Budget insulating new 2×6 frame?
user-901114
| Posted in General Questions on
Building a house in the lakes region of New Hampshire. 28×36 cape on a daylight foundation. Found a builder I like. Gave me the usual quote. 2×6 frame. 10 pitch truss roof. Tyvek. Vinyl siding. OSB sheething. Now I want to get some advise on how to insulate but keep the same budget. Here’s what I’m thinking about for walls from the inside out. Laytex paint>Sheet rock>2×2 horizontal strapping>2X6 frame filled with cellulose loose>OSB>tyvek>vinyl>
Thoughts?
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Hmm how about drywall - 2x6 wall filled with cellulose - osb - tyvek - 2" foam - strapping - vinyl
Stephen,
I would definitely not use exterior foam board with moisture-vulnerable OSB sheathing (and generally advise against "outsulation" in a cold climate).
Your wall section and thermal envelope is one of the least costly methods of achieving something a little better than the R-20 code minimum wall.
But you may be "penny wise and pound foolish" in using the cheapest materials, which saves first cost but makes long-term cost much higher. I always use plywood rather than OSB on my low-cost homes and would never consider vinyl siding (it's junk). Wooden ship lap or fiber cement is far more durable as long as they're maintained, and far, far more green. I always use spruce novelty drop siding finished with solid color latex stain (on all six sides).
If your building inspector will allow it (it meets the International Residential Code), you can use let-in metal T-bracing, housewrap (Typar is much better than Tyvek) and shiplap wooden siding with no sheathing. This allows the walls to breathe much better and will make them more durable while saving materials and cost.
If by "daylight" basement you mean a walkout on a slope, then how are you insulating the living space down there? I would suggest looking into the ThermoMass foundation system that places up to 4" of XPS rigid insulation board in the middle of the concrete wall where it's protected from UV, insect and physical damage and creates both a thermal break and a capillary break in the concrete wall, keeping the inside warm and dry and still giving you a great deal of dynamic thermal mass advantage which lowers heating costs.
I hope you're not going with the lowest bidder for a builder. Quality construction is priceless - cheap construction is worthless.
And, make sure you meet or exceed the IECC minimum R-49 ceiling and R-15 foundation requirements for your climate zone.
Thank you for the replies. Robert. I think the shiplap would cost much more in labor
. Can I switch that and the plywood out with t111?
The foundation is a walk out. West and north are earthed. It's uninsulated and 15 years old.
This builder came back with the highest bid. Good reputation.
Stephen,
T-111 can be an excellent combination sheathing and siding that can later become a nailbase for other siding as money permits.
I installed it on a community land trust affordable home project down in the hollers of Tennessee back in 1983. We used fir T-111and hand rubbed it with oil-based stain. It came out quite nice.
Stephen,
Everyone focuses on different aspects of a house. I would worry much less about the siding option -- over the life of a house, siding is usually changed at some point -- and concentrate on getting a good thermal envelope. For me, foam sheathing provides the best way to improve the performance of a 2x6 wall.
Needless to say, just because the foundation is now uninsulated, doesn't mean it should stay that way. Be careful of thermal bridging; if you install 3 inches of interior XPS on the three below-grade concrete walls, do your best to avoid thermal bridging at the corners of the foundation on the walk-out side.
Would I have to use plywood if I went with exterior foam? I am very concerned with moisture. The land is wet. The climate is wet. The basement looks like it could be wet.. has a pump hole.. i was going to install a solar attic vent in the south basement wall just to keep mold from building up. I have a lot of work to do:)
If I went with t111, would I still use typar? Would foam under t111 be a good option?
Stephen,
Plywood is always preferable to OSB. However, adding exterior rigid foam on top of OSB sheathing will keep the OSB sheathing dryer and will therefore lengthen its life compared to OSB without adequate foam. (Remember, in your climate zone, the minimum R-value for exterior foam sheathing over a 2x6 wall is R-11.25; that means at least 2 inches of polyiso. Read more about exterior foam here: Calculating the Minimum Thickness of Rigid Foam Sheathing.)
Ventilating a damp crawl space or basement sometimes makes the situation worse. Instead of ventilating, consider other remedies, including improved grading and drainage.
T-111 is not a water-resistant barrier (WRB). No matter what type of siding you use, you need a WRB. Typar and Tyvek are both WRBs, but so is asphalt felt.
If you decide to install rigid foam under your T-111, I would advise including a ventilated rainscreen gap between the foam and the T-111.
Stephen,
Martin is an advocate of exterior foam and he believes it keeps the sheathing dryer. I am a strong opponent of exterior foam precisely because it can trap moisture in the sheathing and framing and destroy it.
There are two approaches to creating a thermally-efficient and durable wall system. The one that Martin proposes and that is becoming commonplace among "green" builders is exterior foam to limit thermal bridging, to increase the assembly R-value and to keep the sheathing above the dew point to limit condensation.
The other approach, which I have advocated and used successfully for 30 years, is to thicken the wall assembly, either with cross-hatching or a double wall, in order to improve the thermal efficiency and decrease thermal bridging, control air exfiltration with a well-sealed drywall layer, and allow the wall to breathe to the outside to eliminate any moisture that (inevitably) finds its way in.
Besides the necessity of petrochemical foams in the "outsulation" approach, this dramatically limits drying to the exterior, which is the dominant moisture flow direction in a cold climate. The "warm sheathing" approach works in theory, but if there should ever be a leak into the wall cavity (as happens with every house at some time), it's been proven in both laboratory and field tests that exterior foam will prevent drying long enough to cause serious mold and rot.
A resilient wall system that can tolerate occasional wetting and allow drying will be far more durable over the long term than one that relies on the perpetual perfection of its water control elements.
Along those lines, exterior T-111 is an excellent WRB as long as it's properly installed and flashed at horizontal junctions and maintained over time with occasional coats of stain or paint. You do not need housewrap with T-111, but you will need to apply a WRB membrane over the T-111 if you ever install another type of siding, so that the new flashings will have a drainage plane to tie into.
The least expensive method for improving the R-value of 2x6 walls is to install interior 2x3 cross-hatching and fill the 7" cavity with dense-pack cellulose. If you'd like to use foam board instead, then the better place for it is on the interior. Use 2" of XPS, with seams taped and edges foamed, and that will become your air barrier, vapor retarder, and thermal bridge limiter. But it will not limit drying to the outside the way exterior foam board will.
The absolute worst thing to put on the outside is foil-faced polyisocyanurate foam board, because the foil is a perfect - wrong side - vapor barrier.
I forgot to add that an additional absurdity of exterior foam is, because it won't allow wooden (or even fiber cement) siding to dry out the back side, a rainscreen gap is needed. With a breatheable exterior sheathing, like wooden boards or plywood, there is no need for a rainscreen, which makes flashing far more complicated and problematic.
Stephen,
A rainscreen gap is not an absurdity; it is a valuable asset.
As I'm sure you can tell, you will get a variety of opinions if you post a question on GBA. I strongly advise you to do your own research before accepting the advice of anyone who posts here, including me.
There's a lot of good information available on the Web site http://www.buildingscience.com. The advice in the GBA encyclopedia represents advice that has been vetted by a committee of experienced builders and building scientists; it is trustworthy. Advice posted in our Q&A column represents the opinion of the person posting the advice.
I love this site. I've been reading all of your comments over the years. With everything I've read, here is my plan. Latex paint-sheetrock taped and caulked -2x3 cross hatching every ? Inches - 2x6 - t111 - oil stain.
This presents a false dichotomy between the "opinions" of those that GBA has chosen and those it has not because their equally rational conclusions contradict the current consensus, which is as much based on cost, marketability, easy availability of materials, and willingness of conventional builders to learn as it is on solid science and engineering principles. And almost none of the GBA consensus approaches use truly sustainable methods or materials - and so can reasonably be considered highly biased towards what is broadly acceptable, which is rarely the same as what is best.
In fact, Building Science dot com often offers recommendations which are contradicted by the basic science they present.
So, yes, readers must use some discernment to determine which "opinions" have a solid basis in building science, engineering principles, and sustainable methodology - but it's irresponsible and wrong to suggest that GBA's experts have it right and other, equally informed, educated and experienced builders are merely expressing "opinions".
Robert
Why must you always suggest any building approach other than your own is inferior. Why is that you always need to interject subtle shots at people. Why could you not have simply said "limitations" and instead had to characterize exterior foam as "absurd"? Its almost as if anything but your own approach has no merit..... Most anytime an opinion is offered that is an alternative to yours, you attack it....I know you may argue nothing you said was anything but factual, but your tact and verbal disposition is "woefully" combative, defensive and uninviting to most. You do notice after the word "absurd" was used is when the whole thread took a defensive overtone? I like to see you challenge Martin and others but hate that almost always its done in a way that's degrading and the defensive. These threads almost always lose focus and becoming a pissing match of ego's. Very frustrating.
Don,
There is no "pissing match" until you make it one by attacking my "presentation" rather than my facts, logic and arguments.
I have no dog in this fight. This is never about ego (at least not mine) - as I've said many times these discussions should be about WHAT is right, not about WHO is right. And I have no control over your (or anyone else's) reaction. If you choose to get defensive rather than return an objective argument, that's your choice alone.
And, if you would read my posts more carefully, you might not misrepresent them and take offense at what was NOT said.
I never stated that exterior foam was "absurd". I presented science-based arguments against its use and quoted reputable studies that have demonstrated its highly problematic consequences.
What I DID say was that an absurdity of relying on exterior foam was that it creates the additional unintended consequence of preventing claddings from drying backwards and hence requires the additional cost and complexity of an otherwise unnecessary rainscreen.
It was Martin, in fact, who - rather than argue his position or argue against mine - resorted to labeling my statements as mere "opinion" while his were "vetted by a committee of experienced builders and building scientists" and hence "trustworthy". This was the kind of backhanded ad hominem argument that you mis-ascribe to me.
You would do better aiming your animosity toward the one who actually used underhanded tactics, and taking responsibility for your own reactions.
It would be an improvement if the wall layers described in the original post could be rearranged, with OSB at the interior side of framing and 2x2 or 2x3 horizontal strapping at the exterior. This would address the low perm rating of OSB, because at the interior it just serves the same purpose as vapor retarder primer. It also allows this material, famously vulnerable to wetting, a good chance of staying dry. The added insulation (between the strapping) goes to the exterior of structural framing, which helps protect the framing and reduces the likelihood of moisture damage to the structure (from condensation / sorption). Of course without any sheathing at the exterior you couldn't use vinyl siding, but T-111 still works for this design.
This wall system would be vulnerable to wetting from wind driven rain, so deep overhangs would help. At some point in the future, if finances allowed, the T-111 should get clad in shingles or lap siding over a layer of building felt.
Placing OSB at the interior may not be required if there's not much seismic or wind loading, but it would add rigidity and improve air tightness. Sheathing the inside could be a sore point with your electrician.
Thomas,
That's an interesting proposal. As long as the walls were temporarily squared and braced prior to erection, the bracing could be selectively removed as the T-111 was installed. The T-111 should suffice for rack bracing if attached to horizontal structural-grade framing, in which case the OSB could be omitted entirely since the drywall does a better job as air barrier. The only downside would be that the vertical lap joints of the T-111 would not be fully supported by framing, making them more vulnerable to leakage.
I'm not sure there's an advantage of placing the horizontal framing on the exterior. Interior cross-hatching is a tried-and-true method of deepening the insulation cavity and reducing thermal bridging, or a secondary wall could be built for greater R-value and even less bridging.
I built my first double stud wall house down in the hollers of Tennessee in 1982: an 864 SF 3-bedroom, one storey home that was designed by a consortium of non-profit housing groups to be initially and permanently afffordable. It had a cinder block foundation and truss roof, fiberglass (yuck) insulation and poly (sealed with butyl caulk) vapor barrier.
It used T-111 siding with no additional WRB and 2' truss roof overhangs on all sides. When I visited the community land trust 20 years later, the house was still, as the name implied, Warm & Dry and in good condition. This was in a very damp and deep hollow which got little sun (my boots would mold at night).
What should the spacing be between the cross hatching and is there a subfloor I can live on for a few years? The house is specd for a 20 in deep osb floor truss and advantec T&G subfloor. Was thinking about swapping for plywood and coating with poly. Also. How can I insulate this truss?
Stephen, the vertical spacing of cross-hatching can be 24". If you want to walk on a subfloor, plywood should hold up better than OSB because it's smoother. Better yet, use solid wood planking, which used to serve as subfloor and finish, way back when. But that wouldn't be very airtight. Rolling on a coat of polyurethane floor finish should work pretty well actually.
As for your floor truss, the insulation should hold tight to the subfloor to be effective. If you enclosed the underside and made the assembly airtight, it could get completely filled with cellulose. But the basement would need to be dry or this could be a bad idea.
Stephen,
What do you mean by "osb floor truss"? Is that a Truss Joist with OSB web or are you talking about a conventional 2x4 parallel chord truss?
If there is a moisture problem, the less OSB in the house - especially the framing - the better.
For a one-piece finish floor, you can use tongue and groove 2x6 spruce and give it a finish coat. This will be more attractive and more durable than plywood or OSB subfloor. To insulate the trusses over a damp unconditioned basement, enclose the bottom with foil-faced polyisocyanurate foam board and dense-pack with cellulose. But if the walkout basement is going to be living space at some point, you might want a drywall ceiling (after wiring for lights).
Links please
Robert, could you point us to the "both laboratory and field tests" that prove exterior foam will prevent drying?
I remember reading a link you posted a couple of years ago, but the only moisture problems were in test walls that had 6 mil poly on the inside. The walls dried to the inside when the poly was left off, which is what people would normally do these days.
Logically, exterior foam prevents drying to the exterior. I would like to see any research about wall failures caused by this. Some of the advocates of foam will say there's no such data. Personally I have chosen to avoid foam for a handful of reasons and not based on a known history of failure when used without interior poly. Certainly there is a history of failure with EIFS when applied as a face-sealed system over OSB sheathing.
It's worth noting that the somewhat inside-out crosshatched wall (described above) would allow an extra R-5.5 or so in a cellulose insulated stud cavity with minimal cost and without interfering with drying potential in either direction. Adding wall thickness to the outside of structural framing also makes sense when you insulate the exterior of basement walls.
Doug,
As you noted, I've posted those links many times, and the tests are too few and too limited (that's the nature of lab or field studies).
Which is why, as Thomas suggests, we have to use logic, building science principles and common sense.
What is unarguable are the tested perm values for the various foams (and the inverse relationship between thickness and permeance), and the basic physics that stipulates that a reduction in permeance equals a reduction in drying capacity, while an increase in R-value equals a reduction in the heat flux that fuels drying.
Given that, in a heating climate, the dominant drying direction is inside to out, it is common sense that we should limit interior permeance and air flow while maximizing exterior permeance. The more overall R-value in our wall systems, the more important this strategy becomes because there is less heat flux to force drying and virtually no air flow to carry moisture away.
As Dr. Joe has long explained:
Buildings should be suited to their environment.
The laws of physics must be followed.
Three things destroy materials in general and wood in particular: water, heat and ultra-violet radiation.
Of these three, water is the most important by an order of magnitude.
Critters love wet materials.
No wet materials, no critters.
Things get wet - let them dry.
Things get wet from the inside, the outside and they start out wet.
When the rate of wetting exceeds the rate of drying accumulation occurs.
When the quantity of accumulated moisture exceeds the storage capacity of the material, problems occur.
The storage capacity of a material depends on time and temperature.
The drying potential of an assembly decreases with the level of insulation and increases with the rate of air flow.
As such, energy conservation has the potential to destroy more buildings than architects.
Robert.
In your opinion is foam on the exterior ever a reasonable detail in a heating dominated climate? What if someone wanted to increase R value in the walls in a retrofit situation and could not afford to dismantle all the interior in order to add depth with the cross hatching etc. I mean XPS on the exterior seems resonable in this situation does it not? Or better yet lets say they need new siding? Would it stil be unwise to add XPS following your logic above. in retrofit situations?
Chris,
In retrofits, options are always limited and compromises that would be unwise in new construction must be accepted.
I have never added exterior foam on any renovation project, but have often added interior XPS which is the better location in a cold climate.
But if the interior is not being redone, then adding foam (XPS or EPS, never foil-face polyiso) to the exterior is certainly a legitimate option as long as the potential for unintended consequences is well considered and compensated for.
That includes the need for a near-perfect weather protection system, including WRB, well-draining cladding and flashings, control of interior humidity, and an ongoing maintenance program to keep the exterior weather tight. It also generally requires a rainscreen to keep siding away from the foam, which makes the flashing and sealing details more complicated and more critical.
I should add that it also requires thinking outside the box of the immediate renovation. If it's only a siding replacement, for instance, one must also consider roof overhangs and gutters, and roof ventilation or other ice dam protection, grading and drainage to keep ground backsplash to a minimum, trimming or removing shrubbery that shades the siding, etc.
Robert, What is a good substitute for the osb web trusses on a 28' span? Should I use typar to hold the cellulose in the t111 walls or just tape seams? What is a good window brand for my build? Thanks
Stephen,
As I said, I don't know what you mean by osb trusses. I assume you're talking about Truss Joists (TJIs) which are not true trusses. As I also said, the conventional solution to long-span floor framing are parallel chord 2x4 trusses. Their open web structure allows easy installation of mechanicals.
I also don't have a clue what you mean by "Should I use typar to hold the cellulose in the t111 walls or just tape seams?" The T-111 and the drywall holds the cellulose in. As I said before, you don't need any WRB (housewrap) with T-111. And there's no seams to tape.
I use Pella Proline or their new fiberglass Impervia windows. Best quality for the price, and they now have a choice of four glazing options. I would use the Natural Sun on the south for solar gain and Advanced LowE on the other sides.
Stephen, about your floor framing: if you could run a beam down the middle so the joist span dropped to 14', then the joists could be 2x12s. That would save on framing cost and add close to 9" of headroom in the basement. You would need a few columns to support the beam, making three spans of 12' for your 36' long dimension (could be 6x12 beams). The disadvantage would be not accommodating ducts within the floor framing, if that were the intent. A 2x12 cavity is a more reasonable space to fill with cellulose than a 20" truss or I-joist.
Thanks Robert. I get it now. I just need to make sure the t111 is tight so as to keep the cellulose in.
Thanks Thomas. I think I'd lose headroom because the TJIs I was going to use were to run on top of the sill.
Stephen,
I don't recommend using T-111 directly over studs without a WRB like Tyvek or Typar. Most building codes, including the 2006 IRC, require a water-resistive barrier behind almost all types of siding.
Thanks for looking that up Martin.
The gable ends are perfectly south facing with open floor plans. How many windows would you have on the daylight basement level and main level for a good solar gain, heat loss ratio?
How many r value point would I lose by going to dense pack cellulose in the 2x6 frame?