[Mroussakis]

I want to spray foam my basement . I noticed the rim joist are blocked around the perimeter of the home by wood inserts . Research has told me these are called fire blocks. Used to slow spread of fire into the walls (I think).

can I remove these ? I don’t think I have an alternative if I want to properly insulate the basement .

Thoughts?

i was able to stick
Camera behind the wood insert. As you can see below.

 

[Chris616]

Welcome to the forum.

In a more typical installation the joists are attached to the rim joist, like in the diagram below and unlike in your photo. I’ll be curious to see the explanation from one of the framers about what is going on here. Are you sure that those “wedged” pieces aren’t nailed from the other side of the joists? How often are the bays open like the one that you used to take the "behind the scenes" photo?

 

[Nealtw]

If the joists continue outside ( cantilever) blocks are required.
They may be squash blocks to support jack and king studs or bigger point load.
They could be a repair of some sort.
I would leave them and insulate inside them.

 

[Half-fast eddie]

They may be squash blocks

And they could be blocking to keep the joists from rolling over. Definitely best to leave them as-is.

 

[Chris616]

How to insulate those areas, though? Can't easily drill a hole and and shoot foam in and ensure complete coverage and without the risk of blowing something apart. Can't easily slide fiberglass or XPS foam boards sideways to fill the voids. Hard to use a homeowner available spray foam kit that doesn't have a long wand to get to the end of the cavity. It would have been nice if the builder had insulated those cavities when they were accessible.

 

[Mroussakis]

How to insulate those areas, though? Can't easily drill a hole and and shoot foam in and ensure complete coverage and without the risk of blowing something apart. Can't easily slide fiberglass or XPS foam boards sideways to fill the voids. Hard to use a homeowner available spray foam kit that doesn't have a long wand to get to the end of the cavity. It would have been nice if the builder had insulated those cavities when they were accessible.

My sentiments exactly! I’m in a bit of a pickle here! My spray foam guy flat out said he won’t spray in a hole. Needs to have full access the the cavity.

 

[Nealtw]

I did not see this picture before.

I would cut a 4" round hole in the center of each block and stuff loose fill in there.

 

[Nealtw]

How to insulate those areas, though? Can't easily drill a hole and and shoot foam in and ensure complete coverage and without the risk of blowing something apart. Can't easily slide fiberglass or XPS foam boards sideways to fill the voids. Hard to use a homeowner available spray foam kit that doesn't have a long wand to get to the end of the cavity. It would have been nice if the builder had insulated those cavities when they were accessible.

Joists were to short, I have never seen that before.

 

[Mroussakis]

I did not see this picture before.
View attachment 674278
I would cut a 4" round hole in the center of each block and stuff loose fill in there.

Isn’t that sort of insulation prone to mold?

 

[jim_bee]

Joists were to short, I have never seen that before.

After looking at the pictures again, I think you are correct. It's a new one on me, too.

 

[Nealtw]

Isn’t that sort of insulation prone to mold?

Mold will grow on anything if you allow moist dirty air to get to where the surface is cold enough for the moisture in the air to condense and deposit the dirt in the air on that surface. The dirt is the food but it needs a supply of moisture.
So if you stuff it with insulation of any type and seal it so there is no circulating air you will be fine.

 

[Mroussakis]

Mold will grow on anything if you allow moist dirty air to get to where the surface is cold enough for the moisture in the air to condense and deposit the dirt in the air on that surface. The dirt is the food but it needs a supply of moisture.
So if you stuff it with insulation of any type and seal it so there is no circulating air you will be fine.

But that is sort of the crux of the problem . That inaccessible gap you suggest filling would have air circulating. Outside air which sneaks in through the outer wall of the house and gets into the gap. Which would then meet the warm “wedge” and presto, condensation.
Even if I stuff the gap, and then seal each wedge, warm air from upstairs would be able to permeate the gap, where it would meet cold air, and again, condensation.

maybe I am dead wrong?

 

[Chris616]

Condensation happens when warm moist house air comes into contact with a cold surface, not the other way around. If you can fill the void with insulation and then encapsulate it so that house air can’t get to it (then through the insulation to the cold rim board) you’ll have achieved your goal. There won’t be much air infiltration through the multiple layers of flooring from above, except in specific places like where the forced air duct boot is going through in one photo. That needs to be sealed up. Ditto with the hole that you cut/drill to insert the insulation and the perimeter of the boards that you’re referring to as “wedges”.

I don’t envy you the task of drilling multiple big holes in 2X fir with a hole saw.

 

[Nealtw]

But that is sort of the crux of the problem . That inaccessible gap you suggest filling would have air circulating. Outside air which sneaks in through the outer wall of the house and gets into the gap. Which would then meet the warm “wedge” and presto, condensation.
Even if I stuff the gap, and then seal each wedge, warm air from upstairs would be able to permeate the gap, where it would meet cold air, and again, condensation.

maybe I am dead wrong?

Out side air is the same temp as the out side wood on the house. Stuffing full does not leave cavities full of air that would expand and contract. You only have to worry about water leaks from the outside,

 

[Mroussakis]

Thanks everyone for your responses so far . Very helpful. I’ve sent an email to a structural engineer here in town. I’ve asked if I can just remove the wedges, apply foam, then replace wedges if necessary. This is by far the simplest solution in my case.

I’ll let you know what they say .

 

[Nealtw]

Thanks everyone for your responses so far . Very helpful. I’ve sent an email to a structural engineer here in town. I’ve asked if I can just remove the wedges, apply foam, then replace wedges if necessary. This is by far the simplest solution in my case.

I’ll let you know what they say .

You think that would be easier, I don't.

 

[Chris616]

Well, we’ll throw in our opinions too!

As long as you do them one at a time there is little danger in the joist rolling over since the flooring above is secured to it. I can’t believe that the “wedges” are not nailed in place though, but it might be easier to cut the nails than drill a big diameter hole.

 

[Mroussakis]

You think that would be easier, I don't.

I don’t like filler insulation. The previous owners had it stuffed in every nook and cranny. It was a disgusting black mess. So that’s my rationale.

 

[Chris616]

Yes, it will turn that way if it is not sealed to prevent air infiltration. Because I had the basement ceiling down recently I re-did the rim joint insulation with XPS foam. The 30 year old original had fiberglass pieces with polyethylene covering that was sealed in place. In the bays where the sealing was done well or had not failed the insulation was pristine. Not so much in the bays where there was air moving through. So as long as you do a good job with the air sealing the non-solid insulation would be fine.

 

[Nealtw]

I don’t like filler insulation. The previous owners had it stuffed in every nook and cranny. It was a disgusting black mess. So that’s my rationale.

I understand but what you were seeing is a poor fitting insulation where inside air could sneak passed the insulation and contact the cold surface at the outside.
We we take apart houses even with a vapour barrier we see mold in places like and out let were the air could get around the box and the insulation was lacking around the box. In areas like that the insulation is at least dirty and some times moldy.