Do it Yourself: [DIY] Re: Loft insulation gaps

15 November 2013

[DIY] Re: Loft insulation gaps

Posted by: dan_marsh_99@yahoo.co.uk

> I've been trying to improve my loft insulation a little
> further. I already have about 8" over most areas but
> there has always been a long narrow strip between a
> joist and an adjoining wall that has never been insulated.
> I have always assumed that it was like that to enable the
> house/loft to 'breath'

Hi Dan;

The concept of "breathing" in construction is different
from what lay people use the term for. Air movement
inside of walls/roofs, during cold weather, is a VERY
BAD thing, and should not be promoted. "Ventilation"
of the living space is also a very different subject, and
again should never be confused with air movement
through the walls/roof. During cold weather any
interior air that gets inside the walls/roof is at risk of
being cooled to the dew point, which means that it
would deposit moisture inside the walls/roof, which
will lead to rot, mold, mildew, feeding colonies of
ants, roaches, termites, mice, etc. All "breathing"
of walls/roofs, in terms of air leakage can generally
be looked at as highly undesirable. So, do your
best to stop it from happening.

That being said, there will still always be some
air movement into and out of the walls. We simply
can not stop that, unless our walls are made entirely
of closed cell foam. But, when it is minimized, it
can be handled (by smart construction detailing)
and if condensation is minimal, the little remaining
air leakage can actually have some advantages.
But, do not think of air leaks in wall as "breathing"
which sounds like it is good. It is not. It is "leakage"
and is every bit as bad as that implies. Living space
need "ventilation" (not beathing) Walls and roofs
can (here is the proper usage of that word) "breathe"
moisture via "diffusion" of the moisture through the
actual material, like a sponge effect (not through
air movement)). That is why we no longer use
vapor barriers on the interior side of the building
envelope. We want it to be able to "breathe"
(diffuse) unwanted moisture, to the interior.
But, we do not want air leakage.

If your loft roof is not vented to outdoors,
then your added insulation will be making the inner
surface of the roof boards much colder in Winter
than they were when the roof was uninsulated. It
means they are now spending a significant amount
of time below the dew point, and it is likely that
condensation is forming on them, and being
absorbed, which can cause rot. So, a building
that has stood even for hundreds of years (I see
that you are in the UK ;O) without a problem,
could now develop a serious one, due to not
insulating in a way that protects the structure
from condensation.

> I really feel tempted to insulate this section.

As you should. However, you should protect
the roof sheathing with a layer of impermeable
foam sheathing (ISO or XPS), against it, before
placing your fiber insulation (is it Rockwool?)
It also needs an air barrier and vapor retarder
as the interior finish, to limit air movement in
and out of, and through the fiber insulation.

> I think there's another similar section on the
> other side; should they be left uninsulated?

A good general rule for optimizing cost-effectiveness
in insulation, is to first resolve the areas that have the
least insulation.

> (The eaves are unobstructed).

Good. Do you know if the roof is vented, above
the insulation? If you install rigid foam, as I advised
above, you can have a safe unvented roof (it will not
need soffit and/ridge venting) The foam will keep air
away from the underside of the roof, and will present
a warm surface that will adequately reduce conden-
-sation to a point where it should not be an issue.

Note: that I recognize that there is a great deal of
variation in UK climates, and that many of its varied
climates could be considered "marine", so moisture
is a significant issue. But, I do not know any of the
many specifics of your particular situation, so I am
making many assumption, that may or may not be
incorrect. The Rvalue of the impermeable foam,
mentioned above, should probably be at least
R10 (US) (1½ of ISO, or 2"XPS) which is
double what is recommended for a climate
like Oklahoma, but only half what is
recommended for Massachusetts.

-Laren Corie-
Natural Solar Building Design and
Solar Heating/Natural Cooling/Energy
Efficiency Consultation Since 1975
www.ThermalAttic.com (many new
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