much like insulating subfloor


Old 02-23-00, 06:25 AM
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i have a 12x16 addition resting on piers. the room is insulated, floor to ceiling, and during the winter is moderatly warm 50 degrees. the floor has insulation construction like this, in layers

-3/4 floor board
-2x8 joists filled with pink insulation
-fiber board holding up the insulation
-2 feet of crawlspace, enclosed from the elements, and vented.
-ground, covered with plastic sheet.

i would like to further insulate the floor, and was thinking of doing the following. on top of the carpet, lay a flat framework of 2x4's or 2x3's. in between place rigid pink insulation, cover all with 1/4 or 1/2 board, then carpet. is this worth it? should anything go under the 2x4's, over the carpet (plastic?) should i seal the perimeter of the room with a caulk to ensure no air infiltration etc. i look forward to hearing from you. [email protected]
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Old 02-23-00, 08:24 AM
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Insulate the enclosure around the crawlspace,
an leave it's ceiling open.
Old 07-21-09, 02:17 PM
Join Date: Jun 2005
Location: New Hampshire
Posts: 224
I'd like to revive this post...

I have the same situation, but without the pink in the joist bays or the fiberboard holding it up.

I do have 3 layers of subfloor - 1x6 pine + 1/2" fiberboard + 1/2" plywood + 3/4 pine t&g.

I don't think insulating the enclosure will do much as the heat will escape throught the ground which is at ground level.
Old 07-21-09, 04:12 PM
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Join Date: Dec 2005
Location: VA
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remove all the insulation a spray closed cell foam would be best
Old 07-22-09, 06:06 AM
Join Date: Jun 2005
Location: New Hampshire
Posts: 224
thats what I was thinking as well, but i'm worried about cost.

Any idea how much that would cost? the cabin is 32'x24'. with quite low clearance underneath.

To do it, they would have to use extension poles to get to the back of the cabin as clearance slopes from 3' down to just under 1' at the back.
Old 07-24-09, 09:46 AM
Join Date: Feb 2008
Location: Lake Wales, FL
Posts: 463
The floor insulation as described seems to be pretty good and I wonder if all the work you propose will make much difference.
In the normal way heat rises and the floor is cooler than the room.
Yet heat is attracted to cold and some heat will escape through the floor.
However, you seem to indicate that the space between the joists is filled with pink fibre glass eight inches thick?
Fibre glass is not a very good insulation in most situations, but enclosed as you describe and eight inches thick it does perform well. (not as well as closed cell insulation but in this instance well enough)
I accept that the weak link in your current set up is the wood of the joists themselves, that provide and express route for the heat from the room to move by conduction to the cold of the crawl space, but fixing two inch thick sheets of polystyrene under the fibre board will fix this.
Keep in mind that if you proceed as indicated you will have to move all your doors and associated wood work and probably your heating as well. A lot of work for a very small gain.
Probably laying a decent quality underlay under the carpet will do the trick.
Or better, add the polystyrene as well.
You write "they" why not do it yourself?
Old 07-24-09, 10:34 AM
Join Date: Jun 2005
Location: New Hampshire
Posts: 224

I revived this old post because I have a similar situation to the original poster, the difference being my joist bays are not filled with any insulation at all.

I say "they" when it comes to sprayfoam insulation because I would have no way to get the foam to the remote areas of the underside of the cottage without long extension poles on the nozzles (10 to 15 feet at least) where clearance is less than 12" from the bottom of the joist to the dirt. Also, the DIY sprayfoam kits by most accounts are not great even where there is good access. excavating is not an option because its in NH and the areas of low clearance are bedrock. (guess I could blast it),

The only other way for me to access those areas would be to pull the floor up and creat and fill the cavity from the top. Seems like a lot of work and expense vs hiring a pro with the right tools for the foam.
Old 07-24-09, 12:06 PM
Join Date: Feb 2008
Location: Lake Wales, FL
Posts: 463
Good point, OK if you look into this, I think you will find that because what you propose will be difficult or almost impossible, the only option is taking the floor up.
When the two chemicals are mixed together to make the foam the setting takes place fairly quickly. I do not think that squirting the foam out of long pipes can work, at least not in a way that a contractor will be happy with.
And it gets cold in NH.
This has advantages, in as much as, condensation and wood rot don't happen below 40f. And that's a chunk of your year.
Never the less you should cover all exposed woodwork with waterproof paint, especially the bottom of the joists where the coldest part is.
Filling the gaps between the joists with friction fit polystyrene sheets is the best bet. Then rely on the conducted and radiated heat from the rooms above to keep the bottoms of the joists warm and thereby avoid wood rot or mould.
Old 07-24-09, 12:38 PM
Join Date: Jun 2005
Location: New Hampshire
Posts: 224
rot on the joists is very much my main concern with insulating down there. I already have some rot at the lowest clearance areas that needs to be taken care of from top down.

Are you saying that if I were to have a couple of inches of sprayfoam sprayed to the underside of the floor between the joists would that lead to rot where the wood is covered with the foam?

I should add that I am planning to install a wide plank pine floor on top of the vinyl tile floor, and i'm planning to install a vapor barrier between the vinyl tile and the wood floor. I'd be worried that spraying foam underneath would trap moisture between the floor and the spray foam leading to rot.

Other options considered are rockwool insulation (much better than fiberglass) in the joist bays that I can reach enclosed with fine wire mesh to keep the varments at bay but allow the bays to still breath.

or do nothing, crank the heat and wear slippers. Its a vacation/weekend cottage used more in summer, but I'd like to begin using it in winter as well.
Old 07-25-09, 04:09 AM
Join Date: Feb 2008
Location: Lake Wales, FL
Posts: 463
As I wrote yesterday, as long as the temperature is below 40f the amount of water vapour in the air is so low that as such condensation can be ignored. Its less than 5 grams of water in a cubic metre of air. Whereas the amount of water in exposed wood is very much higher.

I would imagine that the crawl space temperature under your cabin is in the region of 56f for most of the year, meaning that the air, wood, rock and concrete are all roughly the same temperature most of the time.
The problems start when cold air under the cabin is suddenly replaced by warm wet air, the wood still cold attracts the water vapour out of the air, condensation forms and provides a suitable place for mould to grow.
Over the years, peoples perception has moved from lots of ventilation to dry things out (that doesn't work) to, sealed dry areas. The ventilation has lots of undesirable water vapour, whereas controlled ventilation or sealed dry is easier to live with.
Once the water/damp is removed from mould it dies, wet rot is the same, dry rot will try to find alternative sources of water, but will die if it fails.
Sealing the wood with waterproof paint or enclosing it in foam will stop it from coming to any harm. However, if the bottom of the joists are left exposed and cold, then condensation can take place there with the risk of mould etc;
Such water vapour that you produce in your home, will tend to rise and escape through the gaps in the ceiling and walls. It will not move down into the floor unless the floor is very cold. Fitting an almost water vapour proof barrier over the floor will do no harm.
The problem with rock wall and fibre glass fitted under the floor, is that they will both become cold, and being cold will attract condensation, then dirt, and this will then make the adjacent wood cold and wet leading to mould and rot.
A closed cell insulation such as polystyrene, it to all intents waterproof, any condensation on its surface will do not harm and will merely dry with the upwards change in temperature.
Their is nothing nicer than a floor with four inches of insulation under, keeping one's feet warm, especially at little or no cost.

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