Best way to superinsulate?

Started by jimgranite, January 22, 2007, 11:12:14 PM

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Its cheaper than OSB and claims to be better at sealing moisture. - Is this true?
It looks a lot like the stuff down at the Home Depots here, but without looking at it again I'm not sure. As far as moisture resistance, since reading the article posted by John on superinsulation I don't think that's a factor. The resistance to moisture is going to be built into the wall using a housewrap or foam board or combo thereof. How it is achieved depends on your location with regards to climate, humidity, rainfall, temperatures, etc. The black material I'm familiar with definitely does not have the same structural strength of OSB or plywood. A nail driven into it can easily be wiggled back and forth and be pulled out by hand. A nail in OSB will bend before deforming the hole if you wiggle it .


QuoteIt looks a lot like the stuff down at the Home Depots here

Same stuff at Home Depot here in North Texas. I priced it against plywood (not sure now if it was OSB) for wall sheathing - the FibreBrace was like $7 and the wood was like $14.

There is a nice looking pamphlet in front of the FibeBrace touting how much better it is against its wood counterpart.  If I find it I will post back.  Anyway, I believe that is what Rob indicated he used for the initial sheathing.  I would be curious as to what, if any, other material he put on before the siding.

I am considering using it on the playhouse to see how it actually performs.

Who knows, I might be going back and ripping it out later - but I guess nothing ventured is nothing learned.


The $14 must be plywood (CDX ?)  Here the OSB is currently $6.39. Last time I looked the black board was more than the OSB. Code here won't let you use the black stuff at corners, so I figure that indicates a strength deficiency. IMO.


After mulling over fasteners some more, I realized that the bottleneck in heat flow would be the surface area of fastener in contact with wood. I then realized that John said exactly that a couple days ago but I just misunderstood what he meant....sorry John, I will try to read more carefully in the future. Anyway, I calculated that 17 nails with 1.5" of bite would have a combined 12 square inches of surface area. Based on that, the R-value of externally-applied foam would be reduced by only a negligible 1%.

Martyv, it doesn't really answer the question, but the Building Science Corp "Designs that Work" house for Juneau also uses external foam with no cavity insulation. Unfortunately, they don't explain why they chose not to fill the cavities. But it could be that the Fairbanks builder has just modified that design. Yes, yet another link to the Building Science site...lots of good reading there...

Steve, there is a paper discussing R-value calculations and thermal breaks at They say that the area-weighted average of U-values will always overpredict the R-value as it doesn't account for heat flowing sideways within the wall. For wood framing, the error is small. But, when the wall contains metal or other highly conductive components, then the error can be quite large. They got an R-value 86% too big when using that method to calculate the R-value for a wall using 2x6 metal studs.


Question. On the sketch above I'm seeing house wrap on the wall sheathing with the blue foam outside of that. I thought the wrap was supposed to be as close to the cold side as possible which would put it between the foam and the final exterior siding?


Actually looking at John's post on 2/2 the wrap seems to be outside the foam.


Looking at and reading the Buliding Science website,    , the layer of housewrap is placed on the exterior side of the studs as a drainage plane on the outside. The foam layers go over the housewrap. They recommend a semi-permeable latex paint over drywall on the interior wall to allow the inner space of the wall to"breath" or transfer any humidity out to the warm side. They also point out that sealing the top and bottom plates is very important, otherwise all your best insulating is for naught. Another interesting thing I noted was their recommendation to use non-paper faced drywall in the hot-humid climate areas. To reduce possibility for mold I surmise.


"They also point out that sealing the top and bottom plates is very important, otherwise all your best insulating is for naught."

This is one of the reasons I am considering exterior insulboard.  On my last cabin I had the whole interior spray foamed.  Then I went inside at -30 degrees with it still unfinished and heated with an electric unit heater.  I was blown away by the amount of heat loss, (cold air infiltration), at the top and bottom plates and even where king studs were side by side with cripples etc.  All that would be taken care of by exterior insulation.  

After all that...I just got a good deal on a load of three sided now I will build another inefficientt log cabin.  But I sure like log cabins.


If I had to deal with -30 (F) temperatures (although by -30 C it's already durned cold--the scales are the same at -40)  I think I'd do some heavy duty insulation--or have planned for the thermal mass of the building to be self-heating.  Not sure that "both" works well.

My log room at the barn--still doorless--can stay below freezing all of a sunny day when the outside temperature is up into the high 40s.  And it has south-facing windows.  Small but there.  But I expect if I could ever get it up to a reasonable temperature, it would stay there for a good while.  Maybe after the door--and the last of the windows--is in.  But since there's no insulation below ground there it will probably always be cold in winter.

Around here older log cabins often have something on the order of clapboards on the outside, plaster, even wallpaper over very thin boards on the inside, that would work to still use the structure but conceal insulation.


I don't think the location of the housewrap is critical when you are using exterior foam. The foam itself is a weather barrier and air barrier. I would put it over the foam and under any spacers for the siding. It provides both an infiltration barrier and UV protection for the foam.

If it is not exposed for long and it is easier to wrap the wall prior to the foam then that should work just as well.


If you have siding and furring over exterior foam, why have a drainage plane (house wrap or paper) against the sheathing? ... or anywhere?  Isn't the foam closed-cell?

Some recommend embossed wraps, or multi-layered papers, etc, to assure drainage .... at what point does the drainage plane against the sheathing allow so much air to move around that it defeats some of the effectiveness of the exterior insulation?


A closed cell foam installed over sheathing is going to do a good job of insulating the framing and providing an air barrier to infiltration. A drainage plane over this is primarily to break the capillary action of water from wet siding. This could be wood spacers, dimpled housewrap such as some of the newer products, or wavy old 30# asphalt building paper. It doesn't really need to be more than one of these.

These all allow a bit of airflow behind the siding but this is to the outside of any insulation. It probably does sacrifice some of the insulating value of the siding. (Small as it is.)


Just FYI, the references to is  now

here is an example link previously referenced in this thread.  BTW, I do know this thread is dead but I thought this would save others time because there is some really good information referenced in this thread.


Quote from: MountainDon on January 24, 2007, 11:30:35 PM
Welcome aboard!   If you use foam on the inside and must have drywall for fire code, that doesn't mean you can't place your pine over the drywall. No need to get fancy on the joints either, just sorta smooth with no big lumps.

I want to put 2 inches of blue styrofoam on the inside of my roof rafter fiberglass insulation. There are no building codes in my area. Can I safely nail 3/4 inch T&G pine directly over the styrofoam without drywall first?
Dave Raftery


On ours I put 1-1/2" of foam beneath the rafters, fiberglass between the rafters, then screwed osb to the rafters through the foam and then attached T&G to the osb. It's been there at least 25 years. I reshingled last year and pulled some sheets of roof sheathing with damge from waiting too long to reshingle. Everything underneath was fine... In VA.

I see osb was $6.39 in '07, ah those were the days, I think I paid around 5 bucks a sheet when we did ours.


Thanks Don_P,

The OSB is a great idea. I'll probably wait until the price drops below the current $30 per sheet. I still have to insulate with fiberglass under the main floor. I'll do that first and see how much warmer it stays in the cold weather..

Dave Raftery