# basement floor insulation



## jonny80s (Jun 21, 2014)

I'm contemplating whether or not it is worth it to insulate my basement floor.  

I'm finishing a 900sq/ft portion of my basement.  The walls I insulated with rigid foam, foil faced, framed over that now I'm putting in unfaced r15 batting to bring my wall insulation to ~R22.  The exposed wall, walk out basement, I believe is an r13 batting.

I'm heating the space and hopefully the 1800sqft above it with a pacific energy summit stove.

My question is,is it worth it to insulate the slab floor with the same r7.5 rigid foam or am I wasting money as my future father inlaw says?

I'm in northern Connecticut.


----------



## realstihl (Jun 21, 2014)

Insulation is almost never a waste. I don't know how much heat is lost in a floor. Can't hurt to insulate the floor.


----------



## velvetfoot (Jun 21, 2014)

I don't think it can hurt either from a heat loss point of view.  If you insulated below grade walls, why wouldn't you insulate the floor.  It'll feel good on your feet too.  Back when I thought of actually finishing the basement, I thought about it a lot.  There are things to think about.  Like is your floor dry?  Do you want to maintain use of floor drains?  What happens in the event the floor floods, etc.  Since my floor was dry and I had no drains, I was leaning towards putting some foam down and lay some plywood (I think that's what I was thinking) and tapcon screwing down to the floor.  I don't know how it would dry out if it got wet though.  You need a half inch of wood or sheetrock on top of foam though for fire reasons.

There are basement underlayment systems that still allow drains to work, but they may have downsides too.

Plus, you'll lose some headroom, if that's a consideration.


----------



## WiscWoody (Jun 21, 2014)

It'd be nice to do it if you can swing it. I've see some wood blocks for laying over a cement floor before. the lock together and have maybe a one inch space underneath rhat would be good if it was damp. I have a ICF foundation also. Another thing I've thought of is just putting some rigid polystyrene. They make a denser type that can withhold more abuse too.


----------



## Have maul will travel (Jun 21, 2014)

How much are slightly warmer feet worth to you?  You are really not going to lose much heat through a basement floor.  

The temperature difference between inside and "outside" (or underneath) is going to be the least anywhere in your house as the cold air inside will settle there, thus the inside face of the slab would touch perhaps 55-65 degree air and the ground temperature it touches underneath is going to be warm relative to the outside temperature probably 45-50 degrees in mid-winter.   So the temperature differential is more than likely only going to be 5 - 20 degrees even on the coldest of days, which means you are not going to save much heat by insulating your basement floor which, in turn, means that it would probably be cheaper to gain that heat back buying extra fuel than save it with the cost of rigid insulation.  

Your bare feet will be considerably warmer than the air however (98.6 vs. 55-65) and thus the insulation would make more of a difference to them.

I would rather put down a rug, wear slippers and put the money in the bank, but I have been a father-in-law for a long time now.


----------



## WiscWoody (Jun 21, 2014)

In the last basement I finished in Minnesota I had a thick berber installed with a good pad under that. It was a dry basement so no problems there.


----------



## Dune (Jun 23, 2014)

Heat loss through floor=15%


----------



## johneh (Jun 23, 2014)

This is what we use in any basement remodels we do 
It comes in 3 different insulation ratings and gives an 
excellent thermal break and moisture control 
http://www.amvicsystem.com/amdry-insulated-subfloo


----------



## Dune (Jun 23, 2014)

johneh said:


> This is what we use in any basement remodels we do
> It comes in 3 different insulation ratings and gives an
> excellent thermal break and moisture control
> http://www.amvicsystem.com/amdry-insulated-subfloo


Could you try again? The link didn't work for me.


----------



## johneh (Jun 23, 2014)

Looks like I dropped the "r" in subfloor try this one
http://www.amvicsystem.com/amdry-insulated-subfloor


----------



## Soundchasm (Jun 26, 2014)

I got quite a surprise when a minisplit wasn't heating a sunroom built over an external concrete garage.  There was no way that room was ever going to get warm with a 45 degree floor.


----------



## jigger47 (Jun 26, 2014)

This what I laid down http://m.homedepot.ca/product/ProductDetails.aspx?productid=813313&lang=en 
dmx one step sold at Home Depot. Laid OSB t&g over top. Not sure of r value increase but it does feel warmer. Laundry room is it not done as it  has e a floor drain and floor is cooler (bare concrete). I plan on buying a digital temperature reader in the future.  I like the dmx as it allows some air movement.


----------



## Bret Chase (Jun 29, 2014)

jonny80s said:


> I'm contemplating whether or not it is worth it to insulate my basement floor.
> 
> I'm finishing a 900sq/ft portion of my basement.  The walls I insulated with rigid foam, foil faced, framed over that now I'm putting in unfaced r15 batting to bring my wall insulation to ~R22.  The exposed wall, walk out basement, I believe is an r13 batting.
> 
> ...



putting foil faced insulation in the middle of an insulation system really isn't a super awesome idea. there should be exactly one vapor barrier, and that should be directly behind what ever wall covering (sheetrock, etc) you're using.  

as far as the floor insulation... there is a decision to make.... to you want the basement slightly warmer in the winter, or a LOT cooler in the summer (with no external energy input)


----------



## woodgeek (Jun 29, 2014)

Its all about choosing your battles.  The way insulation works, it doesn't make sense nor is it cost effective to insulate one part of your house much more than the others. Because the ground is warm, insulating under your slab to R-10 only makes sense if the rest of your house is already superinsulated.  If not, you almost certainly have better places to put your effort.  Usually this means airsealing the attic and basement.

One rule of thumb for super-insulation in new construction is the 5-10-20-40-60 rule, which is the target R-value for windows, under-slab, basement walls, upper walls, and attic.  Done correctly and airsealed well such a house would use about 2X as much heat as a passive house, 40% as much as a current code-built house, and 20% as much as a typical existing house.  This is roughly the sweet spot (for new construction) with has the lowest cost of ownership.

Getting an existing house to this level is a huge undertaking, wiping out most of the interior or exterior finishes, or both, and costs $50-100k, and is in general NOT cost effective.


----------



## mass_burner (Jun 29, 2014)

We're doing our ~1600 sq ft open basement in a modern minimalistic decor, so the concrete is a plus. I'm begrudgingly insulating the walls, but smooth grey concrete floors are a must.


----------



## johneh (Jul 1, 2014)

mass_burner said:


> We're doing our ~1600 sq ft open basement in a modern minimalistic decor, so the concrete is a plus. I'm begrudgingly insulating the walls, but smooth grey concrete floors are a must.


And that is why you put the foam under the slab


----------



## mass_burner (Jul 1, 2014)

johneh said:


> And that is why you put the foam under the slab



floor is already there.


----------



## 1750 (Jul 3, 2014)

We insulated under the slab with 2" xps when we poured a new basement, and it seems very cozy.   

I guess you wouldn't be getting the same thermal break by insulating above the slab (as it would be inside the building envelope?), though it would seem the rationale would be the same for insulating the inside of a foundation wall. 

However, most people here know more about this than I do!


----------

