Insulating in basement ceiling under porch
brizbot
| Posted in Energy Efficiency and Durability on
Hi All,
I apologize if I am asking a common question. Rest assured I did a lot of research before coming here to no avail.
So here is my situation. My semi-finished basement (concrete floor, concrete walls, etc.) is divided into three sections:
1) Section 1 is under our covered porch
2) Section 2 is under our covered porch
3) Section 3 is under the rest of our house.
I am wondering whether it is a good idea to insulate the ceilings in either Section 1 or 2. I know section 3 is a no-no because it is covered by our house which is heated.
Thanks for all of your guidance. You folks are like wizards to me!
Briz
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Replies
You can SAFELY insulate under all three if you like, but the major benefits would be in #1 & #2.
With #3 it's sometimes useful for heating zone control, but it doesn't take much insulation to achieve that.
There are lots of particulars to get right for #1 and #2, which depend a lot on the construction type and how well enclosed the porch is, your location/climate etc.
Thanks so much Dana. I live in Western PA. I think the one thing I should do is put some squares of foamboard against the rim joists that face exterior (underground). The porch is covered by the rest of our house... I am thinking of putting one of those electric "Amish Style" fireplaces in there which will work nice since the room is like 10 x 10 (and hopefully with the ceiling batt insulation)
Briz,
The problem with the type of construction you are describing comes with wind-blown snow or wind-blown rain. If your porch lacks watertight walls, it's common for the porch floor to get wet. When that happens, it's essential that there be a watertight roof under the porch floor -- between the porch floor and the floor joists.
If the floor assembly of your porch includes a waterproof roof, or if your porch has weatherproof walls, you can insulate.
Thanks as well Martin. Luckily that part of the porch is covered by a roof and surrounded by three foot walls and windows on one side so rain/so is not a real issue.
Most of western PA is US climate zone 5A, except for higher elevation areas:
https://www.greenbuildingadvisor.com/sites/default/files/images/DOE%20climate%20zone%20map.preview.jpg
The band joists and foundation sills can be insulated with foam-board if you air-seal it well with can-foam. IRC code-min for zone 5 for foundation walls & band joists would be R20 total if between joists, R15 if continuous. You probably have sections of both. Since R20 foam is pretty expensive, note that it's possible to get there with R7-R8 foam against the band joist, with R13 batts to the interior side, as long as you make an air barrier toward the interior. The cheapest foam would be reclaimed or factory-seconds goods, which you can often find on craigslist (search the materials section for the terms | rigid insulation |.) With 2" of foam (any type) you would be good for up to R15 of fiber insulation between the foam and the conditioned interior. For the exterior walls (both above grade and below), continuous 1.5" of foam (any type) or more would be good enough dew point control for up to R15 batts (unfaced or fiber faced, but NOT foil faced) in a studwall. Put an inch of EPS or XPS (but not polyisocyanurate) under the bottom plate of the studwall to thermally isolated it from the slab, and limit capillary wicking into the wood.
The material stack up between the porch floor and the basement ceiling matters. Layer by layer, what do you have?
Hi Dana
Thanks for the comprehensive explanation as well. I will be following it closely.
Between the basement ceiling and porch floor is about 2-3 layers wood. The porch floor is wood slats painted with deck paint.