) to about one foot below the outside grade level.  In most cases, this puts insulation and air sealing in the area where it does the most good, at the top of the wall and sill area where it is the coldest and most leaks occur.  Keeping the below-grade area of the foundation uninsulated keeps the ground just outside warm and prevents frost action.  This is especially important for stone foundations that are not reinforced.  This approach also has the best payback as the need for R-value decreases as the depth and natural ground temperature increases. Leaving the bottom of the wall unsealed and warm also allows drainage in particularly wet sites, preventing pressure from building up outside the foundation.

Question: What R-value do I need?
Answer: This depends on the climate zone you are in. Typically, you can use the R-value that is recommended in your area for the walls upstairs. This number decreases the further you go down into the ground because the outside ground temperature is generally increasing.

Question:  What about moisture?
Answer:  If you don't have ground water running into or through your basement, most of the moisture is probably coming from condensation on the cool foundation surfaces.  Reducing sources of moisture in the air will reduce condensation. Methods include:

  1. Install a vapor retarder over the floor of your basement and/or crawl space to prevent evaporation of ground water into the air.  Depending on the soil conditions and drainage, this material may need to extend up the walls to meet the foam insulation.

  2. Keep the windows and vents closed to the outdoors during periods of high humidity.  Summer ventilation can be a major source of moisture rather than providing drying.  Vent the clothes dryer and combustion appliances to the outside.  Products of combustion contain significant amounts of moisture as well as toxic gases. Be sure to provide adequate supplied air to your appliances – see below.

Question:  What about the bulkhead door?
Answer:  It should be treated like any other outside door.  Use a standard insulated door with good gaskets in the plane of the insulation on the foundation wall - outside bulkhead doors are typically not designed with gaskets or insulation.  Remember that the exterior grade goes down to floor level in the bulkhead area and the foundation insulation and air-sealing measures should extend down to the floor level in this area if you are only doing the top in the rest of the basement.

Question:  Should I do the interior walls between the old house and the new?
Answer:  If masonry walls form a “T” with the outside wall, it is generally good practice to insulate for a short distance on the interior.  Masonry is very conductive and will bring cold in through the intersecting walls.   There may be air leakage between layers of stones in old foundations that can be cut off with this approach.
If your stone foundation has multiple layers, for example, granite outside and brick inside, there may be cold air leakage up into the walls above through this passage.  If the inner wall is not structural, removing the top rows of stone or brick will allow the insulation to extend to the outer layer and up to the sill to prevent cold air from bypassing your new insulation.

Question:  What are the issues related to fire, toxicity, etc.
Answer:  If you aren't going to install framing and finish your basement with gypsum board or another 15 minute thermal barrier, you should consult your local code official about leaving foundation insulation exposed.  Commercial codes may not apply to residences, but local codes may address this issue.  Regulations for crawl spaces may be different than basements.  There are a number of options for fire protection in the Fire Barriers section of this web site.  For more information about fire barriers click here.

Question:  If I want an estimate to do my basement or crawl space, what do you need to know?
Answer:  If you provide us with the following information, we can tell you how much to budget for this kind of a project.

  1. Your decisions about the issues we have discussed above.

  2. The lengths of each side around the perimeter of your basement and crawl space.

  3. Do you have at least 18" of space under the joists around the entire perimeter?

  4. The number and size of the windows and doors.

  5.  How far is it to 1' below grade on each side.  Include the sill in this calculation by  measuring from the bottom of the floor?

  6. How far is it to the floor from this height?

  7. The state and town you are located in (Zip code)?

Question: What are the safety and health issues related to crawl spaces?
Answer: The following list includes problems we have encountered in the past:

  1. Open or poorly protected wells.
  2. Resident animals.
  3. Undermined structural walls and piers.
  4. Sharp objects or debris.
  5. Removing old insulation that may be infested or moldy.

Question:  What are the safety issues related to ventilation?
Answer: There are several issues to visit before insulating your basement or crawl space:

  1. Have your house checked for radon.  Sealing the basement may decrease or increase radon levels, but if you are in a problem area it is good to understand the problem and have a professional develop a plan for radon control before you insulate and seal your home.

  2. Make sure there is adequate make-up air for your furnace, boiler, hot water heater, etc.  The best solution is the use of sealed combustion appliances.  If this is not possible, adequate outside air should be ducted to each appliance to prevent "back-drafting."

Question: Should I vent my crawlspace?
Answer: According to Joe Lstiburek of Building Science Corporation:

“Crawl spaces are real simple to understand and deal with. When you vent crawl  spaces you bring in hot, humid air and cause moisture and mold problems. The  ground surface is typically colder than the dew point temperature of the exterior air. The underside of crawl space floor insulation is radiation coupled to the ground surface and is very close to the same temperature of the ground.  Moisture droplets can be seen all over the top surface of typical polyethylene ground covers as well as hanging from the bottom surface of the crawl space floor insulation. Gee, I wonder how all the water got through the poly ground  cover? It must have leaked through the walls. Give me another break. Now, when  the moisture is in the insulation where do you think it wants to go? Where is the air conditioning? Moisture moves to the cold surface. Venting crawl spaces made sense only when you had no air conditioning and no insulation and no crawl  space walls.”


FOAM-TECH has experience based on insulating and air sealing hundreds of basements and crawl spaces over the last twenty years. Our work extends beyond insulation projects to include moisture, radon, and other types of environmental mitigation projects.  Related work includes temperature and moisture monitoring, pressurized fog testing, HVAC system air sealing and testing, and more.