Expert: Michael Curry - 1/6/2010

Question
QUESTION: Hello Michael,

I am hoping you can answer my question.  I have a previously finished attic space with knee walls.

We have access to the knee wall space and currently use it for storage, however, it is very cold and letting in a lot of cold air.  I am hoping to create this knee wall space into a warm insulated area.  

Currently there is insulation on the knee walls themselves and in the floor joist.  However, there is no insulation in the rafters.  

Is it safe to insulate the rafters? I am worried about moisture issues.  I have read that I can install both rigid foam and batts of insulation.  

I would prefer to install the higher R option as this is only an attic space and not a livable space and won't be finished.  

If you can advise me on what materials I would need to finish this project and in what order... airspace, insulation, battens, vapour barrier????

Any advice would be very appreciated,

Thanks,

Tim MacKay

ANSWER: Hi Tim,

Sure you can insulate the rafter space although it will still be on the cold side of the knee walls and ceiling. If you want it warmer you may have to remove some insulation to allow heat transfer to that area. The R value depends on where you live but R28 (7" +/-)should be good, also leave a 2.5 inch air space between the insulation and the roof plywood or you will get an ice build up if it snows.

---------- FOLLOW-UP ----------

QUESTION: Thanks very much for your quick reply.

I have a couple quick question.  Do I need to apply a vapor barrier at all.  Also since I have 2x4 rafters (older home) can I put in larger insulation and just hold it in with something? And what would that something be?  

Thanks and I appreciate the help and advice.

Tim  

Answer
Hi Tim,
You will need to fur out the rafters to accommodate the insulation and use a vapor barrier also. Don't forget to leave an air space. This type of job is a lot of work just to create some heated storage space. An easier way may be to use rigid foam right over the rafters or a basement type blanket wrap.