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Insulating With Foam on the Inside Surface of a Wall

xtal_01 | Posted in General Questions on

OK …. you guys saved me from making a big mistake!  Figure this should be a new post as it it a different question.

I am building a new workshop (about 4500 sq ft) in Vermont.  Standard construction … 2 x 6 walls .. OSB … Tyvek … vinyl siding.

I got worried when I looked at all the blocking, lots and lots of heat loss.

I have 4 almost equal size zones / areas (one is the attic I am not going to heat).

My thinking is keep the entire building just above freezing (say 45 deg or 50 deg) and if I will be an say the machine shop that day, I will just raise the heat in that bay to 65 – 70.

So .. I was just thinking through an inch of foam on the outside before putting up the siding.

As you guys point out … NO!  Without a balance of insulation on both sides of the sheathing I will get condensation and rot the walls.

My budget won’t go for 4″ (R-20) of foam outside.

So I have a few options … can you guys tell me which you think might be best?

1) do nothing … just use R19 in the wall and seal it well.
2) put 1″ of foam just over the studs only and then strapping over than.  This would stop the stop the bridging … give me something to put the interior onto and give me a deep cavity (I could put R26 in them).
3) sheet the interior with 1″ of foam (not just over the studs).  Then I have two chooses:
    a) glue (and nail) the 1/4″ plywood right onto the foam
    b) strap the wall leaving a 3/4″ air gap and then put up the interior.

Again, the budget is tight so I was going to use reclaimed foam (EXP or IOS).

Drywall is heavy (I am building the shop myself) and it takes me forever to tape and mud.  I am planning on something light and easy to put up on the interior … say 1/4″ floor underlay or ??

Any advice is greatly appreciated.

Here is a link to some videos my wife put up on YouTube:

Thanks so much … Mike

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  1. paul_miltenburg | | #1

    Since this is a workshop that it doesn't sound like you'll be using everyday, my vote is option 1. Foam on the inside surface of the studs will make it difficult to securely attach anything to the studs.

  2. brian_wiley | | #2

    Option two is sometimes known as a Bonfiglioli wall, and FHB did a story on it a while back if you wanted to dig a bit deeper.

    That said, I’d vote for option 1 as well. It’s an intermittently used shop. From what I understand from previous posts and discussion here, focusing on air sealing would do the most good in that situation.

    However, I’m not sure that any of your proposed strategies are compatible with using 1/4 plywood as the finished surface as it wouldn’t be air tight. Option 3 may be if you were to tape and detail all the seams as an air barrier, but I’ll leave that to other more experienced members to say.

  3. andy_ | | #3

    Option 4: Put a ton of solar panels on the roof (subsidies? tax credits?) and get a couple mini splits to heat (and cool) the place. I don't know if VT has net metering or what the real numbers could be for you, but it might just work out to be more comfortable, cheaper, and better long term than just insulating.

  4. xtal_01 | | #4

    I looked at heat pumps a few years back. I know they have come a long way but when it gets cold here, it really gets cold.

    I actually have thought a lot about air sealing. Every piece of sheathing hands on a stud or blocking. I was thinking about caulking every bay so I have a good seal. I did use sill seal as well as caulking on the plate to concrete connection.

    Yes, this is just a home shop that I won't be in every day. That said, I will heat it to 45 or 50 all year round. I am a machinist and several of my machines have coolant in them. Plus I do some woodworking and don't want my paint to freeze.

    I am just thinking if I can put say $2K in foam (used foam) on the walls inside and it will help keep it warm than I don't mind the investment.

    I would normally not think of plywood inside ... but I just can't see me putting drywall on a 16 ft high ceiling. Not sure what else would be relatively light, easy to work with and affordable.

    Just as a side not ... 14 x 14 doors .. these will be a challenge. I am thinking of making them. I think I came make them relatively air tight and put in say 2" of foam.

    My wife is paralyzed form the shoulders down. A few years ago I converted an RV so I can do her care while on the road (she had never traveled before we met). I cut a new door ... made trim ... and used automotive style seals. It is wayyyyyyyy smaller but I did get it to seal ... No leaks and no wind noise even at 65 MPG. Designed and built the lift also ... paid $18K for a 10 year old RV ... they wanted $25K for a lift! I did the same in the house. They wanted $40K for an elevator .... I bought an old forklift for $875 ... used the mast ... built my own hydraulics ... total project $2K.

    I am not expecting this to be a model of super efficiency but trying to do the best I can and stay in budget.

    Thanks !!!!!!!!

  5. Expert Member
    Akos | | #5

    Assuming around 8000 heating degree days, keeping the building mostly colder at 50F, that is probably about 1/2 that. The 4500sqft building with 16' walls, so ~4300sqft of walls.

    Your assemblies including thermal bridging from studs work out to:
    Option 1: R17
    Option 2: R24
    Option 3 about the same as option 2

    Through the season, you loose:
    option 1: 4000HDD*24*4300sqft/R17=242 Therms
    option 2: 4000HDD*24*4300sqft/R24=172 Therms

    Depending on your energy costs, you can see if there is a reasonable ROI for the extra insulation. Since the building won't be occupied full time, moisture won't be a problem. I would still recommend a vapor barrier under the slab at least. Also even as little as an inch of rigid under the slab also greatly improves comfort as it will keep the slab much closer to room temperature.

    For air sealing, caulking is hard work and even harder to get right. The better option is to tape the seams of your sheathing with a quality tape (Zip, 3m 8067, Tescon etc). Also figure out your transitions from the foundation to the wall as well as walls to your interior ceiling.

    1/4" plywood is way to floppy to use for the interior. I would go at least 1/2" plywood for anywhere you want to attach/hang anything and 7/16" osb elsewhere. For wall areas that won't possibly see physical damage, you can even look at something like 1/2" foil faced polyiso (your local code might require it to be rated such as Thermax).

    Cold climate mini splits work down to -22F, if your hydro is reasonable, it tends to be the cheapest way to heat a place.

  6. brian_wiley | | #6

    First, nice work on that RV!

    Just to clarify, what stage of the build are you in? From your other post, I was under the impression that the walls were up with sheathing and tyvek on them. Is that the case, or is it currently just the slab that’s been set?

    Perhaps you’ve seen this, but thought I’d share in case you hadn’t.

  7. xtal_01 | | #7

    Thanks ...

    Yes, the pad is in and I have all but two 12 ft sections of walls complete.

    I did put 2" of XPS high compaction foam under the pad. There is also 2" of foam on the side of the pad and where the thickened edge ended, I went out 24" with foam. I did put a vapor barrier over the foam. My first video on youtube shows the prep I did to the pad.

    If you guys have can get to youtube, here are some videos (just a couple weeks behind). They show what I am building.

    Hydro is reasonable here but natural gas is still cheaper.

    Long story but I hit ledge while building the house so I went up rather than dynamite out a basement. This gives me a huge house (the bottom floor is the basement) ... 4800 sq ft above ground. I put foam under (2" all over ... 4" on the edges ... brought it up in the middle of the walls). 2 x 6 walls ... mineral wood batts .... 18" plus blown into the ceiling ... spray foam on the rim. I crawled around the outside and spray foamed where the plate met the concrete ... and then caulked on the inside where the plate hit the floor (home I am explaining it right ... I brought the foam under the floor up when it hit the frost walls so it hits the wall plate).

    In the middle of winter (and we have hit -30 a few nights), I pay about $200

    I keep thinking about the 1/4" ... I know it is thin but if I hang anything, it should be to the studs. I have soooooo much blocking that the largest cavity is 16" x 48"

    I might try to find a few extra $'s .... 1/4" is $20 a sheet .... 1/2" (sheathing not sanded) is $30 ... just hate the looks of OSB inside the shop (a bit vain).

    I like the idea of zip tape .... I don't have the Tyvek on yet so it is still easy to do now.

    That should really cut down on air leaks!

    Thanks .... Mike

  8. xtal_01 | | #8


    Just FYI ... I was on the ladder today putting on the double plate and decided to pull out my phone and take a short video ... shows the project today.

    Thanks ...

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