Crawlspace with cellulose insulation – Go or no go?
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Greetings GBA folks,
From Columbia SC. Please help determine whether blown in Cellulose between the floor joists is recommended.
Here are the parameters measured in the 20 year old crawlspace:
1. Current moisture content of the joists is at 18 %
2. Relative humidity is at 72%
3. Foundation is drained professionally well down to the footers – no current issue with water, or termites
3. Crawlspace is awaiting contractor to install 20mil vapor barrier to 1400 sqft dirt floor area
4. Crawlspace is awaiting contractor to install rigid Silver glo wall and rim joist insulation
5. Crawlspace is vented with brick walls 3′ high meeting the rim joists.
6. Removed 20 year old fiberglass bat insulation— it was starting to fall… and dusty.
Let’s not start a discussion about encapsulation just yet. I just want to know whether blown in cellulose hanging down from between the floor joists will endure and not be detrimental. I understand the stack effect and have observed the air pressure rising up to hit the floor. Carrying with it dirt and dust.
Please help… looking for a better, and safer comfort level here.
Best Regards,
Sam
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Replies
If you are taking all the steps listed above, it would make sense to create an unvented crawlspace. (See this article for details: https://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented-crawl-space)
Assuming you have at least R-5 at the perimeter and ultimately seal the crawlspace, I don't think there is any reason to insulate the floor joists.
Just a couple of questions that might affect your situation and may help other posters to advise you:
Are the foundation walls brick all the way through or brick over a poured concrete foundation?
Is the foundation blocked by plants or fairly open?
Do you have gutters and/or overhangs that keep the brick fairly dry?
I agree with Steve. The unvented space will probably cost less than insulating the floor joists. Based on what you noted, all you have to do is seal the vents and crawlspace door, add some supply air from your HVAC and you are golden.
Regarding the cellulose, I would be concerned about vibration and wood expansion causing it to fall from the joists. Even if they used a spray version with some glue built in, gravity will probably assert itself in time. At least with walls and ceilings you have the drywall working in your favor. Not so in a crawlspace.
Norman - Greenville, SC
Totality Tomorrow!
Fusion,
As I noted in my article on the topic ("Building an Unvented Crawl Space"), a crawl space in your part of the country -- South Carolina, which is notorious for its hot, humid summers -- should never be vented.
I don't recommend that you install cellulose between the floor joists. If you install an adequate thickness of insulation on the crawl space walls (rigid foam or closed-cell spray foam), then no insulation is required between the floor joists.
Norman, Steve, and Martin, thanks
I share your concerns about Cellulose. I do not have enough information yet from CIMA (Cellulose Industry Manufacturing Association) to overcome these concerns about vibration, wood expansion, and just plain gravity. I find it very curious that CIMA is not publicly addressing Crawl space's.... especially in the hot humid south.
However, Am attracted and conflicted about Cellulose because, the feature that Cellulose is treated with Borate to keeps Termites/carpenter ants away. One of the main problems I want to prevent. As well as the fundamental fire safety and keeping the floors warmer in the winter. But I have to agree with you guys, the assurances and test reports are just not available enough to recommend Cellulose as a enduring solution.
As per your comments a about moving to an unvented or encapsulated crawlspace, I am not convinced that sealing a crawlspace is a worthwile enough yet to take on the cost and dependency on making a crawlspace a conditioned space by adding dehumified air to a sealed crawlspace. Sorry, but I observe the vast majority of existing and new construction home are still using vented crawlspaces. Yes, I do agree that we need to keep this moisture out and am fully onboard with the science behind keeping these water vapors and dirt out. But I am not sure the methods about encapsulation survive the cost benefit analysis.
Perhaps, in a subsequent developmental installation, I will seal and make this crawlspace a conditioned space... but the budged does not permit it today.
Best Regards,
Sam
Sam,
It's your house. If you intend to leave the crawlspace vented, I would not spend money and completing most of the steps required to convert it into an unvented space.
In this case, I would put the poly down and affix it to the walls, sealing all edges. As far as insulating the floor, just use whatever makes sense to you.
", I am not convinced that sealing a crawlspace is a worthwile enough yet to take on the cost and dependency on making a crawlspace a conditioned space by adding dehumified air to a sealed crawlspace. Sorry, but I observe the vast majority of existing and new construction home are still using vented crawlspaces."
The ease of air sealing and the resulting higher air-tightness of the walls of a conditioned crawlspace compared to that of the ceiling of an vented crawlspace usually REDUCES the annual energy use of a home. It is (almost always) an operational cost savings to ventilate the crawlspace with a modest amount of conditioned space air, not an increase.
Insulating the walls and keeping the space vented to the outdoors keeps the space (including the joist-edges) colder during the cooling season, increasing the moisture content of any wood exposed to that space. If (against all advice) you're going to keep it vented, put the Sliverglo as a continuous ceiling for the crawlspace, and skip insulating the foundation walls.