# Basement Insulation- put extra?



## Bud9051 (Nov 11, 2015)

Hi Erock and welcome to the forum.
In a basement that you plan on finishing the primary place to add insulation, as pictured, are the rim joist cavities. In addition, when viewed with a thermal camera every rim area I have inspected has been leaking cold air like mad. So caulk or foam the sill plate to the foundation and all wood to wood seams. Caulking is cheap and those air leaks can be hidden behind the ends of the floor joists.

Once all air sealed the next concern is to prevent warm moist inside air from reaching a cold surface and forming condensation. And fiberglass insulation does not block that air. One approach is to fit a layer of 2" rigid foam insulation into each cavity and seal it in place. If you go this route that seal in place step can replace much of the caulking previously mentioned. Then add your fiberglass batt or better Roxul batting for additional insulation.

Bud


----------



## jlhaslip (Dec 31, 2009)

To add to what @Bud9051 said, when you add the rigid foam insulation, use a bead of spray foam to seal the rigid to the framing.
Code where I live requires a fire resistant layer of Drywall against the foam if the ceiling is left open and the foam is exposed. Again, use "fire resistant" foam around the drywall to framing connection. There is a different foam for this application. It is not the same as window/door foam.


----------



## Nealtw (Jun 22, 2017)

Is that a soffet area where the floor goes past the concrete wall?


----------



## Erockny (Jan 6, 2018)

Nealtw said:


> Is that a soffet area where the floor goes past the concrete wall?


Good morning Nealtw, That is a section on where the house extends a little bit for bedroom closet. If the proper term is called a soffit then that is what it is. I am not too familiar with building terms.


----------



## Nealtw (Jun 22, 2017)

Erockny said:


> Good morning Nealtw, That is a section on where the house extends a little bit for bedroom closet. If the proper term is called a soffit then that is what it is. I am not too familiar with building terms.


Actually it is called a cantilever but it is not done like we do here.

We would have solid blocking over the foundation, inside it would look like the rest of the basement. outside the floor would get double the insulation and the bottom would be covered with vented soffet material.


----------



## Bud9051 (Nov 11, 2015)

Overhang or soffit work for wording. Overhangs do have some special requirements, primarily air sealing from underneath and insulation as hanging out away from the basement they receive little heat and that results in a cold floor above. It also creates more cold surfaces for condensation. 

Being new to the house you have no history on any moisture problems, either liquid water or moisture vapor, either of which can become a major problem. You also have concrete blocks which can allow a small water leak in one area of the outside to flow around to many areas on the inside. Most basements have a simple coating of tar below grade on the outside water often finds a way to get passed.

Lots of threads on basement finishing and lots of advice available. Here is a related link: https://buildingscience.com/documents/digests/bsd-103-understanding-basements?full_view=1

Was the house tested for Radon?
What is your approximate location so we can reference code insulation requirements?
Bedrooms in a basement have restrictions and need an egress.
What type of heating system as once you start to close things up you may need to provide outside combustion air.

And more
Bud


----------



## Erockny (Jan 6, 2018)

Bud9051 said:


> Hi Erock and welcome to the forum.
> In a basement that you plan on finishing the primary place to add insulation, as pictured, are the rim joist cavities. In addition, when viewed with a thermal camera every rim area I have inspected has been leaking cold air like mad. So caulk or foam the sill plate to the foundation and all wood to wood seams. Caulking is cheap and those air leaks can be hidden behind the ends of the floor joists.
> 
> Once all air sealed the next concern is to prevent warm moist inside air from reaching a cold surface and forming condensation. And fiberglass insulation does not block that air. One approach is to fit a layer of 2" rigid foam insulation into each cavity and seal it in place. If you go this route that seal in place step can replace much of the caulking previously mentioned. Then add your fiberglass batt or better Roxul batting for additional insulation.
> ...


Thanks Bud, That is very detailed info you have given me. I will do that within the next few weeks and was already looking into Roxul insulation.
Erik


----------



## Bud9051 (Nov 11, 2015)

Nealtw said:


> Actually it is called a cantilever but it is not done like we do here.
> 
> We would have solid blocking over the foundation, inside it would look like the rest of the basement. outside the floor would get double the insulation and the bottom would be covered with vented soffet material.


Neal, I can understand the solid blocking but I don't see the reasoning for the vented soffit material to cover it. In an actual soffit we vent it to allow air flow to remove moisture. Here, venting would just allow infiltration and make the floors colder. Any thoughts?

Bud


----------



## Nealtw (Jun 22, 2017)

Bud9051 said:


> Neal, I can understand the solid blocking but I don't see the reasoning for the vented soffit material to cover it. In an actual soffit we vent it to allow air flow to remove moisture. Here, venting would just allow infiltration and make the floors colder. Any thoughts?
> 
> Bud


Air behind insulation, just like not closing under the floor in a crawl space.
It is planning for the day that moisture gets in the system.
BTW fiber glass insulation only allows air to pass thru when the air has somewhere to go.


----------



## Bud9051 (Nov 11, 2015)

Nealtw said:


> It is planning for the day that moisture gets in the system.
> 
> BTW fiber glass insulation only allows air to pass thru when the air has somewhere to go.


There should be plenty of drying potential with wood all the way around, don't need air flow down there.

As for air passing, wind and changes in pressure would be pumping air in and out of that fiberglass insulation all of the time. Plus, any air leaks above into the walls or house would be inviting the cold air in.

In the energy trade we define a boundary layer to establish the air barrier, often the outside shell, sometimes the drywall. But it needs to be continuous and rigid. Here the exterior sheathing would be continuous with that bottom if covered and sealed.

Bud


----------



## Nealtw (Jun 22, 2017)

Bud9051 said:


> There should be plenty of drying potential with wood all the way around, don't need air flow down there.
> 
> As for air passing, wind and changes in pressure would be pumping air in and out of that fiberglass insulation all of the time. Plus, any air leaks above into the walls or house would be inviting the cold air in.
> 
> ...


We use poly for air barrier so it is continues and sealed.
Are you saying you would solid sheet a vented crawl space or a house built on stilts?


----------



## Bud9051 (Nov 11, 2015)

Neal, we are drifting too much on Erock's thread so I'll just drop this link and run. For further discussion we should open another thread which could be useful to many.

From this link it states that "Air barriers are intended to resist the air pressure differences that act on them. Rigid materials such as gypsum board, exterior sheathing like plywood or OSB…"

Bud
https://buildingscience.com/documents/digests/bsd-104-understanding-air-barriers


----------



## Nealtw (Jun 22, 2017)

Bud9051 said:


> Neal, we are drifting too much on Erock's thread so I'll just drop this link and run. For further discussion we should open another thread which could be useful to many.
> 
> From this link it states that "Air barriers are intended to resist the air pressure differences that act on them. Rigid materials such as gypsum board, exterior sheathing like plywood or OSB…"
> 
> ...


I guess, if a yes or no answer takes to long.


----------



## NotYerUncleBob2 (Dec 29, 2017)

Nealtw said:


> We use poly for air barrier so it is continues and sealed.
> Are you saying you would solid sheet a vented crawl space or a house built on stilts?


There's actually a lot of debate on the value of venting crawlspaces these days. Best practice now seems to lean more toward insulating, sealing, and conditioning the crawlspace rather than venting it. 
I've never built a house on stilts, but I'd be inclined to close up the bottom and treat it the same as if it was a wall.


----------



## Nealtw (Jun 22, 2017)

NotYerUncleBob2 said:


> There's actually a lot of debate on the value of venting crawlspaces these days. Best practice now seems to lean more toward insulating, sealing, and conditioning the crawlspace rather than venting it.
> I've never built a house on stilts, but I'd be inclined to close up the bottom and treat it the same as if it was a wall.


It is more about code and best practice where the house is.


----------



## NotYerUncleBob2 (Dec 29, 2017)

I know that Canadian code specs poly in the walls, or did, or still does? But south of the border we don't use it at all in walls or ceilings. We do have vapor retarders like the faced fiberglass but that's about it on the inside of the house. I guess the movement is now to stop the vapor at the outside of the house and then to prevent the differential that could cause condensation inside the walls.


----------

