Insulating Basement Walls

Hello everyone,

I'm beginning a project to finish my currently-unfinished basement. The house has a block foundation. I'm trying to decide on the best way to insulate the outside walls. From my reading, it sounds like batt insulation has fallen out of favor for basements, so I'm probably looking at closed-cell rigid insulation or closed-cell spray foam (or some combination of the two). I live in climate zone 6, so I need R15 on the outside walls.

The benefit of rigid insulation is its cost-effectiveness, but the downside is that I lose 3 inches (in addition to the 4 inches I'll lose to framing) of the room in order to meet the R15 requirement. The benefit of spray foam is that I don't lose the 3 inches of space, but it will probably cost 3-4x as much as the rigid foam insulation.

I'm self-learning all of this stuff for the first time, so for those of you who are more experienced, what would you do? Could I do some combination of the two (eg. 1 inch of rigid, and then spray foam in between the studs)? Are there other options I should consider?

I live in Minnesota, if that's relevant.

 

Well, the trend that I've seen over the last few years, is "a room within a room" designs-

Instead of studding out the block basement walls and putting in sheetrock and insulation,
you leave a 2' to 3' gap between the block wall and new wall.

This allows you enough space to access the sewer lines and pipes, doesn't reduce the finished square footage THAT much.

Effectively, you get additional insulation from the dead-air between the cold block wall and the new finished wall. Instead of conduction (being in contact with a cold wall) you're only dealing with convection (being in contact with air that is in contact with a cold wall).

 

No, only when people do not follow common sense installation and seal walls with moisture barrier materials which allow water to collect and can cause mold.

Build the walls correct, allow ventilation and batt insulation is a very cost effective way to insulate the basement, something you can not say to foam!

 

I certainly like the price point of batt. If I went that route, what should/shouldn't I do regarding waterproofing?

 

Interesting, I have occasionally seen residential basements where sellers have bought used office cubicle partitions,
ala the movie Office-Space
[url]https://vimeo.com/56886652[/url)

to create a basement office, I guess you could use the partitions as well.



divide up partially finished basements, or to

 

I'm assuming you are referring to the vapor movement, not a water issue, Regardless of the method used you have to have a dry basement.

Assuming that is correct, building interior walls away from the basement wall, not installing any type of plastic, and keeping some gaps above the wall to allow air movement, including moisture, is all you need.

If you trap the vapor it turns into water and then you have a problem!

 

Unfortunately, I don't think that's an option for me. Our local code requires waterproofing/moisture barrier in between the frames and the wall. Wouldn't that require something else in between other than just space?

 

Current convention is rigid foam on the wall and then batts or mineral wool in the framing with no vapor barrier on below-ground walls.

The foam acts as a vapor retarder and thermal break to prevent condensation.

That said, you're stuck with local codes.

 

+2

You want to keep warm moist air from condensing on the cold foundation wall. Rigid foam against the concrete wall (continuous with no breaks, tongue and groove, seams taped) followed by your stud wall with unfaced batts between will accomplish this.

 

So ideally, there's no vapor barrier at any point point between the rigid foam and the sheetrock?

 

The rigid foam acts as the vapor barrier, keeping moist indoor air away from the cold foundation wall. In other words, you are drying to the inside.

I believe another (cheaper) way to do it would be to leave an air gap between the stud wall and foundation wall (like others have mentioned) and install paper-faced batts with the vapor barrier towards the foundation wall. I believe the issue with this approach is you will never get a perfectly sealed vapor barrier and warm moist air will find its way between the walls and condensate on the foundation wall, causing mold etc.

 

The intent is that you dont close off area behind the wall by leaving areas open at the top near the rim joist so that you have air flow.

 

Wouldn't that be against code? In my area, fireblocking is required so that no air can flow from the basement wall upwards.

 

Thank you for all of the replies. I have another question (I'm not going to promise it's the last one).

I have a block foundation, and the tops of the walls are not capped. I'm planning to put some rigid foam up in the rim joists for insulation, but should I also be laying some foam horizontally over top of the block openings to cover those up?

 

You may want to consider using regular batts for the rim joist insulation. An R-19 will be about 6" thick and should be enough to cover the openings in the block. You could use 1-1/2" foam board to cap off the block if you wanted. Cutting foam to fit each an every rim joist cavity sounds like a lot of work for not much added benefit. You'd still have to spray foam around the perimeter to get a good seal, which is messy and ugly. More work than it's worth if you ask me.

 

Fireblocks are required between floors not within floors.