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lclevel_gw

Thermal Bridging

lclevel
15 years ago

I live in southern Louisiana which is considered a hot humid climate, and IÂve been giving a lot of thought regarding thermal bridging, and how to eliminate or reduce it. IÂve read that the surface area of studs could be as much as 20% of a wall. That can greatly reduce the overall R-value in a wall.

IÂve done a fair amount of research and now I would like your opinions on the method IÂm thinking of using.

IÂm open to all ideas, but my main focus at this time is thermal bridging from the studs. However, if you have ideas on how to reduce T.B. at doors and windows I would love to hear them also.

1. Exterior framing will be 2" x 6" wood studs.

2. Closed Cell foam sprayed in the cavities.

3. ½" Rock on the interior.

4. ½" wood sheathing on the exterior.

5. ¼" foam sheathing placed over the ½" wood sheathing to reduce or eliminate the thermal bridging.

6. Hardi placed over the foam.

From what IÂve read, House wrap isnÂt needed since I will be using closed cell foam. Do you agree?

Do you think the ¼" foam will reduce the thermal bridging enough to give a good return on energy savings, or is it over kill?

Thanks in advance.

Comments (11)

  • thull
    15 years ago

    If you really are concerned about bridging, I'd be looking at SIPs. My guess is that sprayed foam in 2x6 walls is going to be $$$$$.

    I'd economize on the insulation with cellulose and up the exterior foam to 1". I'm no expert, but I've seen writeups on homes using the foam as the sheathing. With good shearwalls incorporated as needed, this could work.

    You might also consider incorporating one of the rain screen wall details to ensure that any water that gets behind the Hardie drains out.

    I've also seen some good details on insulated headers to reduce bridging at door/window headers. These incorporate either an air gap or sandwiched foam to minimize bridging.

    See the link for various details.

    Here is a link that might be useful: Building Science details

  • energy_rater_la
    15 years ago

    the thermal bridging will be reduced if you use a 1" closed cell foam board to the exterior.
    a lot of homes exterior walls are solid sheeted plywood corners with 1/2" closed cell foam board and the rest of the wall is 1" foam board.
    You can use a foil faced sheathing board and install furring strips for the hardi.The foil will reflect heat out
    of walls since you will have an air space created by the furring strips.

    if all seams are taped with the proper tape, windows and doors are properly flashed, and all penetrations are sealed
    in foam board then this will act as both vapor, water and air barriers.

    plywood solid sheeting placed on interior of wall will give you the same strenght as exterior sheathing to the
    exterior.

    make sure that you are getting the full 5.5 of foam insulation in the walls. They should overfill the wall cavities and shave it back.

    where in south La are you building?
    if you really are concerned about eliminating thermal bridging have you looked at SIPs panels?

  • sierraeast
    15 years ago

    If you use the furring strip method energy mentioned, remember to incorporate the air space between the siding and the foil into the soffit venting so that the hot air will go into the soffits and up and out through the ridgeventing or attic venting. The hot air has to have an avenue of escape. You also want to screen the bottom of the air spaces so that critters/bugs dont get in.

  • jasonmi7
    15 years ago

    That sounds awfully complicated when you can just buy wall systems that already take care of those concerns.

  • energy_rater_la
    15 years ago

    And it is.
    But all walls are not equal to SIPs!

    I'm in La. also and know one SIPs builder with in
    about a 75 mile radius. ONE.
    There is an old factory that is now a sips prefab home company, but they only use 4" panels for walls and ceiling.
    floors have blown foam :( and instead of sips being roof, it is ceiling. Also they use electric tankless water heaters (gasp!) and straight a/c instead of heat pumps.
    They have a little ways to go before getting that Energy Star certification. But, it is a start.

    Water management of the wall is also easier with the method
    described above. Once that foil/foam board is sealed it performs well at keeping water air moisture from getting
    inside the walls (remember we dry to the interior here-
    vapor barrier to exterior).
    After Katrina Joe from building science was here for several classes. Cool guy. We discussed caulking of laps of hardi vs the wind driven rains the hurricanes bring.
    What I have always recommended is caulking the laps of hardi and leaving only the bottom 4 laps open for water to dry from. By the third class, he agreed that given the rain and the method described above that caulking the laps was not overkill and would be of benefit.

    And you are right Sierraeast..I never thought about bugs &
    critters getting in that 3/4" gap. I'll have to remember that for next time. (and see how much time it takes!)
    As any water that gets in the wall (when not if) the bottom of the wall has to be able to drain it out.

    sorry about that OP...got off track!

  • lclevel
    Original Author
    15 years ago

    Thanks everyone for all the info. I must now ponder over everything that was said.

    energy_rater_l,
    I'm from a little town called Norwood, which is about an hour north of Baton Rouge. Do you know where that is? What area are you from?
    I've looked into the SIP's, but I'm building my house DIY and I'm more comfortable with stick building. However, I'm always looking for better ways to improve building techniques. Building DIY is the only way I can afford to get my Dream Home.

  • dumaspup
    15 years ago

    Just a thought. But why not use ridge vent at the bottom instead of screen?
    Seem to me that some thing like cobra vent would hold up longer and still let the air in and the water out.

  • jasonmi7
    15 years ago

    SIPs are far, far, far easier to build with than stick building, and they are far less prone to all the problems (in terms of energy effeciency) of stick building.

    You might really want to reconsider. The original plan allows about 400,000 points of entry for wind and egresses for energy; in other words...the amount of detail is going to be huge. SIPs, on the other hand, only require that attention to detail every 4 or 8 feet.

    SIPs are easier to build with, are more foolproof with just a little forethought and careful attention, will save more money in the long-term, and in my area, at least, be far cheaper than your original plan.

    Am I trying to sell you on SIPs? Absolutely. It's simply a better, easier product. If you don't know about them; know about them. It's that simple.

  • flgargoyle
    15 years ago

    Just to further complicate things, if I were building in a hurricane area, I would go ICF. If you look at SIP's, check out steel SIP's as well. They don't rot, and termites have a tough time with them!

  • jasonmi7
    15 years ago

    Actually, what I think is even more interesting are the concrete faced SIPs. Tilt-up construction; walls, insulation, interior (if you wish), and exterior all done in 4 hours. Just add steel floor and roof framing, cover, and move in. Well....maybe I glossed over a step or two.....

  • energy_rater_la
    15 years ago

    I'm closer to Lafayette, but have a general idea of where you are. Congrats on building your dream home.
    Your wind zone should be 110?
    You can drop me an email through this site.

    jason..I think we are in toal agreement about sips being
    the superior wall system.

    best of luck OP