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Finishing the Basement: XPS Foam Board Insulation

December 21, 2011

Without any more delay – a new post!

A lot of work has gone into the basement remodel over the last few weeks:

  • Quotes, quotes, crazy quotes from different contractors (electrical, plumbing, drywall, framing)
  • Insulation of the exterior walls with XPS foam board
  • Removal of electrical conduit in preparation for framing
  • Framing! (I say this will exclamation because with all of the waterproofing and preparation we had to do, it was a long time coming)
  • Near-catastrophic plumbing mishap – why aren’t more plumbers certified to work on boilers, and why are you billed for after-hours when you call them at 2:30PM in the afternoon ?!?
  • More plumbing – hot water heater relocated and shower rough-in as I type

There is just as much information out there about the type of insulation to use in your basement as there is about the attic.   As far as I’m concerned XPS (extruded polystyrene) foam-board is the way to go.  fiberglass is itchy and needs an addition vapor barrier. It needs to be installed between the joists instead of behind them. When moisture control is the utmost important concern – don’t mess around. The last thing we want is to spend all this money finishing the basement and then for it to mildew. That’s why we spent so much time and money waterproofing it. Now we are taking the extra step to use the right materials to finish it.

What is XPS? The XPS Association says:

“There are many types of plastic foam insulation material available today and extruded polystyrene foam insulation (XPS) is one of the best for both general and specialized applications. It’s excellent resistance to moisture, imperviousness to rot, mildew and corrosion, controlled compressive strength and ability to maintain insulating power make XPS foam a premiere insulating material for commercial, industrial and residential structures, as well as for critical civil engineering uses.

Thermal Efficiency (Insulating power)
The thermal efficiency of an insulating material* is expressed as R-value. R is the material’s resistance to heat flow. The higher the R-value, the greater the insulating power. The insulating power of many rigid foam boardstock products relies in part on the hydroflourochlorofluorocarbons used in the blowing agent system. Permeation of air into the board can affect the R-value of the insulation. For accurate comparisons of long-term insulating performance (LTTR) it is important to obtain long-term aged R-value data from all insulation product manufacturers. Figure 5 shows a typical R-value curve of XPS boardstock over a 5 year period.

Moisture resistance
A critical factor affecting long-term performance is the ability of an insulating material to resist the intrusion of moisture. Moisture can come in contact with insulation not only during construction, but throughout the life of the building. If absorbed, its effect is to drastically reduce thermal efficiency (R-value). The closed-cell structure and lack of voids in XPS helps the foam to resist moisture penetration better than other types of insulating materials. The excellent moisture resistance of XPS foam insulation has been confirmed repeatedly and consistently in laboratory tests and under field use conditions.

Mold, Mildew, Corrosion resistance
Because XPS foam is essentially a plastic material, it will not corrode or rot or support the growth of mold or mildew. It is resistant to microorganisms found in soil and provides no nutrient value to vermin. These properties make it an outstanding insulating material for below grade applications.”

Well that sounds ideal for a basement installation! In addition to all of the water-resistant properties, it also acts as its own vapor barrier when it is sealed at all joints and edges with tape, caulk and foam.

We used the Owens Corning XPS sold at Home Depot. It also sports the likeness of the Pink Panther 🙂  It’s lightweight and comes in standard 4’x8′ tongue and groove sheets. The installation goes very quickly because it cuts with utility knife and is installed directly against the cinder block with foam rated construction adhesive (very important! Look on the tube to make sure it can be used for foam.) Once the foam is up, caulk or use spray foam (great stuff for windows and doors) to seal along the floor, windows and rim joists.


We noticed while this was all being done by out contractor (saved us sooooo much time, but easily a do-it-yourself job), that the rim joists were insulated incorrectly with fiberglass installed backwards – paper backing facing out. The whole point of the paper backing is that it is a “vapor barrier”, a poor one at that since it is organic material and will mold/mildew if damp. Vapor barriers always face the interior of your space. We spent a few hours on a Sunday pulling all out, and using scrap XPS foam board to insulate the rim joists properly. The foam board is cut to fit and then sealed in place with spray foam.

the black coloring is dirt from trapped in the insulation from air passing through it

Surprisingly a lot of energy is lost through the rim joists, so having this area insulated properly will make a noticeable dent in your heating bills. What is a rim joist? It is the space on the the exterior walls where your foundation wall meets the floor joists of the first floor. Check out this perfect drawing from another home blogger: Dover Projects Blog:

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  1. Basement: Before/After Photographs « A Home In College Hill
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