Question for carpentry guys

Question for all t-man carpenters. I'm currently in the process of building my first house. I'm doing as much of the work on it myself as I can. I want it to be very well insulated because I live in a very cold climate. Right now the house is insulated with Kraft faced insulation. Would I be safe putting 1/2 inch thermax over that and then osb? Or would the double vapor barrier on top of eachother be an issue? I have house wrap on the outside currently. Thanks for any help.

I think Owens Corning has a FAQ page on their site.
I have called them for assistance in the past.

I wish I had done blown in but hind sight should coulda.

I would put the 1/2" foam on the inside, under the sheetrock...shoulda sprayed foam in the wall and been done

To avoid trouble I would check with the supplier and manufacturer. Got to give you credit for thinking this through. A lot of folks would not. I you do something that does not work out you will have a heck of a time getting anyone to stand behind your product but at least you are trying to think ahead.
Mac

When still in that business we tore into many one hundred plus year old houses and the best preserved were always the ones with 3/4" boards nailed horizontaly outside the studs and maybe that old tarpaper type material over that then siding as a finish.
Point being there is a lot to be said for letting a house breath particularly these days with the poorly kiln dried materials on the market.
The only mold issues I recall were caused of an actual rain leak, not general humidity conditions.

Osky

I just built and did spray foam and metal roof and sides. The spray foam has did good so far. It hasn't been real cold but Thursday when I got home from work I turned the central air off. It's been 30 to 40 degrees the last 3 mornings and the temp is still 70 In the house. Its held well the last 3 days. I do wish I would have did a little bit of closed cell in the roof and then open cell. I went with just open cell everywhere. The close cell would have helped with the sound of rain on the roof.

you should not use paper faced with the Thermax over that, I would remove the paper, I also would question why the Osb? I am assuming your referring to the interior ? If this is your finish fine, if it’s not I would omit it and use 1” thermax and then tape all joints a;d nail heads this is now your vapor barrier

I agree with Osky. I would use Roxul ridged board either 1-2” it breathes and doesn’t require any vapor barrier. I would also use a mesh underneath it to help remove any moisture like that used underneath cement board siding. Added insulation in the attic and use flashing properly around the windows and doors, use kick out flashing where needed or you will rott your walls. Flashing incorrectly is one of the biggest problems that needs to be addressed. I recommend you look up how to properly install flashing or suffer the consequences later. Plus If possible use big overhangs to keep moisture off the walls as much as possible. No vegetation within a foot of the house the walls need air flow to dry out

You are correct. Too many tight houses in the last several years that cannot breath.

Yes the interior. I was thinking of putting osb over the thermax and then nailing my tongue and groove pine to the osb. Maybe I should use strapping and nail the pine to that?

Thanks for all the replies so far everyone. Trying to let my house breath but at the same time want to insulate it to the max. Trying to decide the best route. Keep the opinions coming.

The vapor barrier should ALWAYS be on the warm side of the wall.

I fit was me I would do this if not too late, if windows are set it may not work out

Insulate stud cavities and then put 2” on outside of all walls and buck out windows And doors

If it’s too late, put on 1”. Thermax, and then nail tg over it into your studs. The osb only helps if your doing small pieces and can’t nail into studs

I don’t agree with others, you should make your house as tight as possible and make sure it’s 100 waterproof

Then use an air exchange to help remove moisture

Tight houses are only problems when they are not watertight or not exchanging air

My main concern putting thermax over kraft faced insulation is that condensation will form between the two and have no way to escape. Kraft is not a true vapor barrier its only a vapor retardant. It would be a humidity thing and not a leak thing the house is brand new so should be leak free for a while
Am i over thinking this?

Thanks Jack's, unfortunately the windows are set and siding is on the outside already. I was planning on vertical pine so nailing to studs is out of the question

BTW, modern housewrap is designed to breathe similar to Gore- Tex, so it is fine to have housewrap and a moisture barrier behind the finished interior wall. What do you plan to finish the inside of your exterior walls with?

I would remove your kraft paper, If you can sacrice some wall thickness I would use 1” foam over. Tape all joints good amd caulk floor and ceiling corner joints. Then strap it with 1/2 or 3/4 and then foam in between the strapping and then nail on tg

If you could not make wall thicker I would go through work to strap the studs inside wall cavity, notch fiberglass around that, use 1” Thermax and then nail tg

But you could use only 1/2 Insul and strap on top of that and the air space will help

I would not recommend ever setting up a situation where you have two vapor barriers. Sounds like you are aware of that.

I too would have added the foam board on the outside but since it is too late for that You could just sheetrock and fire tape. Then ad two inch nailers with 2" foam board in between. You COULD also run your electrical on this surface ( I'll bet that's already in place)....anyway that will give you more insulation, and nailing surface for the T&G pine, and have your electrical at the finished surface. Probably a bit cheaper too since the rock is cheaper to buy than the OSB.

I am assuming it has been wired for electric and I missed the part about the walls being T&G pine. Gonna be a pain moving all the electrical boxes if you start furring out the walls. I guess that part is up to you. Plenty of insulation in the attic and a good, efficient furnace will do you more good than anything. Using housewrap tape around all doors and windows and caulking anything else that might cause an air leak will seal it up just fine. I don't like Kraft face insulation because it has a tendency to put pressure against the back side of sheetrock causing nail pops. It can also cause your T & G to work away from studs or not get nailed tight, especially if you are using finish nails to nail it off.

I missed the part where you planned to sheath the inside of the exterior walls with osb. If that was the plan, I would opt for white17's plan instead.

I've always been lead to believe that you never mix two types of insulation. Condensation will form between the two resulting in less R value than you would have with using just one.

No problem using two different types of insulation. It is very common to use spray foam as a flash coat to seal up crawlspaces and attics. Then a lot of folks use blown in cellulose on top of the foam in attics. In crawl spaces many people add foam board over the flash coat of spray foam. No problems doing either of those things

It used to be common(i think it still is) for builders to used rigid foam sheathing on the exterior everywhere except on the corners where osb is used for bracing. The walls are insulated with kraft faced fiberglass between the interior finished wall and the foamboard. I have never heard of anyone having a problem doing it that way,. And yes, the vapor barrier is always on the heated side of the walls.

I am not sure if I missed it but what is your wall depth 2x4 or 2x6? If you truly wanted warm walls 6 is the way to go. If they are 2x4 your only going to get so much out Of them. Do what you can and put your efforts into the overhead insulation and your base perimeter insulation. You will lose most out your lid. Get that really well insulated and just as importantly really well vented so moisture is not a problem up there. Your base perimeter can let a ton of cold in if not addressed.

Osky

We can no longer use 2x4 exterior walls here.

You can if you can reach R21.

Looking at the above posts... If you have 2x6 walls with a house wrap on the outside and 6" insulation Kraft face on the inside I'd leave it at that. Make sure you have enough room around your window and door jambs for insulation, do it right, and use good doors and glass. If the interior is a wood tounge and groove type product I'd put it right over the craft face. Done.
I would not put poly any where near the place if I didn't have to.

Osky

I think this is the route I'm going to go with, Osky. They are 2x6 walls and electric is already in. I still may put osb or at the very least strapping because I'm going vertical with the tongue and groove. Attic is well insulated so heat loss out of the ceiling should be minimal.

Thanks everyone for your responses, looks like I will avoid placing two vapor barriers over each other. If it means I have to bring in a little extra firewood for winter then so be it. I would rather have a less insulated, dry house than a super insulated house with moisture issues.

Make the house really tight and then put a HRV in. I built my house and used icfs from the basement to the rafters. If you have the air exchanger it should take care of the moisture inside.

Funny how different places are different. Some areas require a poly vapor barrier and others prohibit it.

Personally, in a cold climate, you couldn't give me a house without a poly VB !

We're it mine, using vertical tongue and groove, the interior osb is crappy backing for nailing. I would just put horizontal blocking in. 2 rows min, maybe 3. 2 x 4s set wide face to the inside flush to the inner studs. Toe nail them in. End nail and stagger if easier.


Osky

OSB is not a good backer for T&G if you are using finish nails in the tongue. OSB just doesn't grip nails very well.

Adding 2x4 nailers on flat between the studs will compress your insulation and diminish your R value.......something you are trying to protect. With batted insulation, you're going to have drafty walls if you simply install T&G without some sort of solid backing.
Skip the insulation board, skip the OSB, and just add 1/2" drywall......taped and mudded with one coat to seal things up.
I would save yourself a lot of work and run those finish boards horizontal. If not, fir ontop the drywall and run it vertical.

Hopefully you electrical boxes aren't installed and rough wire yet, or else you'll be moving everything.

I think engeneers have WAY too much time on their hands

I think 6" inulsation will do just fine despit a couple of 1 1/2 inch compressions where the nailing blocks are. The wood that replaces that 1 1/2 inch of insulation has good r valu itself.

Osky

. I enjoy working with a good engineer ! You can keep all the architects though. They think just because they can draw something two-dimensional that it can be built with wood in three dimensions.

I received a set of signed, stamped prints from an architect for a new museum for the University of Alaska. He had the floor below grade but wouldn't believe me until he sent out a surveyor !

A lot of engineers have actually worked in the trades. Architects not so much.

Lot of truth right here boys.

Don't get me started about architects.

I can't tell you how many times I've had to pore over prints trying to find simple addition/subtraction errors when total inside dimensions don't equal outside dimensions. You would think basic math skills would be prerequisite to being an architect.

One of my current jobs; converting a back porch into a master bath/walk-in closet/dressing room, the existing footing does not meet current code (not deep enough). In my opinion it is deep enough to have no movement. My architect agrees. He was shocked when the code enforcement officer wouldn't let him sign off on it instead requiring a report from an engineer. It was a real blow to his ego! LOL

Electrical boxes are installed but every one of them in the house is an adjustable box so wall thickness can be up to around 2 1/2" should I choose

Would plywood be a better backing for T&G than osb?

I liked working with architects. I usually met them on professional malpractice and breach of contract disputes. Three architects I met that way are still close friends.

Engineers tend to be more arrogant, like doctors. I knew an engineer I used as an expert a few times, and went fishing with once or twice on the Situk River. Lost contact with him. I think he was probably wiped out financially because his firm designed the Anchorage port project, which has incurred a couple hundred million in damages when the new dock failed. I remember when Dennis showed me a model of the new dock system he designed. He was so proud of it. Yet on a real life project the cells were failing even before construction was finished.

Jim

It would be but solid wood, like the horizontal purlins Osky suggested, would be much better. I'm doing a job right now where the original installers failed to hit any studs when they put up the 5.5" straight-lap mahogany siding. As a result hundreds of nails have backed out and the siding is warping, cracking and pulling away from the sheathing. I'm replacing and renailing everything. Granted, you won't have that extreme movement inside but it's always best to use solid nailing.

This.
Snap two or three horizontal chalk lines spaced equally on the inside wall. Cut your blocks for the spaces and nail one spaces block above the line, the next under and so on. Push the blocks in against the paper craft face, if it rips so be it. Stapling the craft face in is not a solid seal anyway. Don't over think it. Again you will lose more heat to draft and cold invasion thru Windows and doors than you ever will thru the walls.
Put the money saved into other needs.
Osky

I,had a business associate ask me to,look at a set of prints recently for a very large storage facility and compound he is going to havie built. The civil engineer who did the utility and water dispersion layouts has the grades running water south, when it's suppose to be running north to the very expensive holding ponds that are required on the property. They thought me a bigger fool than I probably am until I convinced everyone involved to take a second look. Veeeery proud of themselves those engineers and architects.

Osky

Pine has an R value of 1 per inch..
Batted fiberglass is around 3 per inch depending on the brand

It's not just the R value lost in the compression, it's the inconsistency in the crushed product that will create separation from the framing members and sheathing. This separation will leak air, and leaked air will further diminish heat retention.

R value of closed cell spray foam isn't all that impressive, but its ability to totally seal air flow is off the charts. Heat retention is like 96% in the first two inches of application.......much much better than 6" of fiberglass with typical loose installation methods.

You can do whatever you want to do, but if I were doing the job on an exterior wall (inside) with T&G pine, it would be getting a solid backing of some sort to SEAL the draft out. Pywood would work. Drywall is much cheaper and easy to air seal. Unless you are absolutely certain that the exterior sheathing is buttoned up air tight (95% of the time it is not) you need to seal out the drafts on the interior walls or you will FEEL it when the cold winds blow. House wrap is not a wind barrier. It may help some, but it will not keep drafts at bay.

T&G pine by itself nailed straight to studs will leak so bad you'll hear whistling between the boards in some places. The tongue does not seal in the groove (it's not designed to). Magnify that by shrinking wood in the cold dry air and you might as well have window screens screwed on your walls. This is not a theory, I'm telling you from years of experience.
Blocking on flat between the studs is fine for an interior wall. Not a chance I'd allow it on exterior walls.