Rim Joist Insulation?

[forward]

I have an insulation question.  I need to insulate a rim joist and also an unvented crawl space.  I am in a midwest state, cold in the winter, hot in summer and it can be humid.

Lets take the rim joist; the DIY sites and buildingscience seem to say xps rigid foam and spray foam around the edges to seal as best insulation practice.   However, that doesn’t meet code because of fire risk - the xps has to be covered by a fire retarder like drywall. 

On the other hand the fiberglass batt does not stop air or vapor movement, thus it does not meet good insulation practices.

Does anyone here know of a solution or have thoughts on this dilemma?

[James]

Is there a reason you can't apply fiberglass batting covered by vapor barrier?

[Nate R]

Is there a reason you can't apply fiberglass batting covered by vapor barrier?

Still doesn't stop air or vapor movement within the insulation itself. Not sure how big a concern that is in a rim joist, though. Not a lot of height for convection currents to really take hold.

[forward]

Is there a reason you can't apply fiberglass batting covered by vapor barrier?

Still doesn't stop air or vapor movement within the insulation itself. Not sure how big a concern that is in a rim joist, though. Not a lot of height for convection currents to really take hold.

I wondered about that also.  I think what what some feel the downside of that approach is that it's really difficult to get an air seal on that vapor barrier in the rim joist cavity so without a  rigid air barrier and moisture barrier  the air flow will allow moisture to pass to the rim joist and condense.  But maybe I'm over thinking it.

[Nate R]

Isn't fiberglass w/ a vapor retarder also a fire hazard or no?

If not, could you not put in an inch or so of rigid foam w/ spray foam sealing, and then glass batting over it?

And, the alternative is? An exosed rim joist? How's the flame travel on that? :-)

[forward]

And, the alternative is? An exosed rim joist? How's the flame travel on that? :-)

I think you make a good point Nate.  I'm not sure building codes are keeping up with the advances.  I am looking at an alternative of Thermax which is a rigid foil faced product but it gets expensive, thus increasing the payback time considerably.

[GuitarStv]

Don't use fiberglass if you get regular huge swings in temperature.  It's fine as a fire barrier (fiberglass doesn't really burn very well), but will get damp from condensation and grow mold very easily no matter how you set up the poly moisture barrier.  When it's damp, the R value of fiberglass goes down the toilet as well.  Also, fiberglass has a tendency to settle out over time which can cause gaps to appear between the surface you want to insulate and cause tremendous heat losses due to convection.

I'd look into extruded polystyrene panels myself . . . cut them to size and glue directly to the flat surface that you need to insulate with a styrofoam construction adhesive.  They act as both a moisture barrier (as long as you tape the seams) and a very good insulator.  It is more expensive, but you will never have mold problems, and once in place they have better real world insulative values that fiberglass (since they don't sag or absorb water).

[ncornilsen]

Thermax is a great product. I will be installing it in my crawlspace sooner or later. I have a closed crawlspace that serves as my air return plenum, no earthy odor, crawl space is dry as a bone, and my heating bills stay under $150 in the coldest months, and air-conditioning basically never runs in the summer. But, I have no insulation on the stem wall in my crawlspace. I've calculated that insulating it to R13 (2" layer of thermax) would cost me $900 if I do it myself.  It would save me $220 a year using some basic thermodynamics, and considering that my furnace is a rather inefficient 80% AFUE unit.

Thermax becomes cheap when you realize it doesn't need to have a thermal barrier (like drywall) put on it for close-crawlspace applications. Unfortunately, I only need 25 sheets of Thermax, and they have a 48 or 50 sheet minimum order. I'm not sure how to find someone who'd be interested in splitting a pallet's worth with me!

[tooqk4u22]

Thoughts I have about this, but not answers:

1.  Older houses are meant to "breath" and if you seal them perfectly I have heard issues can arise form mold to movement.

2.  Is there really any benefit to insulating the crawl space - heat rises in the winter and in the summer crawl spaces are usually cool.

thoughts on this

[Nate R]

Thoughts I have about this, but not answers:

1.  Older houses are meant to "breath" and if you seal them perfectly I have heard issues can arise form mold to movement.

2.  Is there really any benefit to insulating the crawl space - heat rises in the winter and in the summer crawl spaces are usually cool.

thoughts on this

1.: Yes, that's why you don't use poly in houses like that. Use a vapor "retarder" and not a "barrier." Something that water can still dry through, just more slowly than nothing. (Like tarpaper, for example.) It also depends on how sealed up the other side is. Can water dry to the outside instead? Then you're likely fine as well.

2. Yes, there is a benefit. In fact, it's now essentially required IIRC in many colder areas on new construction.

[DK]

I'm trying to figure this out myself so will be interested in what you do. I had a energy analysis done and that was one of their biggest recommendations to me. Right now it is just fiberglass stuffed in there. I think what I'm planning right now is use caulk to start in any edges or holes I find, then put in some rigid board, then fill in around it with the gap filler spray foam, and put back over top that the fiberglass I removed. Looks like that will cost me maybe $50 to do and should be a huge improvement over it now.

I also looked into getting one of those frothpaks and spray foam it up myself, but that will end up costing about $300, so I plan on taking the cheaper approach.

I was surprised (and disappointed) when I was checking around up there to see measurements, etc, and noticed I have some airflow coming in some areas. :-\

[strider3700]

If you go with the foam,  cut it 1" too small on two sides to make installation easier.  Then use some spray foam. A drop in the middle of the back then press it in place leaving a 1/2"gap all the way around.  Then foam that gap closed.  You'll never be able to cut it prefect enough to be air tight like foam will be.

Not sure what the to about the building code. my basement is wood panelling with a mix of ancient figerglass, mineralwool and xps behind it....

[alwayscurious]

Depending on where you are, and your local building inspector's preferences, you may be allowed to insulate without drywalling by spraying the exposed foam with an intumescent paint. Intumescent paints form a fire barrier by expanding when exposed to high temperatures.

[Midwest]

Do you have to cover with drywall in a crawl space?   I realize you have to cover with drywall in a habitable space (such as a basement), but give the fact this is a crawl space do you have to cover?

I'm not sure of the answer, but think you might want to double check before covering with drywall or fiberglass.

[DK]

To follow up on what I did with mine. I pulled out the existing few inches of fiberglass insulation, then cut up and used 2in foam board, with great stuff foam around the perimeter of it to seal it off. Then I put the fiberglass insulation back up there. I think it cost me right around $75 total for the foam board and spray foam. Since I'm a consumers energy customer (elec only) I got a $20 rebate for doing it too.

I couldn't get to one side since part of the basement is partially finished and the framing didn't allow me to get in there. I also found a handful of holes/cracks that were open to the siding and were letting air in from the outside that I spray foamed shut. Due to the ducting, nails poking through from the first floor subfloor, wires/cords, etc getting some of the foam board in was a pain in the booty, and getting the spray foam around it would have been impossible if I hadn't bought some plastic tubing I fit around the spray foam plastic trigger/sprayer. In hindsight, I wish I would have just went with buying a spray foam froth pack....it would have been an extra couple hundred, but would have gone in much easier and I think sealed better. I ended up having to go back on some of them and spray in more foam where I still felt some drafts where it didn't expand enough.

[paddedhat]

I'm a builder, and I generally build on sealed, conditioned crawl spaces. The ideal solution is professionally installed spray urethane, sealed with a sprayed fire rated coating. It's expensive, and the only local guy who was remotely competititve appears to have gone under recently. I now use 1-1/2" Thermax attached to the walls with Tapcons and big fender washers. The idiot code nazis in this township will not let me use it on the band joist, since in their tiny little minds, the band is part of the wall assembly and needs to be done to the same standards, which means R-21batts. Thermax in now locally available at some lumber yards. 1-1/2" R10 lists for $48 a sheet and I pay about $40. Other builders in the area use 2'x4' Dow  XPS board, then slop a coat of fire rated paint on it. The job looks like shit, and absolutely fails to meet the coating manufacturer's specs. but it's all the code idiots need to see to satisfy their tiny little brains, so it's now SOP in this area. Thermax can be detailed,  on edges and seams, with the shiny silver metal duct tape, and it makes a really professional looking job. I just spoke to an outside sales guy from my local lumberyard. He is now stocking a fire rated thermax with a white pebble finish interior skin. It's a slick way to do a basement, and it isn't any more expensive than the foil faced stuff.