SB: Keeping the Insulation On

[Speireag Alden <Joshua.M.Alden.91@alum.dartmouth.org>]

Hello, all.

     My roof is insulated with mineral wool between the rafters.  It's 
a living roof on a strawbale house.  I'm now planning to remove that 
mineral wool and replace it with foam insulation over the waterproof 
layer on top of the roof.  (My reasons are many, and I go into them 
below for those who are interested.)

     I'm trying to answer several questions.

1.  I'd rather not landfill the mineral wool.  Does anyone know of 
anyone within travelling distance of central New Hampshire / Vermont 
(the upper Connecticut river valley) who can use this mineral wool?

2.  If possible, I'd like to get foam insulation which is impregnated 
with borax, so that the carpenter ants will find it unattractive. 
I'm having a hard time locating a source with a net search, and my 
local suppliers are as usual useless when it comes to anything 
unusual.  Can someone direct me to a supplier who can give me brand 
names, specs, prices, and so on?

3.  My roof pitch is 6-in-12.  I suspect that it would be possible 
simply to lay the foam insulation down and put the earthen material 
back on top of it, and it would not slide.  The current earthen 
material just sits there and shows no inclination to move.  However, 
I'm inclined to be a little more paranoid than that.  I DO NOT want 
to nail or screw through the waterproof layer, which I have gone to 
great pains to keep completely free of holes.  My thought is that I 
could lay salvaged seat belts across the peak and staple them to the 
foam.  I could also use stainless steel strapping, I suppose.  Does 
anyone else have any other ideas or methods which have worked in the 
past?

---

     Now, here's why I'm doing it.  I offer this as a cautionary tale 
for anyone else who is planning something similar to my roof.

     I have a living roof.  The waterproof layer consists of two 
layers of Vycor's Ice & Water Shield, which is just a bituthene 
membrane (poly plastic with asphalt emulsion between the layers).  On 
top of that is a layer of organic material which is gradually 
becoming soil.  We planted it with strawberries, and there are also 
various volunteers.  When I designed the roof, I planned to insulate 
between the rafters with some kind of flexible insulation (mineral 
wool, fiberglass, cotton, etc.)  Eventually, I decided on mineral 
wool, and I installed it.  I put up plastic sheeting underneath as a 
vapor barrier.  I haven't yet finished the underside with sheetrock, 
though I've done a good deal of it.

     The mineral wool is rated as R-4, and there is nine inches of it, 
so nominally the roof is R-36.  However, the rafters provide thermal 
bridging, so the effective R-value is less.

     Here are the problems.

     The roof has a vapor barrier.  You can't really have a much 
better vapor barrier than two layers of overlapped bituthene 
membrane.  Vapor just doesn't go through the stuff.  If you then 
insulate under it, instead of on top of it, you create a situation 
where vapor can get into the insulation and then condense on the cold 
side, right against the plywood decking which supports the vapor 
barrier.  If you could put a perfect vapor barrier under the 
insulation then there would be no problem, but there is NO WAY that 
you can put in a vapor barrier as effective as the one which is on 
the cold side of the insulation.  So, there will always be vapor 
making its way into the roof during the cold season, and condensing.

     The solution to that, of course, is to leave a vent space above 
the insulation and let air flow from under the eaves up through that 
space and out of gable vents.  That's what I did.  However, there are 
several problems with that.  First off, the airflow across the 
insulation necessarily cuts the effectiveness of the insulation 
somewhat.  Second, in order to provide that venting, I had to leave 
gaps at the top of the stucco, inside and out, for outside air to 
flow into the space above the insulation.  We'll get back to those 
gaps later.

     Now, that airspace will pull air from wherever it can get it. 
The outside air, being cold, is far less inclined to flow into that 
airspace than the inside air, which is warm and trying to rise.  So, 
that airspace takes inside air more readily than outside air.  The 
fix for that, of course, is to seal the inside, but that turns out to 
be a pain in the rear involving lots of plastic and caulk, and 
difficult to do while living in the space below.  In the meantime, 
moisture-laden air goes from the interior into that space, where the 
moisture condenses on the cold plywood.  Judging by the dark patches 
at the gable vents where surface mold has started, this is happening. 
I can't tell if it's happening just there, where the plywood is 
coldest, or everywhere on the underside of the plywood.  Probably the 
latter.

     Those spaces for airflow also provide excellent habitat for mice, 
which colonize the roof structure each autumn as they look for warm 
homes.  As they colonize, I have no doubt that they pull out the 
insulation into the airspace in an effort to stop that pesky airflow 
which cools everything down so much, thereby putting a stop to the 
airflow which is supposed to dry out my roof structure.  I kill them 
off each autumn and they're gone until next autumn, but I'm getting 
tired of the carnage.  I'd rather they stayed in their own habitat 
(out in my tool shed, for instance, where I'm happy to let them be). 
I thought that I had the entire exterior sealed well, but they got in 
somehow.  One of the ways they might have gotten in is where the wall 
joins the roof; although I have expanded metal lath blocking access 
to the vents, it would not surprise me if they have found a way to 
worm in past it.

     Either someone on this list or the most recent issue of "The Last 
Straw" remarked that mice tend to colonize the roof structure of 
strawbale houses rather than the walls themselves.  In my experience, 
this is true.  There's no evidence of them in the walls, but we can 
hear them in the roof.

     Now, an earthen roof is best done as an Inverted Roof Membrane 
Assembly, where the insulation is *on top* of the waterproof layer. 
I did not appreciate this fully when I did mine, but I do now. 
Here's why:

     An earthen roof should last just about forever, because the sun 
does not degrade it.  All of the photosensitive materials are 
protected under a layer of earth and plants.  However, in freezing 
climates the sun is not the only hazard.  The freeze-thaw cycle will 
also gradually damage just about any material.  If you put the 
insulation on top of the waterproof layer, then the waterproof layer 
never freezes.  So no damage there.

     An earthen roof, properly constructed, will never need repair in 
the waterproof plane.  However, in this real world, sometimes roofs 
leak.  When that happens, it's extremely helpful to know where.  If 
you have a layer of plastic on the underside of the roof, then the 
leak will appear at the wall.  All that tells you is that the leak is 
somewhere between the peak of the roof and the wall.  On the other 
hand, if your waterproof layer is immediately over the decking, then 
the leaky spot will show right where it is on the decking.  Easy.

     Insulation laid on top of the decking is continuous, so there's 
no thermal bridging by the rafters, which you must either accept or 
handle through details (like a continuous layer of insulation on the 
underside of the rafters).

     If you have no insulation complication on the underside of the 
roof, then you have none of the vapor barrier and condensation 
issues.  The interior wood will always be at around the interior 
temperature, so no condensation will happen.

     If the insulation is outside of the building envelope, it's less 
of a fire hazard, especially if it's foam.  This was actually while I 
chose mineral wool, which simply doesn't ignite.  But foam buried 
under a layer of moist earth is also pretty nearly fireproof.

     If you simply deck with material that you don't mind looking at, 
then you save on finish material.  You can look right at the ceiling 
and the ceiling joists or rafters.  There's no necessity to cover the 
insulation cosmetically.

---

     So, in the, by doing this I'll get:

       Elimination of an intractable interior sealing problem
       Improved roof R-value
       Elimination of mouse habitat
       Visual inspection of the roof structure
       More physical protection for the waterproof layer
       Freeze-thaw protection for the waterproof layer
       Increased headroom in the loft, divided into intervals 16 
inches on center  :)

     I have often remarked to people that not putting the insulation 
on top was my worst decision on this project.  Now I'm going to fix 
it.  I sure wish that I'd figured this one out just little earlier 
than I did.

     So, if you have an earthen roof, insulate on top.  If you do it 
at the outset, it's also a lot easier than stuffing insulation upward 
from below and then trying to seal underneath.

-Speireag.
-- 
I feel most emphatically that we should not turn into shingles a tree 
which was old when the first Egyptian conqueror penetrated to the 
valley of the Euphrates.  -- Theodore Roosevelt 

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