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Best insulation for ventilated crawlspace

For a ventilated crawlspace in a small house near (relatively humid) Asheville, NC, which would be better, Roxul ComfortBatts between the floor joists, or Roxul ComfortBoard nailed to the bottom of the joists, or foil-faced polyiso nailed to the bottom of the joists? Any of these choices would likely be better than the fiberglass batts that are presently hanging at weird angles. I worry about creating an empty cavity if I attach the insulation to the bottom of the joists. I'm not sure I can afford to insulate between the joists and under them. I have read your article on Building an Unvented Crawl Space and decided that is too much work and I don't like the risk of higher radon levels. What are your thoughts about the three options above, and is there some other better option I have not considered?

Asked by Stuart Murray
Posted Aug 8, 2014 8:23 PM ET


6 Answers

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I'm just about 70 miles down the road from you. We decided to have a open crawlspace because the house is so near a pond and there is a risk of flooding. So we did insulate between the floor joists with Roxul. Let me just say, it is a SERIOUS pain to insulate between the floor joists. I've spent hours and hours and hours under the house, air sealing and insulating and then installing a membrane over the insulation.

For example, if you calculate the total floor area for a 20 x 20 house = 400 sq ft, and compare to the surface area of a 4 ft tall crawlspace for the same house 4 x (20 x 4) = 320 sq ft, then you see that overall exterior of the crawlspace is less. It is also likely that it is physically easier to go around the crawlspace as opposed to stuffing or attaching insulation to the floor joists.

Do you have ducts underneath the house?That is a whole different challenge. Based on my experiences in insulating under the house, I think the enclosed crawlspace is easier, including radon mitigation.

Here's a photo of the enclosed porch with Roxul Rockboard (5" roxul batt insulation above it).

roxul rockboard under porch .jpg
Answered by Lucy Foxworth
Posted Aug 8, 2014 9:58 PM ET
Edited Aug 8, 2014 10:10 PM ET.


I agree with Lucy.

Even when it is less expensive to insulate between the floor joists than it is to insulate the crawl space walls -- and it usually isn't -- doesn't mean it makes sense. When a GBA reader says, "I can't afford to do it right, so please tell me the easiest way to do it wrong," my usual response is, "It's time to start putting some money into a coffee can to save for the day when you can do it right."

If I had to choose a method of insulating from your list of options, I would advise you to choose the foil-faced polyiso, installed in a continuous layer on the underside of the floor joists. This method only works if you pay attention to airtightness.

If any GBA reader is interested in reading my articles on crawl spaces and radon, here are the two most relevant links:

Building an Unvented Crawl Space

All About Radon

Answered by Martin Holladay
Posted Aug 9, 2014 4:59 AM ET
Edited Aug 9, 2014 5:03 AM ET.


Martin - I didn't say I couldn't afford to do it right; I said I couldn't afford to insulate between the joists and below them. I did say that it seemed like a lot of work to build the unvented space and I am worried about radon. I'm just looking for honest discussion, and although I appreciate your insight, it frequently comes served with a level of condescension on your part.

Answered by Stuart Murray
Posted Aug 9, 2014 5:32 AM ET


I apologize. It looks like I misread your question.

Detailing crawl space insulation correctly is tricky, and many installations are done the wrong way. So getting it right is important -- more important (in the long term) than initial cost.

I'm sure you'll do a good job, and I wish you luck with your project.

Answered by Martin Holladay
Posted Aug 9, 2014 5:37 AM ET


Thank you Martin. I'm learning and trying to get things right. I'll scour the article on the unvented crawlspace and radon and see if I can get it right. Thanks again.

Answered by Stuart Murray
Posted Aug 9, 2014 5:57 AM ET


Lucy - thank you for your response. Even though it was a pain to install the Roxul comfortbatts, how is it performing now? What kind of membrane did you install over the insulation? What is the R-factor of the comfortbatts you used? Thanks!

Answered by Stuart Murray
Posted Aug 11, 2014 3:02 PM ET
Edited Aug 11, 2014 3:04 PM ET.

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