1″ rigid insulation on OSB sheathing
alfie
| Posted in Energy, Heating & Insulation on
On a new construction project I am committed to going with 1″ rigid insulation on top of the OSB sheathing (2×6 @ 16″ walls). What type of rigid insulation might be suitable?
So far a candidate is:
“THERMAX sheathing is a glass-fiber-reinforced polyisocyanurate foam core with solid aluminum foil facers on both sides.”
Comments?
The siding is going to be Hardie lap siding.
Replies
Just trying to be the first to respond to this with:
"What climate are you in?"
Foil-faced foam on the outside might be fine (warm humid climate) or terrible (cold climate).
In a heating or mixed climate, you'd be better off with just XPS unfaced foam, so at least some moisture can permeate through it when needed at times.
Now, if you wanted to put several inches of Thermax outside, with no insulation inside that (other than closed cell foam), then you're ok.
This subject has been covered every which way. Do the search on it, and leave a few days aside for reading all the posts.
Am in Connecticut. Yeah, saw a lot of posts on the topic.
Depending on exactly where in CT I'd say 1" is too little.
There is no absurdity that human beings will not resort to in order to defend another absurdity. -- Cicero
Thanks for the responses. There will be insulation in the 2x6 walls. I considered thicker rigid insulation, but was concerned about installation and the impact on window and door jambs. 1" seemed more do-able.
Dow Tuff-R is a similar product. In my area, Home Depot carries it in several thicknesses.
Why only 1"? 3-4" would be far superior.
Alfie, I am presuming you want the layer of foam outside to add R5-6 to the overall wall R and to reduce the thermal bridging of the framing. Thus you will be filling the stud cavity with insulation of some sort. Correct me if this isn't so.
If all the insulation is to be the exterior foam, then, yes, 3-4 inches would be needed to meet code. But then you said 2x6 @16" oc.
I imagine code there calls for R20 in new construction. By the numbers only, that can be met with just the cavity fill, and the BI will check it off. By adding to that, you have some objective in mind. What are your goals here? Is this your own house, a spec you are building, or a custom job for someone who has certain goals?
Foil-faced foam is totally vapor impermeable. XPS has low permeability, somewhere around 1 perm for 1", and you cut that in half for every doubling of the thickness.
Much has been said about having too much impermeability on the outside in a heating climate. It calls for a very well detailed vapor retarder inside, so that the cavity isn't "open" to the humidity of the interior of the house. But the VR shouldn't be a vapor barrier (ie. poly sheet), and you should avoid vinyl wallpaper. Think of all the posts on double vapor barriers here.
Sooner or later here, someone will comment that in a heating climate the better placement of foam is on the inside.
Is your intention is to add some quick and easy layer to increase the whole wall R value by a modest amount, while doing the framing "as usual?" Why not do it right and aim for R30-40, thinking further down the road in terms of energy cost? Of course, that calls for more detail than "usual framing."
Thanks for your thoughtful response. I am building for myself and intend to keep the house for a long time. My goal is to achieve energy efficiency and do the best I can from a building science point of view. At the same time, I am not looking to experiment or try something new.Yes, I am attempting to reduce the thermal bridging and increase R value. I anticipate spray foam insulation in the cavity.The “do it right†and go for R30-40 is tempting, but I am concerned that I may not figure it all out correctly. So much information, hard to figure out the way to go. Construction starts soon. So something-is-better-than-nothing may have to do.
With closed cell sprayed foam in the 2x6 cavity the foam board outside should be fine as the closed cell foarm is very low perm.
Dense packed cells could work too.
I think that foam board on the inside would be the best for your climate.
Do a search on "Mooney wall" -- especially if you are buiding it yourself. You will get great R value and low thermal bridging using dense packed cellulose without using foam, but you can also use foam with it if you want.
Billy
Thanks. I have followed discussions on the Mooney wall in the past and it sounds like a great system.
I, personally, think you are on the right track. We have used this system extensively, with great results. 3/4 - 1" rigid foil faced foam sheathing does create a thermal break at all framing members, and also covers framing joint areas that otherwise can be air leaks after framing lumber dries out, such as doubled studs, header/stud connections, etc. This, combined with spray foam insulation, creates a super tight wall system with little difference in labor than conventional construction.
I disagree with the assumption that to do it right you should be installing R30-40 in walls. There is a point of diminishing returns and I think the air infiltration control of this system is more important than boosting the r value much higher. Also consider air sealing your attic with foam at all penetrations and around all air baffles before installing main insulation. We blow in r-50 cellulose in our attics.
Using this system we are seeing total natural gas usage of about $ 6-700 a year which includes heat AND hot water in one story 1400+ sq. ft. homes here in upstate NY. Increasing R value in walls would not significantly reduce these costs. Don't forget to include some type of HRV system to insure fresh air exchange with this type of construction.
Thanks. Which specific foil faced rigid insulation would you suggest? In principle I agree with your view on diminishing returns.I will be insulating the roof with spray insulation, resulting in a conditioned attic. If I am able to achieve my intent, I will have spray foam insulated walls and roof. Yes, I will have an HRV or ERV.Windows - two points here. Putting the rigid outside will add to the labor because we'll have to furr out for installing the windows. Going for thicker insulation would increase the window jambs. For the windows I am going with, after 7 9/16", the upcharge is steep. So I am thinking 1" of rigid makes sense.
I don't think the brand is that important, more what is locally available. We have used both Tuff R and Cellotex brands, same r-values.
Depending on your windows you might not need to fir out openings. If they have a stiff enough nail flange we have had no problem nailing right to the face of the sheathing. The density of the foam is such that the units sit quite firmly. Set them in a bed of good quality caulk and then also flash with a good tape product. I like the Zip guard tape made by advantec. It sticks great no matter how cold and remains flexible. Doors will be anchored through the jambs anyway.
I was thinking type, rather than brand.
Type is foil faced polyisocyanurate rigid foam. 2' o.c. studding does make sense too. This saves money, wood, labor and reduces thermal bridging while still being stronger than a 2x4 wall w/ 16" o.c. studs. The spray in foam also adds rigidity to the exterior sheathing.
Why 2x6 @ 16"?
Why not 24" OC?
Joe H
This one falls in the category of "...just because...". Fair question though.
Hi- Here in Nova Scotia I built my house with 1" of XPS over 7/16" OSB with the house wrap between them. I strapped over that for a rain screen wall and went with pre- finished board and batten siding. The 2x6 framing was filled with fibreglas batts on the inside. The XPS was to combat thermal bridging (our heating season is 7+months) and only increased the window sill size enough for a plant or a cat. The added benefit as a further means to stop air leakage is really appreciated on cold blustery nights. I used type 4 XPS (Dow Corning Cladmate) as its compressive strength kept it from crushing when the end of the strapping was fastened over it. I liked it so much I did the same thing when I built my garage.
Bish; SWASAN ..... Thanks.
Hi, just read your post here, and wanted to ask a couple of questions. I'm in NS (Dartmouth) with an 80 year old house I'm renovating. I have 2x4 walls insulated on the ground floor with fiberglass under drywall, and second floor blown in cellulose under plaster. The main floor has a poly vapor barrier, but it is not continuous. I'd like to add 1 to 1.5 inches of rigid insulation to the exterior of the sheathing (which is tongue and groove pine), and have been trying to figure out what is the right location for vapor barrier on the exterior--and currently am planning to apply it under the foam (against the sheathing), as you have. We're planning to go with clapboard siding.Have you had your house through a couple of winters? Do you think that placement is the right one for our winters? Finally, do you think that the discontinuous poly on the inside will pose a problem? I'll be opening up most walls to install new windows and so could rip away as much as I could reach. Any advice would be appreciated.
Hi, Sorry it took so long to reply. I wouldn't put a vapour barrier on the cold side of the wall, instead I would use house wrap (tyvek or typar). I've heard you can get a paint that acts as a vapour barrier, and together with some good caulking, this just might give you what you are looking for (on the inside walls). I put the housewrap under the XPS (with the seams taped) but it made it hard to see where the studs were to nail the strapping to.Perhaps putting the house wrap on top would allow you to use the hash marks on the wrap to line up with your main studs. I recently purchased a 100 year old fixer-upper and had the joist bays and stone walls in the basement spray-foamed, as well as the attic. I can already see the difference. I highly recommend it. I purchased a do it yourself foam gun and have been foaming as I replace the windows. The house that I had with the XPS under the wood siding went through 8 winters after we moved in and that method worked wonderfully.