Adding usable living space from garage... few issues

Going to cut into my large garage and add a approx. 18X10' area for a mudroom, laundry and half bath. Half of this area has crawlspace under it but the other half is garage slab. My home has a full basement. The house floor is about 12" above the slab, so the addition will be built up to match the house floor. Here is where my question comes into play.

What is the best route for me to seal up/insulate this area under the floor? If I use 2X8 floor joist this leaves around 4-5" of space from the bottom of the joist to the slab. I am planning on extending my exterior (in the garage) walls to the floor. just looking for any ideas here. There will be some plumbing and drain lines in this floor as well. Looking for cost-effective options. I was thinking regular batt insulation and then housewrap attached to the bottom of the joist. I prefer to stay away from sprayfoam because of the cost but I was also considering just filling the whole area under the floor with the stuff.

I live in south-eastern PA so cold weather insulation is a must.

The problem has two questions, will water freeze in the garage, how good is the drainage around the garage.

We don't get the long cold spells here you do and we are required to dig it up and put a footing in below frost level..

The fear is that the frost will raise the floor and settle again.

One cheat would be to run a beam from side to side that would be big enough to carry the wall and half the load of the floor and half the live loads you put in it.
That would be above the floor and would not be damaged with any floor movement..
I would be inclined to install foam board on the concrete if you can keep your plumbing in the joist space running back to the basement.
You still have to to seal the garage so no auto off gasses get into the house.

I don't believe there is any slab movement so I don't think this will be an issue. The inside of the garage rarely gets below freezing and the property has proper drainage as well. the plumbing will be confined to the joist, with the only possibly exception being the drain line where it enters the crawlspace my be just starting to be below due to proper fall.

Although I would not be opposed to cutting the concrete and digging a footer if I had to, I don't believe this will be necessary.

psu927 said:

I don't believe there is any slab movement so I don't think this will be an issue. The inside of the garage rarely gets below freezing and the property has proper drainage as well. the plumbing will be confined to the joist, with the only possibly exception being the drain line where it enters the crawlspace my be just starting to be below due to proper fall.

Although I would not be opposed to cutting the concrete and digging a footer if I had to, I don't believe this will be necessary.

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I suspect a proper footing would be about 4 ft deep out there so anything would be better than that, in my book any way.

If you are in the mood to chance it. there are still some things you can watch out for. If the garage roof trusses are parallel to the wall you want to built, try to miss it so if everything does move the roof is not effected..

If your floor joist are not resting over a foundation, you'd be Ill advised not to place one.

Interior partition walls over a slab are common because they are not subject to floor loading, your's is not the case.

How will the under floor area be ventilated?

Kraft backed batts, for your climate zone will work.

Snoonyb said:

If your floor joist are not resting over a foundation, you'd be Ill advised not to place one.

Interior partition walls over a slab are common because they are not subject to floor loading, your's is not the case.

How will the under floor area be ventilated?

Kraft backed batts, for your climate zone will work.

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That is part of my question (ventilation) was thinking about 2 vents into the garage.

I'm not worried structurally about my plan, I have 5" concrete that is not subject to frost movement, supporting half of a 180sqft area. we will be fine there.

Although a "CARMON" barrier is mandatory between an enclosed parking area and sleeping quarters, I'd still be reluctant to have underfloor vents in an enclosed parking area, I'd put them on the end, even though the natural flow isn't across.

Remove the rim joist or parts of it and run the new joists onto the foundation and the area below your floor becomes part of the basement.

nealtw said:

Remove the rim joist or parts of it and run the new joists onto the foundation and the area below your floor becomes part of the basement.

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This is a great idea. It would be part of the crawlspace. There would be about 12' of area that would be just joist over concrete. I would then just insulate the joist, and the new exterior walls around the outside if I went this route

psu927 said:

This is a great idea. It would be part of the crawlspace. There would be about 12' of area that would be just joist over concrete. I would then just insulate the joist, and the new exterior walls around the outside if I went this route

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For a crawlspace you still want cross venting, you could cut holes in the foundation on each is or you could duct it up and out each side. A little box in the corners either in the garage or the new space, sealed behind the drywall.

psu927 said:

This is a great idea. It would be part of the crawlspace. There would be about 12' of area that would be just joist over concrete. I would then just insulate the joist, and the new exterior walls around the outside if I went this route

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You will need a moisture break between the floor and the bottom plate and you want to keep any water on the garage floor and air from getting in under the additions.
With this product you can get both with one trick.
https://www.homedepot.ca/en/home/p.blueskin-weather-barrier-6-inch.1000403475.html.
With a primer it will stick to the concrete and if you cut the paper and only remove the paper for the portion going under the plate and stick it down with the excess out into the garage. Later leaving the paper on staple it up to the wall as far as it will reach. Remove the paper when you are ready for drywall.

You can almost always find left overs for sale on craigslist.

You're changing non-habitable space into habitable space so if you get a permit/inspection you've got to build to code which is probably going to require a footing and proper isolation from the garage area. Your inspector may approve building on the slab but will probably want you to prove it's thickness. You're also affecting your homeowner's insurance (they will want to see the permits and inspections too) with the added habitable space.

We can help with the methods but until you know what the inspector will want to see with the foundation the rest of the plans need to wait as that will affect almost everything else involved.

Phil

The first thought that came to mind about fiberglass insulation was "heaven for field mice".

slownsteady said:

The first thought that came to mind about fiberglass insulation was "heaven for field mice".

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My thoughts exactly. I am now considering spraying the void with closed cell sprayfoam. At least the bottom and sides. For cost savings I wouldn't do the whole void, I don't think it would be necessary.

Have you ever heard of "encapsulated" fiberglass, which has been on the market for roughly 10yrs.

May be worth investigating.

Snoonyb said:

Have you ever heard of "encapsulated" fiberglass, which has been on the market for roughly 10yrs.

May be worth investigating.

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I have, but I was under the impression this was mainly for dust control

That's one of the attributes. The encapsulation has to be breathable for the product to insulate, another is that when you are working around it the fibers do not get into your clothing and you don't drag it around like blown in.

An architectural facades in shopping centers are usually glued on foam and if you have ever seen them damaged, it's not from the weather, but insect, bird, and rodents, following the food chain.