I'm currently renovating my walkout basement. Walls are already studded. I have two questions related to insulating it:

• I can slide 2" XPS behind one of the walls but another will only fit 1" behind the studs. Can I supplement that with another 1" of XPS in the stud bays?
• One of the foundation walls will be exposed (think mechanical room). Does polyiso require a thermal barrier or is it safe to stay exposed?

My real hope was to have cc spray foam installed but wow did I underestimate how much it would cost... geez. A combo of XPS and polyiso will be significantly cheaper. Any help is most appreciated.

Thanks.

 

It's not clear how you are planning to adhere or fasten the XPS to the foundation wall. You want to ensure a good inimate contact with foundation.

Also, it's important the XPS is well sealed at all seams and butt joints.

As for the question regarding the 1 inch -- you could but the overall R value will be slightly less compared to the 2 inch side.


NO -- any foam insulation needs to be covered with a fire rated barrier such as drywall when located within the living space --- yes that includes the mechanical room..

.Oh yes --- spray foam is pricey but it is the best material and method for insulation/air seal.

 

My hope is to just use adhesive and not fasten it with hardware. Regarding the 1" wall... do you think I should "flash and batt" that wall instead of adding another 1" of XPS. Maybe use Roxul in the stud bays?

What insulation do you recommend for a foundation wall that won't have a studded wall against it? This will be the case in the mechanical room.

Damn. Maybe I should just take the plunge. Tough to stomach when XPS will cost 1/4 of the price.

 

"THERMAX™ Sheathing is a non-structural, rigid board insulation consisting of a glass-fiber-reinforced polyisocyanurate foam core laminated between 1.0 mil smooth, reflective aluminum foil facers on both sides. The reinforcement, along with chemical modifications, contributes to fire resistance and dimensional stability. THERMAX™ Sheathing can be installed exposed to the interior without a thermal barrier and is especially appropriate for hourly rated assemblies."

If you can find this you should still check with your local code officials to be sure it is acceptable to them.
THERMAX? Sheathing

The one inch plus one inch option should be ok, but again check the local codes.

Be sure you have resolved any moisture issues first, once glued there is no going back.

The sealing and good contact Canuk mentioned is very important as any gap that allows basement air to sneak in will form condensation.

Bud

 

Bud9051 --- thanks for pointing that out.
I some times forget of the regional differences between here and there.

Reading the literature more closely , here's a couple of things I came across ....


THERMAX products can be
left exposed to the interior
without a thermal barrier in
most applications.*

• FM 4880 – Wall-Ceiling
Construction Metal-Faced – Class
1 Fire Rated to Max. 30' Exposure
High, 4.25" Thick, 4' Wide, When
Installed as Described in the
Current Edition of FMRC Approval
Guide


Does this mean *in most apllications* that it meets the exposed fire rating at a thickness of 4.25 inches ? If that's the case then 1 or 2 inches wouldn't qualify.



Up here , unless it meets our CAN/ULC standards , polyisocyanurate isn't used for interior applications.
In checking the specifications on the Dow site this product doesn't list it's code approval for use here.

The material standards can be quite tough up here. For example this Thermax product won't and doesn't meet our CAN/ULC-S704-03 & S704-11 which includes .........

the product shall also be marked with a warning: “Caution: This product is combustible and shall only be used as specified by
the local building code with respect to flame-spread classification and to the use of a suitable thermal barrier.”

 

Canuk -- you are correct, the fine print makes you wonder. And, I'm not sure all foil faced PIC is Thermax. That's why we always have to check local codes. Down here every local code department can have its own set of rules. Where it can or cannot be used, how the edges and seams are to be treated, and how it is fastened in place all and more must be discussed locally.

I always like the spray foam excuse, covering the foam is a home owners option, so they spray the job and they are gone. Personally, I'm starting to like mineral wool (Roxul) a whole lot.

Bud

 

So if I don't go the Thermax route, what could I use in it's place? There will be an exposed foundation wall with no framed wall in front of it, so batts are out. Roxul Comfort IS? For the foundation walls that will have framing in front, I've always liked the idea of flash and batt with 1" XPS then Roxul in the bays. I've been reading horror stories about it though. Is this an acceptable method?

 

Flash and batt usually refers to spray foam and I don't care for it due to the irregular surface the foam provides. Using rigid and then a batt like Roxul is fine, except you need to use the correct thickness of rigid. Batt insulation, even Roxul, allows air to sneak through and reach the surface of the rigid insulation. If it is cold enough outside and the rigid is thin enough, that surface drops below the dew point and you have moisture. Depending upon your location, 1" may not be enough to keep the inside surface above that dew point. I'm in Maine and we go with R-10. I use at best R-6 per inch for PIC (polyisocyanurate) as it decreases a bit with time.

Check with your local code folks as some will accept the foil faced PIC the big box stores sell. For some reason I'm remembering that isn't always Thermax. But some are ok with that foil surface alone. Others will want sheetrock or one of the intumesent paints, but they were expensive last I checked.

BTW, no vapor barrier on any of those basement walls.

Bud

 

Last questions Bud... I really appreciate your help. The back wall in my walkout basement is completely above grade and is 2x6 framing. My plan was not to use foam there and just use Roxul and a vapor barrier. Is that not a good idea? Lastly, I'm in Pennsylvania. Given that I can only squeeze 1" of XPS behind some of the studs, I"m wondering if that with Roxul is good in my area?

 

The walkout area that is not exposed to the earth can be treated just like a regular wall that drys to the outside. Thus a vapor barrier on the inside of that wall in your cold country is fine. Below grade a wall needs a place to dry, thus no VB on the inside. The amount of moisture vapor that passes through is very small so easily handled inside. If a VB is added the moisture then accumulates until it matches the outside moisture.

The one inch behind the studs with another one inch between them should be fine. The portion behind the studs is not exposed to any air. There is another aspect of a basement in that only the above grade area are subject to the very cold temperatures. So, technically everything below grade could just be one inch. Unless code demands otherwise. The other part becomes, how much Roxul or other insulation you add. Adding more to the inside again keeps the surface of the right insulation colder. There is a ratio, say 50/50 for the total wall. Half can be batts and the other half rigid There is an article on that somewhere, I'll go looking.

Bud
Here are a couple of links:
How to Insulate a Basement Wall | GreenBuildingAdvisor.com
Calculating the Minimum Thickness of Rigid Foam Sheathing | GreenBuildingAdvisor.com

 

Yes you could and it's done on occasion to increase the performance of insulation in the wall assembly.

Now , to throw a curve a ball --- as Bud pointed out that 1 inch XPS may not be enough to raise the temperature on the room side to a higher dew point. All is not lost though, you could use a lower density *expanded* polystyrene foam board attached to the foundation , Roxul batts in the stud bays with a vapour barrier.
The difference here the *expanded* polystyrene is more vapour permable in that it won't hold vapour that may get into the wall cavity like the *expanded* polystyrene you are proposing.





That's a tough call.
Perhaps that Thermax product may be allowed to be exposed where you are and would solve that .

Otherwise, you might consider applying the XPS rigid foam directly on the wall with 1X3 strapping fastened to the foundation behind, then screw drywall , tape & mud and call it done.





Yeah , that's a tough call.
One thing though, because there is a sizeable space behind the wall framing ( 1 & 2 inches ) is perfect for spray foam. You would have the advantage of eliminating thermal bridging and with 2-3 inches would give a well insulated wall assembly.

 

Yes -- that would be fine.
If you wanted , you could apply 1 inch XPS to the studs instead of the vapour barrier giving a thermal break and the required vapor retarding. This is another method we do to provide a better performing wall assembly.








The only time a vapour barrier should not be used on a below grade wall is in the case of using a low vapour permable insulation material such as the expanded polystyrene , polyisocyanurate , polyurathane or high density polystyrene is used. In those cases the *drying* would be toward the interior.

With more vapour permable materials like standard fiberglass or rock/mineral wool batts need to have a vapour barrier to reduce the vapour drive into the wall cavity, where it will surely condense. Any *drying* will be through capillary action upwards to the portion of the foundation above grade and towards the exterior.

 

Keep in mind 2 factors:

1. There are many different types of foam insulation. The term expanded is a horrible term to use since it actually is very limiting. There is EPS foam that is expanded polystyrene that is not very dense and has the lowest possible insulating value (similar to the expanded polystyrene used in the flimsy, cheap meat trays at stores or coffee cups). The most common construction "expanded" polystyrene is XPS that is controlled and extruded to maintain properties and has much higher insulating properties (about 50% higher) and more strength and stability. It may come tinted in pink, blur or yellow for branding and identification. Polyisocyanurate is a little different than XPS in that is must have foil barrier that helps limit the off gassing and provides some reflective properties that are hard to guarantee over time.

2. If you go by the simplistic market-oriented R-value approach, the insulating values that are touted are based on small sample sizes tested in a very short term laboratory test that began with the "false" values of small fiberglass sample(not real walls) that are short term for the insulation material that will never give that value for wall because of the thermal short-circuiting of the studs, joists, etc plus it does little for air flow control. An R-19 batt insulation can give as low as -9 in some walls.

The unique thing about a basement is that the temperatures seen by the insulation are no where near the outside air temperatures. As an example, in MN we can have -010F to -30F air temperatures, but soil surrounding the basement is about +54F because the thermal temperature of the soil from the property of heat storage of of the soil during the year. There is no air infiltration from cool soil. The concept of mass heat storage is common to most solar heated homes, since temperatures are drastically moderated. It is strange that most homes have more square footage in the basement floors than the basement walls but floors are usually ignored, except for short term creature comfort and not economics. - In the summer, a basement/ low level room (even if not insulated) can be asset to reduce AC costs if the HVAC system is properly installed with low returns.

Unfortunately, codes are written by bureaucrats and some highly financially supported industries that are active on the code committees that I participated in. The requirements (R-value of a wall or floor) are written to be easily enforced with minimum effort without thinking and just looking at the outside air temperature of the area/region. - It is a good selling point for materials and systems and easy to enforce for simplistic building methods.

Dick

 

One thing to say about the low density *expanded* foam --- though it may have a similar R value as common fiberglass batt insulation --- it has more stable characteristics than fiberglass , making for a more superior product. It has it's advantages.

Over the years I've attended many seminars regarding building techniques and materials ( including insulation ). In the last ( dare I say ) 30 years or so , the amount of research and information that has come from the building science community is mind boggling. Reading piles of literature of imperical data doesn't really do justice until you get to actually see some of the *real world* testing --it has and still amazes me. My favourite is research on the holy grail of heating a 10X14 foot building with a single candle -- while not quite yet but close --- and that's in our 30 below zero climate.
Another is the study of cold penetration in the ground ( frost depths ) with the formation and effects of *ice lenses*.

... but i digress ...

 

Well I can tell you from a newbie's standpoint who wants to do it correctly, I agree wholeheartedly with the statement above. I'm work in IT for a living and I often have clients throw up their arms in disgust and confusion. I can relate to that sentiment in the insulation arena. I literally had no idea there was so much science behind it. Much respect. Makes me sorta mad after having an insulation contractor come to my house and recommend using fg batts throughtout the basement. Seems like faulty information given the stuff I've learned on this and other forums.

 

The traditional interuppted method using batt insulation in the stud bay is functional. The methods we are discussing here are just an improvement.

Depends on how that contractor proposed to install the fg batts. Since you have the 1 & 2 inch spacing between the foundation and wall framing , just filling the joist bays and leaving the gaps behind the studs would be a problem -- faulty install.

 

I hope this isn't an issue with XPS as well. One of my walls was framed about 6 inches off the foundation because of plumbing and PVC pipes. I thought the loss of space was worth a cleaner looking wall vs. having to create a soffit. Regardless, as you know I'll be installing 2" of XPS on the foundation, which will will leave 4" gap between the face of the foamboard and the studded wall. I just assumed it was fine since at 2" the XPS will be air tight and be a vapor barrier. Thoughts?

 

You are good-to-go with an air space inboard of the foam (thickness required by code; click on your county, read footnote "c"http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Pennsylvania) and no cavity insulation, similar to this; http://www2.dupont.com/Tyvek_Weather...20Bulletin.pdf

The first R-5 is a biggie when it comes to insulation, chart using R-3.6 here; http://www.google.com/url?sa=t&rct=j...,d.cGE&cad=rja

R-5 worked in MN, though 45% energy savings with higher R for colder locations and additional 5% savings using R-5 under slab, at '99 prices..... possibly more today- lol; Default

Gary
PS. IMO, air seal the joints with mesh/furnace mastic as foil tape may leak as the fb shrinks with age. Canned foam under/over the perimeter and in 1' square grid pattern on concrete wall to limit air leaks to same. Fire-block between foam/frame/concrete every 10' and at top of wall per code for safety; Chapter 6 - Wall Construction Use foil-faced fb on rims w. canned foam, not unfaced XPS.

 

I wish my slab in the basement was insulated. I bought this model home last year so it's all but brand new. There's a vapor barrier under slab but no XPS. Stinks because it's affecting my decision on flooring. I'm going through all this work to insulate the walls correctly but the floor scares me. SOOO many conflicting stories of what to do on the floor. I really don't want to lose the space by using layers of XPS and OSB. I think in the end I'll probably use Ditra and tile with throw rugs. Shame though to have such a comfortable space with cold floors.

 

The question is, what are you going to use the room for? Second question, Where is the heat source located and what is it?
Keep in mind that heat always moves to cold.
Apart from the area immediately under and in front of a typical heater, the floor is only warmed by convected air, that has already lost most of its heat to the upper walls and ceiling. So, very little heat gets down to the floor, little is lost through the floor.
Therefore, a one inch layer of polystyrene, with a fully floating layer of OSB that is t&g and glued, and is totally isolated from the slab and walls, should do.

 

It's going to be a rec room/theater room. I plan to spend a lot of time in this area. I'll be running 2 feeds and 1 return so it will be climate controlled. It's a walkout so it has sliding glass doors. Adding 1 1/2" or more to the floor height (XPS, OSB, finished floor) would screw up the transition at the doors. I thought about heat matts under Ditra and tile. Not sure how that would do on a cold slab though.

 

To clarify --- as Gary in WA posted is correct. The space between the *room*or *warm* side and the back of the studs is fine providing there is insulation ( and enough ) behind , contacting the foundation.




Think of your mechanical room situation --- if you were to apply 2 inch thick XPS directly to the wall and have no wall framing -- the wall is insulated is everything is fine. Then if you decide later to put up framing , things haven't changed as far as the insulation is concerned.

In other words ---as long as there is enough insulation continuously applied and sealed to maintain a high enough temperature on the *room* side to keep a higher dew point , there won't be condensation issues.

For example -- let's say the temperature in the basement will be 68 F --- the relative humidity ( RH ) is 50% --- the *room* side temperature of the insulation needs to be at least 49F --- any lower and the RH will reach it's dew point and condense.

Now, if the RH was lowered to 40% the dew point lowers to 43F --- and so on.
There is a direct correlation between temperature , humidity and the dew point.

Here is a link for a dew point calculator Dew Point Calculator


Sorry I wasn't clear what I meant here.

The most common problem I've encountered with insulating basement walls with traditional batt insulation --- people simply stuff the stud bays with the batts with absolutely nothing in any gap behind the back of the stud and the foundation. Put up the walls and call it done.
In some cases people are under the misguided idea you are supposed to leave a gap ( air space ) between the insulation/wall framing and the foundation.

This is wrong in so many ways.

First -- it's counterproductive. Having an air space behind ( on the cold side ) reduces the insulating factor you set out to achieve -- creating convective loops ( stack effect ) --- especially noticeable with batt insulation

Second -- because of the dew point mentioned earlier -- the warm moist air that will enter ( and it will ) the wall cavity will drive through batt insulation to the cold wall and condense and will freeze if cold enough. This creates all sorts of issues --- mould being one of those.

It's a waste of time and money when it's done improperly.

Having said all that and in the particular case about the contractor suggesting batt insulation ----

As you described you have gaps of 1 , 2 and apparently up to 6 inches between the wall framing and the foundation.
How was the contractor going to deal with those gaps of 1 & 2 inches behind the studs since he would be using fg batts ? I would guess either he wasn't going to do anything or just try and stuff some fg into those gaps. Neither would be effective and incorrect.
In which case you would have condensation issues and reduced insulation value for the wall assembly because of convective loops and the dew point being too low.


Clear as mud ?

 

Canuk - Yep, makes total sense.

So let's use the mechanical room as the example again. There will be an interior partition wall (not sitting against the foundation) that separates the mechanical room from the renovated portion of the basement. If it put batts in the stud bays of that partition wall but leave the foundation exposed in the mechanical room, would that be sufficient? Or must I still insulate the foundation with polyiso, xps, etc? Hope I'm making sense... hard to describe.

 

Since I don't know exactly the layout I've included a rough drawing of a typical layout.
Leaving the walls in the mechanical room uninsulated will just mean that room is cooler than the finished space.
Putting insulation in the partition walls will *slightly* make a difference thermally.
Reason being it's *open* ( not sealed ) to the relatively cooler space in the mechanical room. However, the mechanical room itself is considered conditioned space being that it's within the heated envelope of the house --- so, there isn't any concerns of the dew point issues with condensation per se.

Usually though, insulating the partition walls is for sound control from the mechanical room and there will be *some* thermal benefit so it's not a total loss.


What is important is the area of the insulated exterior wall meeting the uninslated wall. You want a tight seal at the point where the two meet -- preventing the issues discussed previously , of having warm moist air behind that.





I believe with many current codes , technically ALL the exterior walls of the basement are to be insulated. But, every jurisdiction has their requirement.

So I can't answer if you MUST insulate the mechanical room walls --- but if it were me -- I'd do it.


Hopefully this helps.



Ah -- just got the user name connection.

 

Canuk, you rock. Your drawing is EXACTLY how I'm framing things out. It seems, though, I have bigger fish to fry now. I mentioned that a few walls against the foundation were already framed. Well this evening I yanked fg from the rim joist that the builder installed and started inserting XPS and "Great Stuffing" the heck out of it. Wall #1 no issues other than that stuff isn't so great to work with. I had sh*t everywhere but I can deal with a mess. I go to wall #2 which runs parallel to the joists. Blocking was used between the the last joist and the rim joist so there was someplace to secure the wall. That's great... but I can't get XPS in there and I certainly can't get my arm up there to foam it. Soooo.... what now, lol? I can either destroy the wall (which I REALLY don't want to do) or use a different product in the rim joist. Recommendations? Roxul? You've been an awesome help already... this newb really appreciates it.

Yep, I'm a nerd

 

lol ---- yep the foam-in -a-can is a lot of fun.
That's why I use the low expanding foam ( used for window & door sealing ) as much as possible --- it doesn't keep coming out of the the plastic tube like the high expansion stuff for filling gaps. You'll be a pro when you're all done.

Sure -- just use the Roxul in those areas. Does Roxul in the US come faced ? If so then that would be a choice.

We don't use faced batts up here ( it's been removed from the building code thankfully ) so I wouldn't know if it 's different for the US.

Glad to help.

 

Naw -- just a specialist in a special field.

 

Fantastic. I'm sure it will be MUCH easier to work with too. The Roxul here has no face. I watched the DIY video on their website. They recommend R22 for the rim joist. Luckily my local Home Depot had it in stock. It will work out in the end because I plan to use the Roxul R22 for the walkout 2x6 wall. Rough night... lol. Expletives abound. The dog thought I was losing it