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Crawlspace insulation???

kazba's picture

I have a question for the experts.  I am remodeling a house in Santa Barbara, CA.  The entire house was built on footings and the crawlspace under was never insulated.  To insulate the entire underside of the house is obviously a large project, so i was thinking that it would be easier to just add the insulation to the top of the existing plywood floor.  My thoughts are to add a layer of xps to the entire house, seal it, and then just put another layer of plywood on top of the xps.  Another problem with the house is that the current floor sounds hollow (because it is) when you walk on it.  It makes sense to me that this layer of rigid foam insulation might fix this problem, but i was wondering what anyone else thinks.  Obviously, this would raise the floor a couple inches, but this is no problem for my remodel because i am changing all doors and windows and raising the entire ceiling.  I haven't seen any articles that talk about insulating above a crawlspace like this and assuming that there is probably a reason why...but it just makes sense to me.  Does anyone see any problems that i haven't thought about?  Thanks-Brian

kaz (post #207272, reply #1 of 12)

Santa Barbara sounds like a very pleasant warm comfortable place.

so, the obvious-why insulation?

The only problem I can see is that you would seal that subfloor at the top and give the potential for moisture to collect in the subfloor itself.  If the crawl is dry, then I guess that wouldn't even come close to happening.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Yeah, not clear that you need (post #207272, reply #2 of 12)

Yeah, not clear that you need insulation -- certainly not much.

How thick is the plywood, and what sort of floor covering is on top of it?


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

If I understand correctly, (post #207272, reply #9 of 12)

If I understand correctly, the problem is that the floor sounds hollow. XPS might, you say, solve this. A layer of XPS on your floor would, to some degree, reduce the flow of heat from the living space to your unconditioned crawl. Because XPS is a vapor retarder, it would, presumably, also reduce the flow of vapor from the crawl into the living space above. Sound transmission is a separate issue. This, from Dow Chemical: http://dow-styrofoam.custhelp.com/app/an... "STYROFOAM™ Brand Extruded Polystyrene Foam Insulation does not perform well either at absorbing sound or at blocking sound transmission as compared to a variety of materials that have been designed and/or are sold for acoustical control/sound absorption purposes. STYROFOAM™ Brand Extruded Polystyrene Foam Insulation is an ineffective sound-deadening material." If you want to insulate your crawl, insulate your crawl. If you want to change the sound of your floor, do that.

Thanks for the advise, I am (post #207272, reply #3 of 12)

Thanks for the advise, I am hoping to kill 2 birds with 1 stone...but as others posted, i probably don't even 'need' to insulate the crawlspace.  I'll look into a better way to fix the 'hollow floor' sound. If anyone knows a good way to do this, i am all ears.  The floor coverings will be a mix of wood, tile and carpet.

kazba (post #207272, reply #4 of 12)

For the carpeted area-better pad, perhaps in addition to Homasote-1/2".

For the wood, MASS-of one sort or another. 

Tile-again, mass-mud set possibly-or thicker subfloor (which is best at 1-1/4" TCA recommended) then durock.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Another layer of material, (post #207272, reply #6 of 12)

Another layer of material, with adhesive between the layers, should fix the "hollow" sound.  The material can be plywood or some sort of composite (Homosote, OSB, hardboard, etc).  Since mass is useful in this regard, you could even use drywall, except that you'd need a thin layer of something else over it for durability.

Hint:  Before you start such an effort, do a search-and-destroy mission on squeaks, driving deck screws into the existing flooring where necessary to stop them.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Hard to beat concrete for (post #207272, reply #10 of 12)

Hard to beat concrete for sound-deadening. Some condominiums I've been in--over-under, rather than side-by-side condos--had thin concrete slabs poured over a subfloor. You'd need to think through the details, engineering details most importantly, i.e., whether your structure can bear the increased load. But it's not necessarily prohibitively expensive: the condos I mention were low- to moderately-priced, but, then again, they were engineered for, not retrofit for, concrete floors. And the concrete would necessarily be very wet, which raises a number of concerns, but one pour and you're done.

Yeah, I was in an apartment (post #207272, reply #11 of 12)

Yeah, I was in an apartment in the 70s that had about an inch of concrete on the framed floors.  Very quiet and solid.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

no offense intended.. (post #207272, reply #5 of 12)

A hollow sounding floor is one of the least important complaints I've ever heard of by a homeowner.

Assuming of course that insulation there is otherwise superflous.

 

Just sayin'...

.

Insulating floor. (post #207272, reply #7 of 12)

Heat always moves to cold.

It makes sense to have the insulation on the inside of a room, as there is little sense in heating the joists and floor.

Laying a fully floating layer of polystyrene, covered by OSB or plywood is the best you can do.

You have, carpet, underlay, OSB, insulation. existing plywood floor, joists.

This way there is no heat bridging between the OSB and the joists. Result floor surfaceis maintained at room air temperature and feet are always warm.

Water vapor is programed to move from warm to cold as well. About 200 times a year warm wet air arrives outside our home and we see it as frost or dew on our trucks and grass.

This warm wet air is pulled into our crawl spaces by the "stack effect" inside our homes. Having a completely sealed floor with no holes stops the stack effect and helps keep our crawl spaces dry.

Warm wet air only lands as condensation on cold surfaces - surfaces below dew point, wood floors and joists are warmer than the ground outside.

 

 

 

 

 

 

It makes sense to have the (post #207272, reply #8 of 12)

It makes sense to have the insulation on the inside of a room, as there is little sense in heating the joists and floor.

There is no net heat loss from "heating the joists and floor", vs having them on the "cold side", so long as insulation and air sealing are equivalent.  Yes, there is a "cost" involved in bringing a "thermal mass" up to target temp, but once there no heat is required to maintain that mass at that temp -- heat is only needed to replace heat lost through insulation.

If your concern is purely heating efficiency the decision should be made based on which configuration allows the most effective insulation.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Heat loss. (post #207272, reply #12 of 12)

The joists in a typical home are attached to the frame, foundation and  the roof.

The frame etc amounts to many tons of wood, through which  heat moves by conduction.

Paying to heat the joists, frame and roof,   wastes  money.

This over the life time of a building amounts to a high cost.

Insulating the warm air within the home from the floor, ceiling joists, and the walls saves this cost, which over the life time of a home amounts to a lot of money.