I am working, doing audits, with a contractor who tends to make basements, crawlspaces into conditioned space. Especially when all or most of the duct work is in the crawlspace. He does this by covering the floor, side walls and piers with heavy plastic. He then uses rigid foam insulation on the walls, foams the joints and above and seals the vents and access hatches. Then we duct seal and re-insulate.

Has anyone been doing this for a while? Any unentended consquences?

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That works great.  Check out www.crawlspaces.org

I have a company that does this as well.  It provides great results keeping the humidity level in the crawlspace down and decreasing overall air loss to the crawl. What is he using to "condition" the space?  Dehu?  House air?

we have been doing this on retrofit and new construction since 2000. works great, just make sure the crawlspace is in negative pressure to prevent radon or other things from getting in the home. www.eeba.org has the builders guides by Building science institute specific to climates with the details all laid out, use this as a resource to get all the details correct. In our northwest dry cold climate we preffer spray foam insulation when we can and seal the vapor barrier to the stem wall ( no need to run all the way up just over the cold joint) then we draw out house air and vent it outside, this keeps the crawl at negative pressure, temp and humidity track with the home. Still there are alot of other details to get right as well, like the management of the outside water, wether or not to tie in with radon control, types of heating systems, how to treat the space durring construction in the wet season, etc.

We have been advocating this for years. By sealing crawlspaces you gain in two ways:

1. First from a comfort factor. If you have a cold crawl, then you have a cold floor, heat rises so the area around the floor is colder, making your feet, and anyone laying on the floor to watch TV, etc. uncomfortable.

2. Being more uncomfortble leads to higher setting on the thermostat, which means higher energy bills.

Usually we have been using spray foam insulation directly to the walls, then duct sealing the duct system with mastic. We normally would not insulate the ducts, because they are now in a conditioned space.   



Why duct seal when the ducts are brought into conditioned space? Maybe to improve air flow to the registers?



Hi Derin

We would normally seal the duct to make sure we are getting the most efficient, and cost effective return for our customer. Normally return air is not taken from  sealed crawl, so any air that would enter the supply air system could have some contminants, and the temperature of the crawl is usually less than the temperture in the living space that could also affect the supply air being. It's also more professional, and sets you apart from the competition.

Have a Great Day!!!!!!


I wonder where you are working.  Here in North Carolina, some 30-40% of the houses are built with a sealed crawlspace like the one you are describing.The only question I have is about the last word in your paragraph.  What do you reinsulate?  Not the floors I hope.  Once the insulation is on the walls and the space is conditioned, the crawlspace acts like a little conditioned basement.  And, lo and behold, it is more energy efficient than a vented foundation.

Scott Cummigns - Dow Building Solutions




No, I meant that we seal and re-insulate the ducts.


Good.  I thought so.  If you really want to see the 'Bible' of crawlspace conditioning, you can go to advancedenergy.org and go to Crawlspaces under the Quick Links tab in the header.  There is more information there than you can believe.


Scott Cummings

Dow Building Solutions


Scott, I have been reading the information on that advancedenergy.org site and I'm confused.


Here's a quote: "In Flagstaff, researchers monitored energy use throughout a single heating season. While the homes with insulated floors used 20 percent less natural gas than the controls, those with insulated foundation walls used 53 percent more."

I'm still reading the full report. but if the control homes have no insulation in the floor or on the foundation walls, how can insulating the crawl space result in a 53% increase in gas use.  What am I missing?


Excellent Question Bud.  Advanced Energy originally did their pilot work in North Carolina where it's hot and humid.  Years later, they did investigative work in Flagstaff and Baton Rouge.  What they found tracks well if you keep the following in mind:  Energy savings from a sealed crawlspace comes in the hot months.  The cold months end up being a wash or a slight increase in energy cosnumption.  The theory is that during the winter, you are heating more space so your consumption goes up.  During the summer however, you are keeping the house dryer witha  sealed crawlspace so your AC can work much more efficiently.  In a hot climate, the savings in AC usage dwarf the slight increase in heating season comsumption and your net bills are about 18% lower for the year.  Flagstaff does not have a very hot/humid climate but does have stiff winters.  Voila.  No significant energy savings.  However - and this is important - the crawlspace still remains dryer, free of pests, and generally provides better inddor air quality.  Oh, and it keeps the temperature and humidity on either side of your hardwood floors equal so there is no cupping or crowning of hardwoods.


That's why I told Evan I would be interested in knowing where he lives.  The sealed crawlspace craze has been popular in NC, SC, GA, VA, TN, and MD.  Some out west and in the midwest but not so much since basements dominate there.

Thanks for the reply Scott and that's is pretty much what I expected, they increased the space and heated it, thus more energy use.  But that still gives the wrong impression for what we should be doing way up north, I'm in Maine.  Now, we don't have a lot of crawls (certainly not vented ones), but we do have a lot of unintentionally heated basements where the furnace and ducts find a home.  I know it isn't exactly the same, but we ALWAYS insulate and air seal those basements and if a crawl would do the same.  In no way would there be a 53% increase in heating costs, so I'm looking for how they heat their control homes and what changed when they created their test homes.

My concern is that many who read the summary will never connect with what was done different to create those results and simply start advising people to insulate their floors and not the foundation walls.  In fact, I have been running into that type of advice and perhaps this study is where it came from.  Just an opinion right now, but something is not right in Denmark or the crawl space test in this case. 

I will finish the full report and post my findings which ever way they point, it is just that our experience up here is so opposite what their conclusions are stating.  We also don't have as many termites :).




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