Insulating floor?

[dja4260]

My home as a 12x11 den off the master. It was built on a slab with an small exterior entrance. The floor isn't insulated. In the winter the floor is freezing. The room does have a ventless gas heater.

I was thinking of finally cleaning out that crawl space and adding insulation. I can get rolls of R-19 for dirt cheap.

Questions:

Would just filling void in floor joist with paper backed insulation that is stapled be enough? Should I double up on the insulation and really pack the void tight?

is more better?

Am I over thinking this?

 

[QuietPortage]

R25 to R30 is recommended for floors in your area, although anything installed between the joists will help. Don't pack it too much; it shouldn't be compressed.

Spray foam would be my choice for this, although it is more expensive.

 

[seanf]

Don't pack the void tight with insulation, you lose r value that way.

I would use polystyrene foam board. When you say slab and crawlspace I assume you mean that there is a gap under and the floor isn't just a concrete slab on grade? R19 should be approx 6 inches thick, depending how large your joists are if 2x8 I would put 2" polystyrene foam board then your R19.

Alternatively depending on your floor covering you could install this product Dri core I know I linked to the Canadian Home Depot but you should be able to get it in the USA, I used the plastic backed stuff instead of the foam backed stuff for my basement when I finished it, its a bit pricey but it does give you a bit warmer of a floor.

 

[dja4260]

Floor joist are 2x8. There is a 30 inch space between concrete floor and bottom of joist. The whole area and floor stays dry. I was going to insulate the walls as well.

I can buy foam board for cheap. Cut it into strips and glue it to the bottom of the floor, inbetween the joist. Then I'll add insulation on top of the foam board.

Sounds reasonable. Thank you.

 

[seanf]

get the rigid foam board tho not the white stuff that breaks into balls, rigid foam is something like R3.5 per inch compared to 1.7ish per inch for the white flimsy stuff.

 

[137_Trenton]

I have a large addition built onto my house. It's built over the previously existing concrete slab which used to be the patio. The addition was built 33 years ago, and back then it was the norm to put vents in the walls and vent the area underneath. Doing this they put insulation on the underside of the floor, and I can tell you the floor was always freezing cold in the winter and quite hot in the summer. I finally got sick and tired of it and ripped out the vents and mortared in new cinder blocks to block the holes off, then I ripped out all the fiberglass insulation from the underside of the floor, and spray foamed the entire walls and sill area between the joists. It's great in there now... just like the rest of the house. You could insulate the floor, but it's more important to insulate the walls/sill area.

 

[Ihatetochangeoil]

Don't pack the insulation in tighter than it is meant to be installed. You get insulation from trapped air, not solid fiberglass blocks.

You CAN install R19 over R19 if you can get R19 rolls "dirt cheap" as you say. This will effectively give you R38.

Considering you're in Chicago, if you have access to the subfloor from underneath, I'd heat the floor from underneath; THEN insulate it, so it would be comfortable to walk on with bare feet in the winter. For northern climates, I have seen these electric subfloor heating elements sold by the foot in Menard's, but I can't remember the manufacturer's name.

Google or Bing search "electric floor heating elements." https://www.searchlock.com/search?q=electric+floor+heating+elements&tsrc=b

Be very careful if you use "spray foam" insulation; some brands expand as they cure...Don't use this around windows, removable vents, etc.

 

[QuietPortage]

When I suggested spray foam, I didn't mean the individual cans that are meant to fill gaps. All varieties are meant to expand, some more than others. Using the appropriate material is key. For broad applications, I'd pay a local service to apply it, although there are DIY kits available. The benefit to spray foam is a moisture barrier and airtight seal.

 

[dja4260]

I just finished.

I was able to find R-19 for $10 per 50 sq ft roll. It fit nicely and then I covered the whole floor joist with 1 inch R-5 foam board. I did the walls as well with the left over R-19. I used 4 R-19 rolls and 5 R-5 foam boards. I doubled up the R-5 with the extra.

I had a friend that helped and the process went smooth.

 

[QuietPortage]

You're a man of action! Nice job.

 

[dja4260]

I didn't notice that the slab is sinking. I'll make those repairs when my father comes into town.