Hello,

I've been renovating an 1835 post and beam farmhouse in the southern tier region of New York state.

I'm trying to figure our how to get more insulation in rough cut 2x4 framed exterior walls.   The studs are irregular, many not plumb etc.   I have most of the walls fiberglassed with R13 batts, but know this isn't optimal.

I was thinking to use 1 inch or so xps foam board over the batts before sheetrocking, but I've read that this can cause water issues...  Any feedback about this?  Temps drop to zero once and a while.

The other option is the do some kind of furing out of the walls and try to add R19 or higher batts.  Spray foam is too pricey for me.  I don't know if anyone does wet cellulose there...  These option would be better because of the irregularity of the studs...

Any feedback appreciated.



The other insulation issue I have there is a 20" high uninsulated crawl space that radiates cold into the kitchen and dining room.  Using fiberglass in that kind of application sounds like a nightmare...  I was thinking of cotton batts, but cutting them, especially if the joists are oddly spaced seems really tough as well.    Any feedback on experiences of insulating tight crawl spaces? 


Thanks.

What is the siding & sheathing construction like?

If those are full-dimension 2x4 studs an R13 batt doesn't have sufficient loft to fill the depth of the stud bays. This allows HUGE amounts of convection inside the cavity, cutting into performance dramatically.  You really need to fill those up so that there's a little bit of compression when the interior wallboard (or XPS) goes on.  Splitting R11 unfaced batts would get you there.  Compressing an R19 batt is another approach- an R19 batt has the same amount of material as R19 batts per square foot (!)- it's just much lower density. Compressing it to 3.5" yields ~ R13, but at a true-4 it'll be about R14-15.

At ~1.2 perms, a single layer of 1" XPS  on the interior is almost a class-II vapor retarder. While this allows ~3x the amount of moisture transfer of a kraft facer, if it's a plank sheathed shingle or clapboard exterior you'll probably be fine, and if it's vinyl siding you'll definitely be fine in that climate zone.  If brick veneer with a ventilated cavity, it depends.  If clapboard directly on studs (no sheathing), post some pictures.  If the R13 batts have kraft facers (~0.4 perms) that are well sealed to the studs you're already in pretty good shape from a wintertime moisture-diffusion point of view, and adding the XPS will not appreciably change that.

The most important thing is to make it AIR TIGHT, taping/caulking/foaming all seams & penetrations in your XPS.  If you're not sure about the exterior stackup and want a bit of insurance, a layer of Certainteed MemBrain (tm)  between the interior XPS and the sheet rock will reduce the moisture diffusion by more than half during winter months, but will open up during the more humid spring weather to promote faster drying.  MemBrain is treated as a class-II vapor retarder (sub-1-perm), despite having variable permeability, but by being more vapor-open when the RH is high, it releases moisture from the wall cavity more quickly than it can get in.  If that is your moisture control approach, be sure not to actively humidify the conditioned space to more than ~35% during the dry winter weather or you'll be defeating it.

The better-approach to insulating the crawl space is to insulate the foundation walls, foundation sill, and band joist with ~2" of closed cell spray foam.  Insulating between the floor joists puts the joists at risk of mold. Stopping the air leaks at the foundation and band-joist/beam etc is more critical. Laying down a 10mil poly vapor retarder on the ground and sealing it to the foundation before the spray foam keeps the ground moisture well controlled, as well.  If the pros won't touch it, this can be a (summertime only) DIY project using a 600 board-foot kit (Tiger Foam, Fomo-Foam, etc.)  If it's just a small area, there are smaller kits available as well. With an insulated air-sealed foundation the crawlspace temp should pretty much stay in the 60s year round in your climate.

Thanks for your reply Dana1. (By the way, you gave me a huge help understanding my boiler project a year or so back.... so far my 7 unit, isonene foam insulated, building is running beautifully on a Triangle Tube 175 with a 60 gallon sidearm.)


The sheathing is 1/2 osb, house wrap, cedar siding.

So you would recommend compressing r19 batts in the 2x4 cavity? I thought there is supposed to be a small air space between building paper and fiberglass? The fiberglass will never be tight at the edges because of so many out of plumb studs, wierd angles, triangles etc in the framing. This is why i'm wanting to go for the foam board, which I can seal carefully...

Crawl space... what about a framing around the crawlspace walll (we are talking 24") and using r38 batts? (i have a bunch extra)

Ideally there would be an air space (drying plane/rainscreen) between the exterior cladding and the cedar siding.

No need for an air space between the OSB and the fiberglass. Seal up any penetrations and air loss pathways in the wall prior to insulating.

You could frame out the crawls space but it needs to be PT and fiberglass is not a great insulation for any potential moisture contact. You could do the stem walls with a rigid foam or spray foam.

Posted By Bisseti on 20 Nov 2011 07:41 PM
Thanks for your reply Dana1. (By the way, you gave me a huge help understanding my boiler project a year or so back.... so far my 7 unit, isonene foam insulated, building is running beautifully on a Triangle Tube 175 with a 60 gallon sidearm.)


The sheathing is 1/2 osb, house wrap, cedar siding.

So you would recommend compressing r19 batts in the 2x4 cavity? I thought there is supposed to be a small air space between building paper and fiberglass? The fiberglass will never be tight at the edges because of so many out of plumb studs, wierd angles, triangles etc in the framing. This is why i'm wanting to go for the foam board, which I can seal carefully...

Crawl space... what about a framing around the crawlspace walll (we are talking 24") and using r38 batts? (i have a bunch extra)

R19 batts in a standard 3.5" 2x4 cavity perform identically with R13 batts, which is OK if installed perfectly, but not "great".   R15 batts trimmed perfectly would do "great" (yeah, right, PERFECT, like THAT has ever happened! :-) ) as long as they filled the cavity depth completely, and they're a bit thicker (but not a lot thicker) than R13 batts right out of the bag.

Wet-sprayed cellulose (scrubbed flush to the interior stud edges) does a better job of filling in around all anomolies, and generally performs better than low density batts by eliminating gaps, and having better air retardency than even R15 batts. To get there with a fiberglass product, dense-packing Optima or Spider to 1.8lbs+ density would have similar air-retardency and a slightly higher R, and it's harder to dense-pack in a bunch of cut up odd-sized bays, whereas spraying cellulose while it's open does a pretty good job for cheap, and is way more air-retardent than low density fiberglass. It has similar thermal performance to half-pound Icynene, and is usually much lower in cost. Wet sprayed goods won't settle the way dry blown-low-density celluose would, but isn't as air-retardent as dense-packed cellulose (or Icynene). It's something of the "value option"-  only slightly more expensive than batts (maybe even cheaper in an awkward oddball framing situtation), and better performing on average.

To insulate the crawlspace wall  & band joist you have two issues- the foundation needs to be able to dry toward the interior (or you risk rot at the foundation sill), and it has to keep the cold-edge of the mini-studwall above the dew point of the conditioned space air (which you're coupling the crawlspace to.)  To do this you need somewhat-permeable foam between the foundation (or band joist/sill) and the fiber insulation, and keep the ratio of the foam-R/fiber-R in a range that keeps condensation at the foam/fiber interface well controlled in winter.  Too much fiber R==colder stud edge & foam== more condensation.  It's easier to do with spray foam than rigid in a tight crawlspace, but you can't use just R38 batts or you'll have a mold problem.  With 1" of XPS with 1-part foam-sealed edges & seams you can get there and use R11-R13 unfaced on the interior, but it could be a lot of cut'n'cobble with a lot of potential for air-leaks.   2" of closed cell foam (or 3" of the mid-density higher-perm 2lb foam, MD-R-200) gives you R12, and you can skip or put off the interior fiber until later if you like. 

If you like the cut'n'cobble concept and can rip 2-footers easily to fit, rigid 4" EPS roofing foam would give you ~R16 at about 10cent/R/foot if buying virgin stock, but there are commercial reclaimers that can cut that by 2/3.

I would insulate the crawlspace ceiling by adding an air barrier, boxing it in and filling it with cellulose. Make sure the crawlspace is then well vented to the exterior. Moisture or mold were never any problem when I had this design. The ground tends to reach the average air temp and it takes sustained moisture for mold to grow.

The summertime dew points in the NY southern tier below the Finger Lakes (most of which is US zone 6) are well above the subsoil temps- you'd have to open it up almost to a pier-foundation to get the ground temps high enough in summer. Even if well ventilated if the building is air-conditioned the exterior edges of the joists would be at temps sufficient to grow mold. Unless you opened the crawlspace ventilation massively and put up an inch of EPS below the joists there would be at least some summertime mold risk.

In a 2' tall crawlspace the amount of material required insulate the crawlspace wall & band joist/beam is usually far less than what it takes to insulate the floor & beams/joists.

I've seen crawlspaces in similar climates, with reasonable venting and a vapor barrier on the floor work well (zero mold). But I wouldn't do it as you get to hotter and more humid climates. Or if there are ducts under the floor.

amount of material required insulate the crawlspace wall & band joist/beam

I suggest floor insulation too - which means more and more expensive insulation. Plus a fan to force the crawlspace to exchange air with the house. Otherwise you are trading potential summer moisture problems for winter/spring/fall ones.

R10 on the crawlspace floor might make sense in a 25 year NPV (maybe even 15 years if propane/oil/electricity is the heating source), but only if you were planning to put in a slab, and only if you're going to R20 on the crawlspace walls, and R25+ on the first floor walls. Insulating & sealing the foundation walls earth-couples the house to the subsoil and fixes the floor comfort issues and mitigates the mold risk, but in an antique house like this there are usually better places to spend the money than crawlspace floors.

It's pretty cool subsoil (48-50-ish) but not permafrost, and well above the dew point of wintertime conditioned space air (~40F, for 35%RH 70F air). The mold risk is still going to be in summer when outdoor dew points are high, but that's manageable at relatively low cost with a small dehumidifier, (or active ventilation with air-conditioned air from the first floor), provided it has a ground vapor barrier. The summertime surface temp at the crawlspace floor would likely stay above 55F, maybe even above 60F if it's in the 70s on the first floor, with R1-2 of subfloor + finish floor, and the crawlspace air temp would be in the high-60s to low 70s. The average mid-summer dew points are in the 60-65F range, so it would be good to air condition/dehumidify the first floor to 55% @ 75F or lower (for a sub-57F dew point.)

The summertime surface temp at the crawlspace floor would likely stay above 55F, maybe even above 60F

Agreed, with the right crawlspace floor insulation and airflow to the heated space. Crawlspace wall/floor insulation, complete air sealing (interior and exterior) and a dehumidifier should also be considered.

thanks for this info! So 4" of eps foamboard on the walls might get me in the right direction? That seems manageable.

Another couple points. The house has a stacked stone foundation, post and beam framing, one basement window, and bilco type door, not sealed well...

I need to understand more about dew point stuff. When the outside temp above gets warmer than the basement (spring especially), I have a bad problem with condensation on fiberglass batts I put between floor joists in the part of the basement that is full height. (the crawlspace in under the circa 1850 "addition"). I'm thinking the solution is to open up another of the old window wells to get cross ventilation and maybe add a humidifier.

But.. do you all feel I am mistaken to use batts between the floor joists?

If you are insulating the walls, you are going in the direction of converting it into a conditioned crawl. As such, ventilation would not be part of the equation in most scenarios.

I think you need to get the crawlspace floor sealed tight against moisture (i.e. vapor barrier sealed to the walls) and get the ribbon boards and air leaks sealed up. As long as you care controlling the vapor drive and accumulated moisture in the crawlspace should be minimized.

Just make sure you either vent to somewhere or use a dehumidifier. A sealed cavity with neither is asking for problems.

+1

If you are treating it as conditioned, it needs supply side air.

Posted By Bisseti on 03 Dec 2011 01:02 AM
thanks for this info! So 4" of eps foamboard on the walls might get me in the right direction? That seems manageable.

Another couple points. The house has a stacked stone foundation, post and beam framing, one basement window, and bilco type door, not sealed well...

I need to understand more about dew point stuff. When the outside temp above gets warmer than the basement (spring especially), I have a bad problem with condensation on fiberglass batts I put between floor joists in the part of the basement that is full height. (the crawlspace in under the circa 1850 "addition"). I'm thinking the solution is to open up another of the old window wells to get cross ventilation and maybe add a humidifier.

But.. do you all feel I am mistaken to use batts between the floor joists?

In summer the outdoor dew points in that area the average in the 60s, sometimes spiking to the 70s, and there may be ground moisture being added to that, raising the dew point of the air in the crawl even further.  Any air in the crawlspace that contacts materials at the dew point or lower will have a moisture accumulation/condensation event.  Since the crawl space is earth-coupled thermally, the temps in there could easily be below 65F when the outdoor dew points are north of 65F, especially if the house is air-conditioned. 

With batts between the joists and no crawlspace/basement wall insulation the winter coolth lowers the  crawlspace & basement temps going into the spring, which doesn't help.  If you ventilate, it only exposes the space to more outdoor air humidity, unless you do it SO massively that the temp in the basement tracks the outdoor temp.  By insulating the  walls the average winter temp is raised, raising the temp of the floor below a few degrees, making the number of summer hours that it's below the outdoor dew point fewer, and  by sealing it it makes those condensing hours less relevant, because the source of humidity (the outdoor air) is restricted.

A vapor retarder on the floor is mandatory to control ground moisture drives, then insulating & air sealing the foundation walls will have the desired effect(s), both winter & summer.  Leaving the batts between the joists would lower the heat loss, but also lowers the temp of the crawlspace.  Insulating the crawlspace & basement  floors and removing the batts would be even better, since it would keep all of the exposed & susceptible wood in the crawlspace/basement at the conditioned space temp.  But even insulating just the basement walls and not the floor would limit the summer condensation, and by air sealing it the summertime RH down there could be controlled with a room-dehumidifier.  At 60% RH & lower the moldy smell goes away, and condensation simply can't happen.

When looking at room dehumidifiers, note that the 70pint versions are about 50% more efficient than the 30 pint units..  Some of the digitally controlled versions will tend to short-cycle like crazy and overshoot the set point due to the sensors being inside the housing (sensing the moisture evaporating off the coils at the end of a cycle.)  The old-school mechanical control systems don't do that, but can drift a bit more over a season needing some fine-tuning to keep it on the fine edge.  Keeping it 50% or less adds significantly to the seasonal power use.

If the house is air- conditioned, ventilating the basement & crawl with conditioned space air will normally keep the RH well controlled down there.