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Underside of floor joists material for cellulose insulation


Eklmn's Avatar
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Join Date: Jul 2004
Posts: 78
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02-02-14, 01:47 PM   #1  
Underside of floor joists material for cellulose insulation

- Cold room, wood floors, no insulation.
- Idea is to close underside of joists to dense pack with cellulose insulation
- One foot below floor is sand, it is dry there (no water).

Question 1. is okay to use cellulose?

I read that cellulose is pretty good to use even in mind wet. Cellulose is not spreading mold and fungus.

Question 2: what material to attach underside of joists to hold the cellulose?

Thanks.

 
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calvert's Avatar
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02-02-14, 07:20 PM   #2  
underside of floor joists

Good questions, I don't know how you are going to get a material adequately attached to the underside of the joists easily considering that dense packing cellulose requires a fair amount of pneumatic pressure to be done correctly.

Do not under any circumstances install cellulose that will be subjected to getting wet.

Is this an old home with an air porous foundation. In your case I would try to figure out some scenario whereby you could get a couple inches of foam sheathing with a foil facing installed to the underside of the joists. This would give you an R-value across the entire assembly rather than just the cavities. Insulate and air seal the rim joist as well.

A wood floor is never going to be above the air temperature of the room and that is always going to be a lot colder than your body temperature.

 
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02-02-14, 08:08 PM   #3  
The room is located between house and garage. It's not heated.

>> I don't know how you are going to get a material adequately attached to the underside of the joists easily considering that dense packing cellulose requires a fair amount of pneumatic pressure to be done correctly.

I could remove wood floor, nail wood planks (other material?) underside of joists, fill up space with
- cellulose GreenFiber Blow-In Natural Fiber Insulation-INS541LD at The Home Depot
or
- Roxul ComfortBatt Roxul ComfortBatt R-15 Unfaced 47 in. x 3 ft. 11 in. Batt Insulation-RXCB351525 at The Home Depot
or
>> you could get a couple inches of foam sheathing with a foil facing installed to the underside of the joists
- agree. I am considering foam also as an option

>> Do not under any circumstances install cellulose that will be subjected to getting wet.
I do not think that cellulose would get wet, but who knows, why?

Attaching example image how it would look:
Name:  floor_insulation.jpg
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calvert's Avatar
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02-03-14, 04:22 PM   #4  
Eklmn, I am unclear why you want to insulate the floor if you are not heating the room?

How would you propose nailing material up to the bottom of the joists with the minimal space you have below? Swinging a hammer working from above to nail up boards could probably be done but it will be frustrating.

I'm sure I could conceive some weird plan to get foam attached to the bottom of the joists but again, if the room is not heated what would be the point.

One other option to put a solid bottom to the floor joists would be to use a clip designed for the drywall industry to hold 1/2" or 5/8" plywood or osb cut to fit between the joist, dropped down to the bottom of the bay and nailed through the clip into the side of the joists. These are the "corner-back" clips made by the "Prest-on" company. You can check out their site and just imagine using them with the plywood instead of drywall.

Even if you get the boards or some other material attached to the bottom of the joists I am not sure you would be able to achieve a true dense pack cellulose installation with the use of the machine you might get from HD. Not to say you can't get a respectable job...just not sure you could get dense pack.

The Roxul batts would be fine but make sure you do a good job of air sealing and insulating at the rim joist.

Cellulose may have certain mold inhibiting properties but getting it wet would certainly diminish its ability to resist fire and mold and also permanently affect R-value.

 
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02-04-14, 08:22 AM   #5  
Calvert, thanks for your posts.
>> unclear why you want to insulate the floor if you are not heating the room?
I have two cold rooms, one of which is heated, so I want to add some insulation to save on energy coins. This project will not save money in long term, but add some comfort.

>> cellulose installation
throwing away this idea, thanks for explanation!

The cold room has patio doors that I plan to replace with a wall and regular door for better insulation. I will start with the wall then with floors as I am skeptical now about insulating floors after your posts.

 
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