Rim joist insulation: What did the builder do?
BobTheWeekendWarrior
| Posted in General Questions on
Greetings,
I am working on finishing my below grade basement in southern Minnesota. Home was built in 2014. I have been planning to hire professionals to spray 2″ of foam along my rim joist. Upon further inspection I have noticed that the builder insulated the rim joist.
From the outside in: 1 1/8″ thick manufactured rim joist material. 1″ of foam insulation (appears to have some kind of front and back facing). 1/2 piece of OSB which is visible from the interior.
I have not seen this kind of rim joist insulation and after researching on the internet cannot find any mentions to this method. Anyone know about this method?
Photo attached.
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Replies
He didn’t do much.
To be kind, let’s say he attached 1 inch polyiso to the rim board.
The 1/2 OSB on top does little, not sure why it’s there other to protect the polyiso from mechanical damage ?
The whole assembly has an R value of 8, at the most. Which is inadequate IMO. Plus there is nothing sealing your rim joist from the outside and there is undoubtedly tremendous leakage going on.
I recommend you consider adding 2 inches of closed cell insulation to address both low R value and lack of air seal.
Bob,
I'm guessing that this is a manufactured product -- probably a product from Structural Wood Corporation called Insulated Rim Board. According to Structural Wood Corporation, this product is rated at R-8.
Bob,
Weyerhaeuser makes a similar product called TJ Insulated Rim Board. See the photo below. Your builder might have installed this product.
.
Thank you everyone for confirming the product/method and confirming I should proceed with spray-in insulation for air sealing.
Very similar to the detail I used on my own house. It did look similar until Carpenter Ants ate the foam.
Looks like I am getting 2"-3" of closed cell sprayed from the rim joist down to the sill plate and onto the top of the foundation. That ought to be some good air sealing.
What are the benefits of closed cell spray foam (1.5", installed by pros) vs XPS (2") on the inside foundation walls? I am looking at about a 30% cost difference. The outside of my foundation has a tar/rubber like water barrier sprayed on and 1" of rigid foam, not sure what kind (disregard my past posts where I indicated I had more on the exterior....I was wrong)
The advantage of 1.5" of sprayed 2lb polyurethane over 2" of XPS is that it's continuous and air tight, no extra detailing needed that might be missed or deteriorate over time.
Another advantage is that 2" XPS eventually drops to R8.4 as it loses it's HFC blowing agents over time (adding a significant greenhouse gas punch in the mean time.) With 2lb polyurethane blown with low climate impact HFO1234ze you'd be about R10.5 @ 1.5", with 2lb foam blown with (high warming) HFC245fe it'll be ~R9-R10. It's not clear if 2lb blown polyurethane loses it's blowing agent is as rapid as with extruded expanded polystyrene (some say it's there for the duration- I'm skeptical) or what the fully depleted R-value would be, but initially it's substantially higher per inch.
Dana thank you for articulating those benefits. I am assuming these kind of benefits would be hard to connect with tangible results like increase heating costs by X% or reduced long term maintenance costs by Y% or $s.
I certainly like the cost saving of rigid and do not mind the installation effort. But I am happy to pay 30% more for sprayed if I can be sure there is a tangible result. I would be able to feel 30% more money in my pocket. :-D
I am not an expert so take it for what it is worth, but I went to Home Depot and purchased one of the 2 part poly spray kits and spray foam all of my rim joist bays myself. The job was not that difficult and using the spray I was able to make sure there was insulation behind obstacles (register boots, wiring, plumbing, etc..) that would have been almost impossible to get with ridged foam board. A Significant savings over having someone spray it. Plastic painters drop cloths are a must, the foam does splatter and drip some.
In my area closed cell foam runs about a buck a board foot, installed for any job over 1000 board feet. Spray foam kits only come close to matching that price point with 600 board-feet kits. The smaller kits are significantly more expensive. Only when it's a very small job (under 300 board feet of material) do the kits start become a cost savings.
The DIY 2- part kits all use less-than-benign blowing agents.