How should I insulate my basement?

Question

My basement is CMU blocks and concrete floor.

I had water issues, so I waterproofed it with an interior french drain, drainboard at the edge and a sump pump. This wouldn't take care of dampness so I added a dehumidifier.

I plan to finish the basement and here's my understanding of how I should go about it:

I think I'll avoid vapor barrier so I don't get mold problems. It gets quite humid around here. I'm in Washington DC on the east coast and if I interpret correctly the following: http://www.house-energy.com/Basements/Interior.htm, I shouldn't use vapor barrier. I have a few minor cracks in the wall maybe I should fill those may be not.

Rigid extruded polystyrene installed directly on the wall. Two inch R10 perhaps tucked into the drainboard of the french drain. This way any moisture will go down into the drain.

Leave some space (inch or two) then build the wall from the ground so it's completely detached from the wall.

Not sure what I'd do with the floor; maybe would insulate it maybe not. Height is only 7ft so any inch of height is precious.

Here's an article about this in BuildingScience.com: http://www.buildingscience.com/documents/reports/rr-0309-renovating-your-basment

This is all theoretical, I've never done anything like this, so I'm looking for feedback. Also, would I need insulation on the ceiling? I like the idea of spraying fire-rated foam, is it an overkill? Guess, I'd need soundproofing if nothing else.

Edit:

Since I'm starting with the walls, here are my first-priority questions:

• Fill cracks or not?

• Vapor barrier or not?

• 2 inch rigid foam or just 1 inch and then more insulation next to the dry wall? (I'm thinking all insulation better be away from the dry wall to avoid any condense around it)

• Do I tuck it into the drainboard?

• Do I glue the insulation or is there another method of attaching?

• Use spray foam to insulate the parts of the wall between the studs? Or is the rigid foam going to be fine? (I'm thinking the spray would fill all gaps, therefore'd be better)

Answer 1

Why would you need insulation on the ceiling in a basement? Since the ceiling is touching a heated space itself, there is no need if you will then be using the basement too. Insulation there should be done only for soundproofing, unless you have radiant underfloor heat installed, in which case, the insulation should have been done already as part of the installation. So insulation really only offers soundproofing value. So the question is, how much noise will be generated there?

Counterbalance that with the rationale for leaving this space accessible. I've often enough needed access to the underfloor regions of my home, for wiring changes, plumbing changes, repairs, etc. So any insulation may be better done as fiberglass batts anyway.

As far as the walls go, filling the cracks is a reasonable idea, IF they are stable. If the walls have moved due to frost heaves, etc. and might continue to move, I would ensure they are stabilized before you cover the wall over. A buckling foundation wall is not something good to have under your house.

Answer 2

A few things I'd like to add make clear.

1. There is no need to insulate between floors. However, due to the Stack Effect, you may want to seal large access holes that were left open during original construction. An example is under a bathtub. An osb board cut to the space and air seal around. This will help reduce the temperature differential between floors if needed. Insulating the floor joists may actually enhance the temperature difference between floors.

2. XPS, extruded polystyrene, changes it's Perm Rating depending on it's thickness. Using 2" lowers the perm rating to the vapor barrier level. I strongly suggest following buildingscience.com and use 2" xps with airsealing techniques. Also, be absolutely sure you seal all around the 2" xps. It is imperative that no warm air from the basement comes in contact with the concrete wall during winter. Failure to do so will lead to condensation and water problems. Also, the vapor barrier will also keep your interior from gaining humidity during summer.

3. If you ever smell something bad, it may be coming from your sump pump. Houses normally produce a negative interior pressure which pulls gasses into the house. Look online to find how to air seal around them. This will further reduce a source of humidity.

4. I'd only worry about the cracks if they were structural. If they're big, you may need an additional assessment. If they aren't a structural problem, there won't be a negative affect from sealing them with the proper masonry sealant.

5. Don't forget about the box sill; the joist that goes around your house. Make sure to at least have that air sealed. Spray foam is the simplest and most effective option; though pricey.

6. Use plenty of adhesive, not in splotches, when applying the 2"xps. continuous beads and adhere to the clean surface of the wall. Since to code, you must have a finished face over the 2"xps. 2x3 or 2x4 stud wall will work. They can be touching the foam, though any touching will create a thermal bridge. Since them temperature difference between the basement and ground temperature isn't as great as above ground, It would be cheaper and easier to wire the walls if they were to be left empty. If anything, batt insulation would give the quickest ROI if you did choose to fill with insulation.

7. So for the wall construction. I would use 2"xps to the wall, tape and seal. Create a 2x4 wood stud wall 24" o.c. (2x3 and thinner have a tendency to warp). Have that touching the xps and left empty for ease of wiring. Insulate the box rim with expanding foam, (not the gap filler, an actual applicator is needed).

8. If you are worried about not enough insulation, add batts to the stud wall. Because the xps creates a low perm rating at 2", moisture won't be a problem with this type of insulation. When installing the stud wall, be aware to not let convection become a issue. don't allow a gap behind the stud wall to travel up and over it; nullifying the insulation you've placed in there.

9. Get the walls done first. The sooner you start saving on your energy bill, the sooner your ROI will be reached. If you still have problems with humidity, higher an energy auditor. They will identify your source of humidity. Then tackle the floor issue.

And for the person wondering about ground temp and insulating the basement.

1. Why people insulate the basement in the Midwest? Have you ever done any energy calculations?!? These are required by code with new builds. Get schooling and learn something. The frost line differs depending on where you fall in the average ground temperature map, but normally goes down 4' during winter. that's freezing 4' down. Not adding at least an R10 to the walls and R5 to the basement makes it very difficult to pass energy efficient standards. Not having insulation in the basement and having the rest of the house sealed, insulated, and properly controlled, will create about half of your energy losses. Also, proper air sealing and insulating prevents humid, moldy, smelly basements.

Answer 3

I am in the same situation, except I don't have the interior french drain (or "drain tile"). If I did, then I would do this:

• moisture barrier (e.g., that heavy plastic sheeting with dimples in it) on the block wall, with the bottom tucked into the drain board.
• a layer of rigid foam insulation on the room side of the moisture barrier.
• spray foam the rim joist area, covering the tops of the rigid foam.
• 2x4 framed wall as you described.

I have seen conflicting info on whether or not to put a layer of rigid foam under the framed wall. I know - it seems like it would get squished - but I saw it from a reliable source (which I cannot find just now... I thought it was buildingscience.com).

Answer 4

I am in a similar situation, and I believe you are correct.

In some cases a vapor barrier helps, but in some cases it does not. I installed rigid foam insulation on the basement wall, and then strapping, and then blue (mold and moisture resistant) gypsum drywall sheets.