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gatorfan 06-02-2010 04:24 PM

Insulating Floor, No Sub-Floor
Hi all,

I'm slowly renovating our house, built in 1910. The home is built with stem walls and piers over a crawlspace. The first floor decking is all heart pine, but they are nailed directly onto the joists with no sub-floor beneath. A few questions:
  1. The crawlspace is extensively vented with large sections of stem wall cut out and filled with vertical 2x4s spaced 3-4" apart and covered with screens. Our HVAC is a retrofit (obviously) and uses (primarily) flexible ducts through the crawlspace to heat and cool the first floor. Should I consider blocking off the crawlspace vents and insulating the entire space? Or should I just insulate between the joists? Below the crawlspace is bare ground, not concrete.
  2. What's the best way to block air infiltration from the crawlspace into the house?
  3. What sort of vapor barrier should I use, if any?
We're located in west-central Florida (zone 2), so DOE/Energy Star calculators indicate a R-value of R13-R19. I'm also curious to hear opinions on the plastic sheeting systems that are designed cover a dirt floor and attach to the sills to act as both a vapor and air barrier. We do not have a moisture problem-- it's very sandy soil and bone dry year round.


Perry525 06-03-2010 04:45 AM

You want to insulate against heat gain?
And or cold?

To avoid heat gain, you need a white roof, a reflective membrane just under the roof and insulation under that.

Followed by insulating all walls facing South, East and West.
And under the floor.

Avoiding heat loss during winter needs a different approach.
Insulation needs to be as close to the rooms as possible.

In either event, the vents in the crawl space need to be blocked and all holes, cracks and gaps filled.

As you do not live in the crawl space, the insulation needs to be as close to the floor as possible and it needs to include and cover the joists and any other wood attached to the floor.

The insulation needs to be polystyrene tightly fitted below the floor.

The floor should be taken up, the joists covered with waterproof 3/8 inch ply and the perimeter carefully sealed with foam, then a layer of plastic sheet, then replace the floor.

The normal water vapour barrier is placed under the drywall, to stop the water vapour created in the home from entering the wall to reach the cold outside, and then condensing on the wood framing and starting mould and wood rot on its way through this is where the majority of days and nights are warmer indoors due to heating, and cold outside.

Where the majority of days and nights are warm and wet outside and the inside is kept cool by air conditioning, then the vapour barrier is fitted on the outside of the framing as the outside water vapour will try to reach the cold inside, and will create condensation inside the walls.

It is always best to ignore recommendations, as todays recommendations will be superseded tomorrow, when it is recognised that todays recomendation was not good enough.

Look back at history and compare how your home was designed and built, with todays standards and with what Europe and other countries are doing today, and recognise they need to improve to morrow.

People do not seem to understand how water vapour is made, where it comes from and where it goes. You are lucky to have sandy soil and no water vapour problems. You do not need to lay any plastic sheets on the ground in your crawl space.

handyguys 06-03-2010 11:14 AM

You will want to insulate the floor. Use faced batts, vapor barrier UP, towards the finished space. They make wire rods to hold it up just for this purpose.

That leaves the ducts in the crawl space. They too should be insulated. Your options are to either replace them with insulated (ideally ridged ducts) or wrap them with insulation. If you are lucky then they are already insulated.

Leave the ventilation in the crawl space as is. Thats good.

Your two projects (insulating ducts and insulating floor) don't need to be done at the same time. The floor is your priority in my opinion. Then the ducts.

lastly - The heart pine IS your subfloor, its what they used back then. ;)

gatorfan 07-19-2010 08:14 PM

Sorry to ask my question and then go MIA for a while! Long story....

Anyway, my first priority is to get some insulation in the attic floor. I have major air infiltration issues as well, so I'm tacking those as the next priority, even before looking into wall insulation. I'm sure that adding insulation to the floor will help, but I'm concerned that I'll still have significant air infiltration through it as there is no subfloor. Will the stapled facing paper be enough to effectively seal it?

Will this problem ultimately be moot once I have the floors redone and a bunch of poly overcoating added? These floors are original and have had some serious shrinkage causing large (up to 1mm) gaps.

The ducts are insulated, but only modestly.


gatorfan 07-19-2010 08:39 PM

Sorry, to answer Perry's question, I need to fix both heat and cold. Energy bills are high in the summer and the heat pumps can't keep up at all in the winter. Our biggest problem is air infiltration, not insulation. We had FPL come by for a duct leak test (blower door), and he couldn't get the house depressurized below -18. I can't remember what he said he needed but was ~50 IIRC....

I'm looking at replacing the doors and starting to seal cracks around the windows. We have an out-of-service boiler/radiator system that I'm slowly dismantling and filling holes where they passed into the crawlspace and attic. I've already removed the massive 6x3' louvers to the whole house fan (a great idea in some places, but not that useful in FL when a/c is available).


HillbillyRob 10-16-2010 05:36 AM

I have a modular home in NC we get temps from 2 to 107 degrees.
They did not bother with moisture barrier on the ground in the crawl space so we have moisture / mould issues. I
had planned to put down moisture barrier and insulate the crawlspace walls this summer, but had health stuff going on and no budget.
The last house I was in was 1920 stem type craftsman.
Here is what we did. Moisture barrier on the foundation walls, and on the dirt, I added wrap insulation to the ducts, they had modest insulation on them.

I closed up the vents used liquid nails to attach styro furring strips to the brick, then glued Reflectix followed by 2" blue styro panels. In the attic was blow in and I put mylar on top of that . Our ac bill dropped from 375 to 140, in winter our gas bill dropped from 190 to 60.
I also used insulated curtain liners from Lowes (20 for standard and 25$ for long windows).
Plus I put a solar powered gable end fan in the attic.
All told we spent about 1200 for the fan/curtains/ insulation.
The floor itself had batt ins pushed up between the floor joists.
The rods are about the same thickness as coat hangers..and they fall out after a time. I took some plastic pallet tie strap and stapled those across the joists.
All this was a bit of a pain as i am partly disabled.
I took it in stages, but it was well worth it..2 am bathroom runs to a cold bathroom brr. The new owner - landlord liked it so much that he jacked up the rent by almost 400$ ..we left.

gatorfan 10-19-2010 07:13 AM

Thanks, Rob. I think I'll eventually do most of your list as it seems to jive with what I'm reading elsewhere. I think the biggest thing I can do is to get some insulation in the attic, but the knob and tube has to go first. We have a 200A main panel someone added at some point, but most of the circuits just feed into the K&T box. I'm doing some prep now and hope to start converting circuits to romex before xmas (starting with 2nd floor!). Luckily the early winter is mild here, so there's a chance I can get the 2nd floor done before it gets really cold (for FL anyway)....

I put a hole in a wall for the first time a few weeks ago to add a new electrical box. Not only is there no insulation in the walls (which I assumed), there's not even any exterior sheathing. I could see the inside of the clapboard! Projects, projects. :D


DrHicks 10-19-2010 08:25 AM

If I'm understanding your question correctly, I'd say you'll want to insulate between the floor joists.

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