How much foam on outside walls …. or any ????
Hey!
I am totally confused and I am hoping someone can either answer my question or direct me to someone (or a green building site) that can.
I am building a 4500 sq ft workshop. Standard stick construction … 2 x 6 walls (16 ft high) … on grade pad ( 2″ of foam under it plus 2″ on the outside thickened edge and 2′ outward below grade where the wall ends). I have a 24″ energy heel build into the roof trusses .. so say 18″ of insulation will go in there with a 6″ air gap (planning on metal roof).
I have split the building into three bays …. I was going to keep it at say 45 deg (just so my coolant, paint and such do not freeze) … and then heat the part of the building I am going to work in that day to say 65.
I live in Vermont … cooooooold in winter (I have seen -30 here at the house) …only 5 days above 90 each year.
I was going to just use fiberglass in the walls.
Then I got to thinking after seeing a friends place, why not put an extra 1 1/2″ of foam on the outside. Windows are not in yet so easy to buck out with a 2 x 4 on flat.
I had a friend come over who is a lot more knowledgeable on eco building.
First he said not to do this at all …. he said with the two 14 x 14 garage doors, heating is not piratical. OK .. he is probably right but I am going to heat the building so hmmmm.
Next advice he gave was to only put 3 1/2″ of fiberglass inside the walls if I am going to put foam outside. He said you want a balance of insulation on each side of the sheathing … something about the point where the moisture will condense? I think he put 6″ of foam on his house but that is out of my budget.
Is this true?
I had planned on 5 1/2″ of fiberglass inside and 1 1/2″ of foam outside.
Any suggestions or ???????
I am just worried, I have so much blocking in my walls that I need something as a break between the vinyl siding and walls.
Thanks …. Mike
PS … here is a link to some videos showing the project … https://www.youtube.com/channel/UCYAzdvTKwlfPxOTVmYMoAjg
Replies
Your friend is right about the balance of the exterior and interior insulation, although 6” of exterior foam is overkill. I live in Vermont too, and depending on whether you live up in the Kingdom or down on the Massachusetts border, the ratio will be different. Here are some good guidelines about exterior/interior insulation from Efficiency Vermont: https://www.efficiencyvermont.com/Media/Default/bbd/2019/docs/presentations/efficiency-vermont-bbd-rethinking-the-rules-on-minimum-foam-thickness.pdf
In general my thoughts are, first, you should use more foam in your slab, at least double, second, if you’re building such a large building and can give up a few more inches all around, a double stud wall with either cellulose or fiberglass will be cheaper than foam. My 2cents. Where in VT are you?
Hey!
I am in Jericho ... about 20 min from Burlington.
Pad is in ... poured it 8 years ago when I started building the house, so the 2" under it is fixed.
Building walls are also finished. They are single stud 2 x 6 ... really that is all that is in my budget.
The only reason I was looking at foam is because of all my blocking .... lots and lots of wood. You can see this in my videos.
I thought maybe 1 12/" of foam would give me a good isolation layer before hitting the wall sheathing ... but am I going to cause myself a water problem with condensation?
Thanks .... Mike
As the chart I linked to says, all of Vermont is in climate zone 6 (I'd argue a bit with that, I'm in Addison County, on Lake Champlain, so a bit warmer than you are in Jericho, and we're both in a warmer clime than the Northeast Kingdom), so R-11.25 for exterior foam on the outside of a 2x6 wall. In the real world that means 2 1/2" of XPS or 2" of polyiso. (But look into the problem with polyiso's performance diminishing in cold weather, it could be a deal breaker.) If it was me I'd look at the cost difference between adding expensive foam or a 2nd 2x4 wall. If I went the foam route I'd put 2 1/2" of XPS, or Gutex if I had the money, https://foursevenfive.com/blog/nature-does-it-best-gutex-wood-fiberboard-insulation/ , then fiberglass or rock wool on the inside wall, but my gut says I'd save money by the 2 wall solution, fill the 2x6 cavities with fiberglass or rock wool, add a 2nd 2x4 interior wall and either blow in cellulose or rock wool/fiberglass that.
Thanks ... definitely something I had not thought about.
FHB has a great article on this. https://www.finehomebuilding.com/2010/07/15/rigid-foam-on-the-roof-and-walls
Thanks .... I will take a look tonight
Last day of good weather her in VT ... might even hit 70!
Mike
Mike (oxtal)
A few thoughts based on what I have learned, some of it the hard way.
First, check out the "ABTG Wall Calculator". It helps you get a sense of how the wall components play together to control moisture , or to grow mushrooms.
I gather you have already put up your exterior wall sheathing. If not, rigid insulation on the outside can be what holds the stuffing in. Shear can be accomplished with let-in bracing, or put your shear panels on the interior.
The "perfect wall" has all the insulation on the outside of the sheathing and frame. The inside of the rigid insulation is ~room temperature. If it is a closed cell foam, and you add fluffy insulation on the inside of the foam, you lower the temperature of that inside foam surface, and the inside air that is next to it cools to that lower temperature. That increases the relative humidity of that air.
You can calculate the temperature of that interface using the respective R values, indoor and outdoor temperatures. Then pull up a hygrometric chart, and you find your way to the RH, dew point and so forth.
Now go back to the ABTG Wall Calculator.
Plastic foam is frowned on, but I used 3-in. of expanded polystyrene (Type 9 R4.3 (75ºF) anyway for my energy refit. This is over 2x4 walls with blown fiberglass (R 2.2 to 2.7). My outside foam insulation to inside fluff insulation ratio is probably 1.3 . In CZ 5 at 7200 HDD/year that is high enough to not have an interior vapor retarder. But this house in the southern part of CZ5 and real HDD is about 5500. Even though my EPS has a Class 2 vapor retarding permeance of 0.6 p, I do not need to add a vapor retarder at all. I could put a coat of vapor retarding paint on the inside of the wall for added assurance.
Unfaced EPS is not a suitable water resistant barrier, though a faced EPS could be. XPS (and now that isn't the sin it was last year) can be your WRB (Weather resistant barrier), overlap the intermediate layers and tape the final seams with a really good tape. Add your vertical structural battens, and put up the siding.
A temporary shelf at the bottom lets you play Tetrus with 4x8 sheets and minimizes fasteners needed to hold the foam while furring is applied. Foam right over the window openings. Cut those out as you install windows.
When you are ready to, you can install the fluffy insulation. Go back to the Wall Calculator. If you did 4-inches EPS you could do 5.5 inches mineral wool in CZ6, 9000 HDD and do a low Class 3 VR paint (one coat of SW VB paint/primer);
Garage doors are an issue, but you can insulate the panels (XPS) , maybe run a WRB on the inside as an air barrier across the joints, Make sure the door to frame seals actually work.
Polyisocyanurate has a pentane condensing problem. It works as an inner layer, or as the only layer. I looked at it as exterior insulation, and its performance was comparable to EPS at 25ºF. I don't care how good it is at 75ºF.
Price of XPS and EPS work out to about:
Bulk XPS $0.10 per R•ft², about $0.14 retail
Bulk EPS $0.04 per R•ft², about $0.10 retail with foil
At 120,000 R•ft² I'd find a EPS expander (for lack of a better term) and put on a good WRB (Ours remained exposed all winter, no tears, no trouble, though well protected from sun)
Good luck
WOW ... great information ... thanks!
I made a lot of mistakes already ... mainly because of my budget and listening to others. I wish I had not put sheathing ... OSB at $50 per sheet earlier this year. I wanted to go with foam but was talked out of it.
The calculator is great .... I will play around with it a bit.
I am at the mercy of the used foam market. Right now I could get some used 3 1/2" EPS for a very reasonable price ... $20 per 4 x 8 sheet. That gives me about R12 and I could buck everything out with 2x4's
I did mineral wool in the house ... was thinking about fiberglass in the shop because it seems to be half the price ...but again I will have to take a good look.
I will have to play around with the calculator a bit.
Thanks again!
Go for the mineral wool! It costs more, but it's much easier to install well then fiberglass is. I've done both and I try hard to avoid fiberglass now.
Thanks ... I used mineral wool in the house ... it was soooo much easier. I am just surprised at how much more it seems to cost.
I was thinking last night .... I will need about 4500 sq ft (walls not including ceilings).
The issue again (as always) is money.
Rockwool for a 2 x 6 wall is $1.80 at Lowes
Fiberglass for a 2 x 6 wall is $0.70
So I am talking almost $5K more !!!!
If they are both about R20 (sometimes I see R19 .. sometimes R23) is there really a difference in the two or ?
I am even willing to think about blowing insulation into the wall ... I have not checked the cost and I am always worried about setting.
Thanks!
Mike
Cellulose is a good way to go, your blocking won't be an issue because you or the contractor will install the fabric over the interior of the studs and blocking, so they can see where to cut the holes for the hose. Cellulose, if applied by a professional, will not settle. The Home Depot type blowers aren't powerful enough (because they run on 115V) to force the cellulose in at 3.5 pounds per cubic foot (dense packing) but the pros use 230v blowers for just that reason. Dense packed cellulose remains dense enough to never sag. As for cost, pros buy truckloads of cellulose and pass 'some' of that savings on to you. We've used Bugbee Insulation in Williston on a couple of projects, they're pretty good, they also do fiberglass and spray foam. A smaller outfit may possibly give you a better price.
When I built my house, I acted as the general contractor and subbed a lot of the work.
I went for bids ... had Bugbee and Matt Zambarano both quote the job .... Matt was a bit cheaper.
He did a good job ... only a few small issues I had him come back for ... 5 1/2" mineral wool in the walls ... spray foam on the rim and the gable ends .... blown in the ceilings (almost 24")
I could easily blow in before shutting up the walls.
Might be worth looking at.
I was going to insulate the house myself but in the end, the quote I got was almost the same at what I was going to pay for materials.
Thanks .... Mike
PS ... you must be near Jericho ... you are welcome to drop by and look at the project.
On the lake near Vergennes. If I'm up that way I might check in on it.
Since your building a shop and not a house the standards for house wall insulation likely won't apply.
Condensation potential in a wall cavity is driven by moisture vapor in the building. I'm going to guess that since you installed insulation under the slab that you also installed a vapor barrier sheet. So there's not moisture coming from the earth. That leaves leaks in the roof or walls from the outside and moisture from activities and people inside the building.
I'll bet you have the exterior tight so that leaves interior moisture.
Will you be working with a lot of water based finishes, adhesives or cleaning a lot with water during the heating months? If not you will probably be fine with any insulation combination - no exterior insulation or a combination of any level of exterior insulation to the cavity insulation.
And since you're probably installing the cavity insulation yourself you'll be willing to put in the little extra time to do a decent job with fiberglass and the performance will closely match the mineral wool. So you'll save the $5000.
And do a good job installing a class 2 vapor retarder or vapor retarder primer on drywall to help throttle any interior moisture from entering the walls too fast. Air sealing is a must but I'm sure you're on top of that.
Then monitor the interior moisture level / dewpoint - especially during the first year since there's a lot of water in the slab. You may need to run a dehumidifier the first winter to keep RH <40 or so.
Does that all make sense?
That makes complete sense!
Yes in all respects. I give myself a bit of a pat on the back when it comes to air sealing. My house is 4800 sq ft and only costs me $200 to heat per month here in VT (6" walls ...mineral wool ... caulked in every sheet of drywall to the studs ... and lots of spray cans of foam and caulk in every crack I could find).
The slab does have foam and a vapor barrier .... and has been down for about 8 years now.
Only me playing in the shop so no great moisture load even once it is built.
Really in the end, I am just trying to get my best "bang for the buck".
Rain rain rain .. then snow ... for the past 6 days! Cold (25 deg) this morning but SUN! Heading out to get the last section of all up.
$22K of trussed got dropped of yesterday!
Thanks .... Mike
I'm just reading this forum for the first time, great info. I live in Ontario north of Kingston, I think about the same winters as Burlington.I built my workshop 20 years ago, same cement pad and 2x6 construction as yours. To provide more insulation and a thermal break, I toe nailed horizontal 2x4s on the inside, on edge on 2' centres. This was done after installing the insulation in the 2x6s.Next I ran the electrical wiring by laying it on the 2x4s, which was really easy as there was no drilling. Then I installed the 3.5" fibreglass batts , 6mil poly vapour barrier, and gyprock sheeting. I was worried that the toe nailed 2x4s would not support the gyprock sheeting properly but after 20 years it's fine. It should be noted that installing fibreglass batts without the wiring in the way is really easy and makes for a tight insulation job. Good luck.
I will have to give that some thought. Might be a good alternative to foam on the inside.
Interesting thought about wiring also. I decided I would just run trough and conduit on the inside walls (as you said, much easier than holes ... plus I can decide where I want plugs, machines and lights after the building is done).
Side note ... I love Kingston. To get from VT to Welland (near Niagara Falls to visit my family), we sometimes go over the lake ... gives me a chance to stop at Upper Canada Village, the Military Communications and Electronics museum and Old Fort Henry (I keep missing the pumping station ... only been once years ago, seems to be closed when I go through). Just hate having to go around TO. My niece just graduated from Queens ... mining engineer.
Thanks ... Mike
Thanks .... Mike
Small world. When the kids were at home { over 45 years ago} we lived in Montreal and spent a lot of weekends and vacations camping in Vermont. In particular we went to Smugglers Notch near Stowe and a small island in Lake Champlain near North hero. We used to visit Burlington all the time for dinner and a drive-in movie. We'd get back to the campsite around 1or 2 in the morning. Great times. Makes me think I'd like to visit that area again.
Cheers