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NateHanson 09-10-2007 02:52 PM

insulating under first floor
Our house has a 4' basement, with gravel floor, and the foundation is insulated, but I'd like to insulate the first floor, as I think we're losing a lot of heat that way. The floors were fairly cold last winter.

Our floor has 2x4" truss-beams on 24" centers, and I don't know the best way to insulate this. Do I need a vapor barrier? What's the best way to suspend the fiberglass under the floor? How do I prevent having open spaces between the rolls of fiberglass where the truss-beams are?

How would you insulate this space?

NateHanson 09-11-2007 07:05 AM

Any help?

Would rigid foam insulation be a better bet for this application? Use construction adhesive to glue it to the underside of the subfloor between the trusses?

concretemasonry 09-11-2007 08:47 AM

insulating under first floor
The vapor barrier (actually vapor retarder) goes on the warm side of the insulation (top) for a heating climate.

Rigid will not harbor the usual bugs and moisture that fiberglass will. You can always glue it and foam any joints.

Keep in mind that when you insualte the floor you also eliminate any cooling benefits of the crawl space in the summer since the soil in a great temperature moderator.

NateHanson 09-11-2007 09:10 AM

That's a good point about the cooling. Although, up here in the land of 75 degree summers, and zero degree winters, heating is much more of a priority than cooling. Heck, we had the wood stove lit twice in the last week of August on cold mornings this year. :)

With the moisture and installation headaches of fiberglass, and the void space in the trusses that will allow airflow into the fiberglass between each roll, I'm leaning towards using rigid foam. My hesitation there is primarily price. I seem to recall that 2" PS foam was running about $25 per sheet! And I'm going to need about 40 sheets of the stuff. But I suppose I'll save time on installation.

Any idea how many inches of foam would be suitable for this purpose? The basement was probably 50 degrees last winter. Probably end up cooler after insulating the floor. But the temp diff isn't huge, so would R10 (I think foam is R5 per inch, right?) be sufficient?

AtlanticWBConst. 09-11-2007 09:36 AM

As mentioned already, Rigid foam board is the way to go.

Here are some links that may help you regarding thickness and R-values:

NateHanson 09-11-2007 09:55 AM

I hadn't considered fire problems. One of those links said that unfaced rigid insulation sometimes requires drywall covering to meet fire codes.

Do I need to use the foil faced foam for this basement space? I've got wiring and a furnace down there, so I'd rather not create a fire hazard. (although the floors WOULD be nice and warm!)

concretemasonry 09-11-2007 10:00 AM

insulating under first floor
You really do not need that much insulation (unless it is a psychological thing).

You are only insulating between the 50 degree basement and the interior (70 degrees?) - that works out to 20 degress at maximum. For above grade walls, you have a much bigger difference, possibly as much as 90 degrees, which will require much more insulation for the winter.

In Minnesota, we have much bigger temperature swings (-30 to +100), so the earth is a very valuable source of natural temperature moderation. We can go from heating in the winter to a 30+ day period in the summer without turning off the air, if you want humidity control. - Many people overinsulate in the wrong places (like basements).

NateHanson 09-11-2007 10:18 AM

The Owens Corning site showed R-19 for insulating floors, but I don't think they were talking about floors over an insulated basement.

So for a 20-30 degree temp difference, is R-10 a good ballpark goal?

And any thoughts on the fire-proofing issue? Is the pink stuff safe to use on a basement ceiling without any covering?

scrapiron 09-11-2007 07:01 PM

Although expensive you might want to consider a rigid spray foam. There will be absolutely no air movement from basement to living quarters, it is a vapor barrier, and 2 inches will give you an R rating of about 14. I agree that a 20 degree difference doesn't seem like much however it is a continous draw on your interior heat. Around here a fire barrier is required only in living areas.

digler101 09-27-2007 12:09 AM

Nate-I know your post is several month old. I was wondering how your project turned out? I have a simular crawl with gravel floor. I used R-19 under floor, and have not put down any vapor barrier down yet, time hasn't let me do that yet. It seems the more I read about this project the more I get confused on the best solution for insulating this area. My basement has a 3'x3' opening to the crawl. It's low crawling to the back of the house where I'm able to sit. I tell ya, After spending 8 hours in there just putting the insulation up I felt like I was hit by a Mack truck! Now comes the vapor barrier any suggestions for laying the 6 mil plastic down? It's a vented crawl and I'm told to cover these vents in the winter and open in summer? Shouldn't I just seal them since the crawl opens to the basement/living area? Thanks

NateHanson 09-27-2007 12:36 AM

Actually my post is only 2 weeks old, so I haven't gotten to the project yet.

In my basement, plastic sheeting is just laid out on top of the gravel.

digler101 09-27-2007 12:52 AM

I put the insulation up in the floor joists just need to get back in there and lay the plastic. Just a bit worried that I'll get new problems because I may have done something wrong (sealed vents properly, no insulation on duct-work, etc.). It'll be years before I know probably. One concern is radon.

NateHanson 09-27-2007 09:11 AM

I'd definitely insulate your ductwork. It's counter-productive to insulate the floor (making the basement cooler) while leaving exposed ductwork. You'll lose a lot of heat down there.

I've also been told to put in a vent that can be opened in the summer (to decrease moisture) and closed in the winter.

moneymgmt 11-29-2007 01:14 PM

Bringing this back up because I have a similar issue with my crawl space. Block walls, sand floor, and about 3' from ground to subfloor. There is plastic sheeting down and now I need to insulate.... somewhere. The block walls have 4 vents (each approx. 8"x12") in total and only 2 of them close. The local insulation warehouse recommended using foam board on the walls and covering right over the vents, leaving the ground and subfloor alone.

Above, there are comments that insualting the floor while the space gets colder is useless so I'm wondering if insulation the walls is the better way to go? What about the vents, do I need to keep them accessible? I looked into spray foam and the figure I heard was $4/ft, if that's the case that's not an option.

digler101 11-29-2007 05:05 PM

Insulation in Crawl
I followed what everyone here said, and hoping I've done it correctly as far as insulation the crawlspace. My crawl is attached to my basement and the entrance is approx. 5' feet up a concrete wall. I went in, finished the under floor with R-19. Went back a week later and insulated the HVAC vents running to the back bedrooms. I then went ahead and finished the floor (gravel/sand) with a 6 mil plastic. 3/4 of the crawl is finished with the plastic, and I still have the remainder to finish. 6 mil despite what the people at Home Depot told me will puncture, especially if you have gravel you're installing over. I sealed the two vents off for winter, and so far so good. I'm replacing windows next, which I hope will top off the heat loss the winter months. The other night we had some strong winds and felt air coming through, that's why they are getting replaced. My crawl is similar in height (about 3'-4') as the last poster, so movement can be somewhat restricted. I just wondered about the walls which are cinder-block and whether or not I should have insulated as well????? Good luck!

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