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Old 10-07-2011, 10:34 AM   #1
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Crawl Space Insulation Best Practices

Hey guys -

I've been doing some casual research over the past year or so about insulation. I have a house on a lake and as a result its very very humid for a good part of the year. Also, the house isn't terribly well insulation, and to add insult to injury, the only heat is electric baseboard heat. We're not there full time in the winters, but when we are we stay on the first floor and do our best to keep the heat low. Also, we drain the house and turn off the heat when we're not around during the week.

We experimented last winter with leaving the heat on in the kitchen and living room at 50 degrees. When we got the $400 electricity bill for the month, we decided that leaving the heat on wasn't an option.

SO. A lot of backstory I know, but I wanted to explain the situation. I would like to insulate the crawlspace beneath the house. Mostly because I want the floors to be warmer, and I want to reduce the drafts, and hopefully seal up a few holes down there as well where the spiders get in (wishful thinking I know).

I have an estimate from one contractor that goes as follows:

1. Spray foam walls with R10 sealed to vapor barrier on floor $3822
2. Fiberglass floor joist with R30 fiberglass insulation. $1592 (Not an option in my opinion)
3. Spray foam floor joist with R30 open cell foam. $4525

Based on this info alone, what do you guys think? I would estimate the crawlspace is 900sq ft.

Thank you for your thoughts


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Old 10-07-2011, 02:01 PM   #2
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Lets start with the easy bit, how much of a DIY person are you?
You can start by blocking out those spiders, just cover the whole floor with tightly packed sheets of polystyrene anything from one inch to three inches will make a difference, three inches is better. Fill any small holes with spray foam from a can. Then cover with t&g glued oriented strand board. Finish to choice.

The idea is that warm air rises, as it does it pulls cold air in from outside, that is where your heating cost comes from - continually heating cold air from outside, that then disappears through holes in your ceilings and walls.

Having done the floor, move on to the walls and ceilings, again cover the walls and ceilings with sheets of polystyrene, topped with dry wall.....can you do this?
It is not difficult, do a bit when you can and save a lot of the outside walls, then the ceilings, just stopping the warm air from escaping will change your life.


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Old 10-07-2011, 02:43 PM   #3
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I consider myself a handy DIY sorta guy. Better with electrical and carpentry. Overall though, I'm not sure this is a project I want to do on my own. The crawl space isn't remotely even, very rocky, and slopes from 2' to 4' of clearance in different areas. Its not a smooth sandy surface, so I don't think 3" sheets of poly would work well as I still need to access all areas of this crawl space frequently in the winter (to drain the pipes)

I suppose my main question is on the benefits of a vented vs unvented crawl space? It looks as though codes are shifting towards unvented crawl spaces which to me would be an indication that this is a better approach? Does R10 on the walls with Poly on the floor = R30 in the joists?

Do I need to fill the joists to achieve an R30? I thought I'd read somewhere that closed cell foam only needed to be 2"-3" thick to achieve this high of a rating. This particular contractor didn't want to insulate the ceiling and if he did, would fill in all joist cavities. This would necessitate me moving a LOT of wiring. A lot.

So yeah - sorry if I wasn't clear. This is really a question on methodology. I wasn't necessarily looking to validate the quotes he'd provided (though I was surprised at how much more insulating the joists was (50% more $$)

Thanks for the reply!

Last edited by sorebikr; 10-07-2011 at 02:43 PM. Reason: Courtesy (adding a thanks!)
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Old 10-08-2011, 06:14 AM   #4
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all new homes built from 2016 inclusive will have to be of Passive House standard, our Super insulated, with very low air leakage
0.6 air changes per hour.
To do this all holes need to be sealed, including those in and round the crawl space.

Knowing how
it is to work in the crawl space my proposal was to lay the sheets of
on the floor and to cover them with OSB t&g boards.
This stops the loss of heat from your comfort zone into the crawl space via the floor and joists.
The main loss of heat from a home is first holes, second by conduction.
This floating floor is easy to install and avoids being fixed in any way that will create a heat bridge.

Filling the spaces between the ceiling joists with polystyrene, requires care and is time consuming and messy if you use a saw, using a hot wire is a lot quicker and clean. Polystyrene starts to melt
There is no need to move any wiring, just work round it and fill any gaps with foam from a can.

Keep in mind that the best way to insulate a ceiling or wall is to fix the sheets of polystyrene below the joists or on the inside of the wall, to stop the heat being conducted through the wood, wood is not a good insulator and it makes up a very large amount of ceiling and wall, merely placing insulation between the joists/sticks means that a quarter of the ceiling/wall is not insulated....that's a very large loss area. I
three inches below the joists and as much as you can fit in between the joists for a really cheap to heat and comfortable to live in home.

The water vapor that we produce in our homes by cooking, washing, breathing, sweating, having animals, indoor plants, drying programed to move towards cold.
Given the chance and a hole, that water vapor will move into a wall or roof, condense on the nearest cold surface and may start mold and or wood rot. Having a layer of three inch thick polystyrene enclosing your living space will ensure that the surface never reaches "dew point."
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Old 10-08-2011, 07:40 AM   #5
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Personally I would go with option (1) putting foam insulation on the walls and poly on the ground.

When its all said and done there is usually more said than done
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