Winnipesaukee Forum - View Single Post - Open Cell Spray Foam

DickR · 01-06-2018, 11:04 AM

Spraying foam upward calls for a certain amount of clearance below the floor for the applicator to crawl under and work. While closed cell foam provides more "R" value per inch, roughly 6 to 6.5, it must be applied in "lifts" of no more than about 2" at a time, due to the need for the heat released in curing to be dissipated by conduction. Open cell foam provides typically 3.6 to 4.0/inch of R value, which is similar to what you would get with a blown cellulose or fiberglass insulation, but it can be applied in thicker lifts. I'd certainly advise waiting until warmer weather, for best results. Also, with either open or closed cell foam or any other insulation, you won't get the same overall R value as you will between joists, due to thermal bridging of the framing. The R value of framing wood is roughly 1.2/inch, only a third that of porous insulation or just a fifth or less of what closed cell gives.

Considering the difficulty of application in a tight space, you might consider having a contractor do a dense-pack of borate-treated cellulose through holes cut in the sheet goods underneath the joists, much as is done when insulating a wall from the outside through holes cut in the sheathing. As a side benefit, the borate treatment of the cellulose provides a deterrent to rodents and insects.

After installing insulation between the joists, consider also installing a vapor-permeable insulation layer below the joists, such as Roxul board (https://www.homedepot.com/p/Roxul-Co...1224/206789748). This will provide an additional amount of insulation below the whole floor assembly, eliminating a lot of the thermal bridging effect of the framing.

Finally, another option is to change the skirt material to something that will keep out the weather and insulate those walls, especially if you have any utilities down there. For sure, apply that 6 or 10-mil vapor barrier over the ground. If you do go the route of building a weather-proof and insulated skirt, and the ground is flat or nearly so, consider also laying down at least 2" of closed cell insulation board, then the vapor barrier, and finally some course material like crushed stone over that for protection of the insulation and vapor barrier. With the skirt and ground insulated, there would be no need to insulate between the floor joists.

01-06-2018, 11:04 AM	#7
DickR Senior Member Join Date: Apr 2008 Location: Moultonborough Posts: 735 Thanks: 4 Thanked 254 Times in 166 Posts	Spraying foam upward calls for a certain amount of clearance below the floor for the applicator to crawl under and work. While closed cell foam provides more "R" value per inch, roughly 6 to 6.5, it must be applied in "lifts" of no more than about 2" at a time, due to the need for the heat released in curing to be dissipated by conduction. Open cell foam provides typically 3.6 to 4.0/inch of R value, which is similar to what you would get with a blown cellulose or fiberglass insulation, but it can be applied in thicker lifts. I'd certainly advise waiting until warmer weather, for best results. Also, with either open or closed cell foam or any other insulation, you won't get the same overall R value as you will between joists, due to thermal bridging of the framing. The R value of framing wood is roughly 1.2/inch, only a third that of porous insulation or just a fifth or less of what closed cell gives. Considering the difficulty of application in a tight space, you might consider having a contractor do a dense-pack of borate-treated cellulose through holes cut in the sheet goods underneath the joists, much as is done when insulating a wall from the outside through holes cut in the sheathing. As a side benefit, the borate treatment of the cellulose provides a deterrent to rodents and insects. After installing insulation between the joists, consider also installing a vapor-permeable insulation layer below the joists, such as Roxul board (https://www.homedepot.com/p/Roxul-Co...1224/206789748). This will provide an additional amount of insulation below the whole floor assembly, eliminating a lot of the thermal bridging effect of the framing. Finally, another option is to change the skirt material to something that will keep out the weather and insulate those walls, especially if you have any utilities down there. For sure, apply that 6 or 10-mil vapor barrier over the ground. If you do go the route of building a weather-proof and insulated skirt, and the ground is flat or nearly so, consider also laying down at least 2" of closed cell insulation board, then the vapor barrier, and finally some course material like crushed stone over that for protection of the insulation and vapor barrier. With the skirt and ground insulated, there would be no need to insulate between the floor joists.