# Should I insulate my floor joists?



## Donthaveaclue (Jul 14, 2021)

I bought a fixer upper I got half the subfloor and floor joists in. Roughly 16 inch from ground to bottom of floor joists. HVAC is in the ceiling. Should I insulate the floor joists? Kentucky weather.


----------



## mark sr (Jun 13, 2017)

If this is a crawlspace - then yes it should be insulated.


----------



## SLSTech (Jan 19, 2021)

Depends - answer above is correct unless you encapsulate the crawl space & instead insulate the walls which in many cases will be the better option


----------



## chandler48 (Jun 5, 2017)

IF you don't encapsulate, you will need at least a 6 mil if not a 10 mil membrane over the dirt, and yes, insulation would be a good thing. Figure how you will install it first. At 16" many bodies won't fit in that space post flooring.


----------



## gkreamer (May 8, 2020)

This is an interesting topic with lots of conflicting info out there...

I agree with both posts here. This is on my to-do list as well. From what I've read the BEST option is to pour a concrete floor in the crawl ($$$) over 6mil poly and then insulate the walls with either spray foam or rigid foam board. Air sealing the foundation vents is a must too.

Next best option is seal foundation vents, 6mil poly minimum, insulate walls, and add a dehumidifier and drain if you have a water problem.

But my local govt wants spray foam the entire wall (no termite inspection gap at the top) and a vent and intake to the HVAC in the crawl with poly and sealing vents. The guy said "most of the humidity comes from the attic".

Then CrawlspaceNinja.com recommends poly, wall insulation, dehumidifier, french drain and exhaust fan.

To quote Austin Powers, "now I've gone cross-eyed"...


----------



## chandler48 (Jun 5, 2017)

gkreamer said:


> But my local govt wants spray foam the entire wall


Since when does the government have a say so in what you do with your crawlspace???


----------



## Nealtw (Jun 22, 2017)

I would like to see a cost benefit report that includes working in a 16" space. 
Adding insulation to the outside walls adding HVAC to the crawlspace and running a dehumidifier does not sound cheap when ventilation is free. 
Crawling around in the dirt to put up insulation between the joists and finding a way to keep it there better save you a lot of money per year to pay for that. 
With or with out insulation of any kind the crawl space needs to be dry.


----------



## Nealtw (Jun 22, 2017)

gkreamer said:


> This is an interesting topic with love ts of conflicting info out there...
> 
> I agree with both posts here. This is on my to-do list as well. From what I've read the BEST option is to pour a concrete floor in the crawl ($$$) over 6mil poly and then insulate the walls with either spray foam or rigid foam board. Air sealing the foundation vents is a must too.
> 
> ...


All our new houses have a 4 ft deep crawlspace with a 2" skim coat of concrete over poly, that might explain why we have mostly basement houses.


----------



## gkreamer (May 8, 2020)

Nealtw said:


> All our new houses have a 4 ft deep crawlspace with a 2" skim coat of concrete over poly, that might explain why we have mostly basement houses.


My crawlspace is about 3', four courses of CMUs plus mortar joints. I'd like to get a 2" slab poured, it's relatively level down there.


----------



## Nealtw (Jun 22, 2017)

gkreamer said:


> My crawlspace is about 3', four courses of CMUs plus mortar joints. I'd like to get a 2" slab poured, it's relatively level down there.


Sounds easy if you say it fast.


----------



## gkreamer (May 8, 2020)

chandler48 said:


> Since when does the government have a say so in what you do with your crawlspace???


I called to ask about the termite gap and that sent the guy on a diatribe regarding requiring a configuration that was nearly the opposite of what I've read on crawlspaceninja.com and BusildingScience.com and crawlspaces.org. I was also told a permit is required for this work.


----------



## gkreamer (May 8, 2020)

@Donthaveaclue - apologies for the thread-jack!


----------



## gkreamer (May 8, 2020)

Nealtw said:


> Sounds easy if you say it fast.


@Nealtw it's all easy and already done in my mind, lol.

The driver for this is my wife's health, trying to cut down humidity and any other nasties from the stack effect. She has asthma and unfortunately a bevy of other health/breathing problems.


----------



## Ronnie833 (Jan 9, 2021)

The trade-off is a bit more energy efficiency in exchange for mice and mildew.


----------



## Nealtw (Jun 22, 2017)

Ronnie833 said:


> The trade-off is a bit more energy efficiency in exchange for mice and mildew.


It has nothing now, I didn't read a complaint about mold or mice?


----------



## gkreamer (May 8, 2020)

I cannot speak for @Donthaveaclue , but I don't have mice or mildew.

I did get 4 quotes from crawlspace contractors that varied from $9000-$25000, hence the interest in DIYing it.

I can afford either the labor OR the materials, but not both. Well, based on the materials cost I really can't afford the labor.

But as @Nealtw said, the objective is to make the crawlspace dry. That should prevent mold and improve indoor air quality.


----------



## sscherer (Dec 3, 2021)

Some months ago I decided to insulate the rim joists with 3/4" EPS panels, foam around them for a vapor barrier, then insulate with 3" rockwool. Some past building science research indicated insulating cinderblock foundation walls more than a foot below grade wasn't cost effective or code-required in the Southern USA. (I'm in Georgia just N of ATL.) 

Code DOES require the 3" termite inspection gap below the sill and rim joists. The encapsulation/vapor barrier installer covered up the earth and walls and left the termite gap. 

The first thing we did on buying this house was put a 70 pint dehumidifier into the crawl space.

Anyone who represented that "most moisture comes from the attic" is blowing smoke.


----------



## chandler48 (Jun 5, 2017)

We also have to deal with the Southern Building Code on older houses which require venting a crawl space. It makes sense in our climate, but not in others. I'm further north of Atlanta than you, probably. When encapsulating those vents must be occluded.


----------



## gkreamer (May 8, 2020)

@sscherer - thanks for sharing your experience, it's helpful!
@chandler48 - since I'm roughly 550 miles northeast of ATL I'll definitely be blocking off vents! I'm 50 miles south of DC in Fredericksburg VA.


----------



## chandler48 (Jun 5, 2017)

gkreamer said:


> I'll definitely be blocking off vents!


Yeah the further north you go, the less effective venting is in crawlspaces. Here in the mountains our breezes almost completely evacuate the moist air in crawls, so they work well. Once you start closing up vents, however, you must encapsulate completely. I don't think there is any middle of the road.


----------

