Hi all,
Thought I’d try this here. No response over at Energy. I’m new, I’ll find my way around before too long. This may be old hat but I just want to be sure.
Is there a recommended construction detail of installing an insulated and heated space over an existing porch slab. The house is between 40 and 50 years old and the slab is original to the house (no vapor barrier under the slab). The 7-8 inch step down is to be elliminated. Overall dims 12′ x 16′
Here’s my take:
- Poly vapor barrier.
- 5/4 PT sleepers at perimeter and on 24″ centers fastened to slab.
- Fill voids with rigid insulation
- PT Rim Joist at perimiter (trimmed to eliminate slope).
- One side bolted to house.
- PT Floor framing resting on sleepers.
- Attach framing ot sleepers
- Bat insulation between floor joists. (Air space required?)
- 3/4″ TAG sub floor.
- Bearing walls.
- Finsihed floor.
- Poly wraps up over rim joist all 4 sides.
This is the method I used (about 10 years ago) on a slab elevated about 30″ above grade to repalce a botched job that resulted in standing water under the floor. This next job is very close to grade (3″ – 4″) and has been seen with heavy condensation.
The standing water was due to the siding being inboard of the slab by about 1/2″. I believe it was rain water and not condensation.
Is there a better way? Are there any pit falls with this method?
Thanks,
Marc
Replies
Your best bet would be to start anew.
Why go to all that work on a questionable slab, especially a patio without footings. That would not pass muster in FL with the Building Dept. and probably not too many other locations.
Your location has a lot to do with your foundation construction needs but with a simple slab on grade and 12" excavation (FL figures) you are looking at removing about 2.5 yds of old concrete and pouring new about 6 yards. A little over 4 tons of concrete rubble to cart off would be cheap at any price to obtain a good underpinning.
It's actually a screened in porch. I'm sure is has footings. I'm just looking for resources that define how to vapor barrier and insulate the new floor.
Marc
Marc,
Where are you building? Humidity and insulating requirments differ across the states. I can better answer questions after knowing this. It sounds like humidity and condensation are a concern and it should be. You don't want to create a mold farm.
Per the last response: Check to see if this slab does have a perimeter footing. I've seen too many screen porches built on flooting slabs. Sounds like your new structure might need more support for that intended for a screen porch.
I'm in central New Jersey, the house is about 1.5 miles inland from the coast. I'll have to dig to make a definite call on the footings. Footings here must be 36" below grade. There is a complete roof over the porch, Plywood ceiling, knee walls and 3 - 4x4 posts in each corner and screens. There is a continuous header on the 3 open sides. I was going to support all that and add the floor and new floor to ceiling windows.
Marc
Marc,
Lets assume the footings are adequate and the old structure is strong.
You had mentioned a 7-8 step down to the slab. I would use 2X8's for the joists and place 16 oc. and a minimum of 3/4" tg ply for the floor. I would put down a perimeter mud sill, say a 1X4" (green treated), tied down to the slab. The 2X8 ' won't span 16' so block the joists and add green treated shims from the bottom of the joist to the slab at the mid-point. lets see 3/4" + 71/4" + 3/4" = 8 3/4" might be too tall. You could go to 2X6 joists, but at that point I would start from scratch and the insulation listed below will not work.
To insulate I would use a R-19 fiberglass bat. Would allow >2.0" of air space to the slab. Install with fraft paper to ply floor. Cover the slab with 6 mil. (min) poly to prevent moisture from slab getting to the insulatuion. Also it is important to ventilate the rim joists all around the perimeter to allow the area below the insulation to breath.
Their might also be a chance you could use "closed cell" insulation, like polystyrene to cover the slab, but I've never tryed that, maybe someone else has an idea.
Good luck,
The slab is pitched quite a bit and actually goes from about 6" to 8" along the 12' dimension. After I compensate for the thickness of the floor and mud sill (sleepers) there's not much space left. I have to trim all the joists to get a level floor. Insualtion becomes a problem because it will be hard to maintain a 2" air gap with tapering joists. Maybe I can use the same thing they use between roof rafters to keep the air gap. It's thoes arched styrofoam panels.
I might have overlooked the ventilation, thanks.
Would you install the mud sills on top of the vapor barrier and bolt through to the slab? Or install the sills first?
Marc
Marc,
You might be better off reducing your insulation to a 3.5" bat instead of compressing it with a eve channel. When you compress fiberglass insulation you reduce it's effective R value. You might consider using styrene insulation instead. Reducing the size of the joists sounds like a nightmare to me. Remember I would not suggest building this base with 2X6 construction and it sounds like that too would be too tall.
Your mudsill question:Install the mudsill over the poly vapor barrier and bolt through.
With only 6" to work with, you might be better off with new concrete. Take to some concrete professionals in your area about pouring 3 to 5" over a 3"styrene base on the old slab, or start from scratch with a new insulated slab. This would also be a good choose if you are considering infloor heat.
Good luck.