Rim Joist air sealing/insulation
I have read through most of the threads here about rim joist insulation but I have not seen this angle taken as of yet. This is for a currently unfinished basement. Home was built in 64. I am interested in a couple different methods in doing this. The first method is just sealing all air leaks at the joists(with caulk or canned spray foam) around the whole foundation. After the sealing, you would then take fiberglass batts and place them into the open cavities and cut them for a snug fit. You end up with 1 side of the fiber glass exposed, poking into the living space. Since fiberglass is not flammable, this does not cause a code problem. The second method would be to get a fire rated spray foam kit online (tiger foam, handi foam, etc) and seal the rim joist that way. Both methods require the same amount of prep time. The air sealing/batt route is way cheaper but I am not sure how good of a job this would really do in terms of insulating. It is also more labor intensive. The spray foam version would do a better job but the cost is more. Spray foam kits for 200 sq ft coverage are about $350. Batts and canned foam are way cheaper. I have roughly 70 sq ft. I would have spray foam left over to maybe insulate the cat with : – ) Actually, I would just end up putting a couple inches deep of spray foam for the rim joist, until I run out.
The basement has areas above and slightly below grade. It gets very cold in the fall/winter due to air leaks. I live in western New York.
Is the first method good enough or do I forget it and just get the spray foam kit? OR is there a plan C?
dennis
Replies
Well, firstly, I would not use caulk as an air sealant. I use a foam gun, but then I apply it often.
Sure as April brings green apples, Riversong will come out with better advice, but I tend to use foam sealant in the crack and either Expanded Polystyrene cut to fit and foamed in as well, or mineral wool to enclose the space.
It's not the "cold" by itself, it's the air movement which causes the issues.
I'm also not sure I agree fiberglass is non-flammable. I use mineral wool precisely because I want to create a one hour fire rating.
Quality repairs for your home.
AaronR Construction
Vancouver, Canada
AaronDoes not using XPS foam board cause a problem because it would be left exposed and it should be covered by a thermal barrier? Granted that the amount of material exposed is not a lot but it is still exposed.I can use foam for the sealing but I was just throwing out possible suggestions of what to use. Pure silicone seems to work fairly well but expanding foam is good as well. I would really need to buy a foam gun to apply the material precisely so as to not waste as you do when you have the straw based applicator.thanks for replyingdennis
One problem with insulating the rim area with a porous insulation is it's awkward to cover it with a vapor retarder afterward. Kraft backing or poly has to be stapled as best as you can around the framing to keep inside humidity, low as it may be in winter, from the rim. Without retarding the water vapor movement to the now much colder surface of the insulated rim, you've got a condensation issue.
The foam will seal against air leakage, as will pieces of rigid foam sealed in place with can foam.
DickI do not disagree with your comments at all. What I would like to say is that if I go the rigid foam route, there is a problem with that material. Technically, it does require a thermal barrier and the basement is not finished and I am not sure when that would happen. I am talking about the pink/blue XPS stuff you can get at HD, Lowes, etc. It would be clumsy to have to install drywall all along the rim joist area. On the other hand, it would only be a couple total sheets of material but still, the thermal barrier issue is present. If the house burns down, I don't think the insurance company is going to be reasonable in that case and that I did not do due diligence.There is a material called thermax from Dow that is supposed to be fire rated but I have not been able to find it at a local place that has it in stock. It is also pricey and requires a min qty. I only need about 3 sheets at most. I am really leaning towards a spray foam kit because I just don't see an easier way out of this. Thanks for the reply.dennis
I think the thermal protection requirement for foam has an exception for the rim area. So I googled on it, came up with this from the Corbond site:===================
a. Rigid spray-in-place perimeter floor header (box sill, rim joist) insulation.
(Thermal barrier requirement exception at floor header for class 1,1.5-2.0 lb. spray foam when applied at 3.25 thickness or less per IBC/IRC).
===================Foam at that density is considered closed cell.
There is another site that agrees with Corbond's wrt to spray foam.http://www.buildingscience.com/documents/information-sheets/4-air-barriers/info-408-critical-seal-spray-foam-at-rim-joistIt looks like that I should go the spray foam route. If anybody is curious who I am considering to use, here they are. All will be the 200 bd ft version (or close to it).http://www.tigerfoam.com/products.phphttp://www.sprayfoamdirect.com/products/component/page,shop.product_details/flypage,shop.flypage/product_id,17/category_id,6/manufacturer_id,0/option,com_virtuemart/Itemid,1/http://www.fomo.com/handi-foam-two-component.aspxthe last one you actually have to buy somewhere else online.dennis
Caulking the cracks and fiberglass insulation sounds fine to me. That is done all the time. However, I agree w/ the other poster FG may not be 'fireproof'. Not a good idea to leave FG exposed to a habitable space (or even occasionally habitable).
In addition, you should install a vapor retarder ... e.g. 6 mil plastic, which as a poster pointed out is some meticulous work, also ... but definately doable. I don't think poly has a flame spread rating, either, so you still may have a fire issue.
I'm not familiar w/ the fire rated spray foams you mention. Do they actually insulate or just fire protect? Often fire protective insulation does insulate, but doesn't have the R-val/inch that insulation does.
I did the foam and bat method you mention. It worked fine. The layer of foam is approx. 1" thick. You could always use the left over foam to air seal other leaks in your house like penetrations into the attic from wires etc. If I had to do it all over, I would have ordered the larger foam kit (600 bd ft) and used it in my attic etc. I used up the 200 bd. ft. kit just for my basement. The 600 bd. ft. is a lot better bang for your buck. Just something to consider.
Edited 11/11/2009 1:01 pm ET by ejazz2095
Edited 11/11/2009 1:01 pm ET by ejazz2095
Edited 11/11/2009 1:02 pm ET by ejazz2095
I should have been more clear in my description of air sealing but my intentions were to just air seal around the four sides of the "joist box" and then apply batts. So this would mean that you would not have spray foam over the whole joist area. You just air seal the cracks and then friction fit batts. So yes, fiberglass touches the wood and there is not vapor barrier to speak of.I just had cellulose blown in the attic (after I had air sealed the attic up) and the guy who did it also does rim joist worked. He mentioned to me that he just does the air sealing and then friction fits batts. He says he has never had problems. I do not dispute his claims but I was thinking that moisture may be a problem and that maybe not enough time has passed to bring this out. I could be wrong about it all too.
I'm in the process insulating the header similar to what you describe.
I did buy the foam gun and it's great. Money very well spent. I would not want to use the foam with the straw applicator.
I've sealed with foam, FG and then 6 mil poly sealed with acoustical sealant.
Someone here posted a pic of folding the poly prior to installing it. I've found this is working best for me. The first couple were messy but now that I've figured out a process, it's going smoothly. I do a half dozen each evening.
Here's the process I found works best.
Cut a piece of poly about 5" wider than the joist space and about 6" more than the height.
Fold and cut as shown in the attached sketch. Fold one side by eye with 1-1/4" flaps. Then measure to match the joist space and fold on the line. It doesn't need to be perfect. Fold the top by eye.
Apply sealant to the floor, staple through the folded top edge in the center.
Pull back one side, sealant then staple.
Repeat with opposite side.
Staple the side 'ears' to the floor.
Trim to the bottom of the joist and fold back the flap.
Seal and staple bottom edge.
The red tape visible is only there because the next joist bay was only 4 inches and I just taped in some poly.
Good luck,
Adam
All methods will work fine. I've also seen spray foam to seal the edges, friction fit fibreglass batts, then that was held in place with friction fit styrofoam sheets and that was caulked. Overkill I'd say.
Local inspector says the spray foam is ideal. Forms thermal barrier, forms moisture barrier, and can be exposed....of course it has to be fire rated.
Do the spray foam...i'm spraying mine!
Friction fit styrofoam sheets ... must be covered by gyp bd, though, right? And not just gyp bd ceiling, but gyb bd "in substantial contact" with the insulation face. I forget the exact code language, but that phrase seems to have been the key phrase that has always stuck with me.
Foam board & Great Stuff is a good solution - plus firestopping
Foam is the best solution to insulate and air-seal your rim joists. We use the "cut and cobble" approach with 2" thick pieces of XPS foam board. This is faster and cheaper than all-spray foam. It is better to cut the pieces a bit loose so they are easy to install -- then seal the edges to block air leaks with the 'Windows and Doors' version of Great Stuff foam, which remains flexible after it cures to avoid cracks as things expand, contract and shift. (Do not try to stop air leaks by trying to get a tight fit, which is rarely tight enough.) 2" thick foam is required in many parts of the country, to stop condensation.
Thermal barrier exception - maximum 3.25" thickness: Another important point -- the IRC and IBC building codes allow you to install exposed foam to insulate rim joists without a thermal barrier cover, e.g. 1/2" drywall. The maximum thickness of the foam is 3.25", however. (See IRC 314.5.11) Two layers of 2" thick foam board would be a code violation and fire risk, for example. This could also lead to a 'failure' tag from your inspector and lots of time consuming rework. (The inspector on our last job was aware of this limit and quizzed me about what we had installed. Fortunately, I had read about the thickness limit.)
Read up on fire blocking too: Modern homes burn 8 times faster than old ones, and exposed foam without firestopping is a key reason. Firestopping must be installed along the top of a basement wall, for example, next to the places where exposed foam may be used to insulate the rim joist -- and above the wall and stud bays that may also be insulated with flammable foam board. Do a Google search for "How to Firestop Your Basement" by Contractor Kurt -- a great summary of the key steps. We often stuff Roxul rock wool into the space at the top of basement stud walls and the adjpoining rim joist pockets (over the foam board), to seal this connection. Rock wool is much denser than fiberglass, molds kind of like clay and stays where you put it. Check with your inspector first, to see what he/she will approve; some require standard fireblocking, e.g. with pieces of plywood or OSB at the top of the stud wall.
I hope this is helpful.
Mark