Insulating walls (and a flat roof?) with perlite?

[Garald]

Simply because I'm having trouble falling asleep, I started to simultaneously

(a) play around with ChatGPT

(b) fantasize about possible improvements to the family home - in the duller, non-darkest part of a South American country. I should explain conditions there - temperatures don't go under 10C, so nobody has heating; that would be all right, except modern brick construction comes with zero insulation (unlike older adobe construction, which unfortunately tends to fall apart during earthquakes). So, during a couple of months in 'winter' (humid, sunless, no proper rain - more of a nasty autumn) inside temperatures drop to 15Cish and people feel mildly miserable (or bury themselves under piles of scratchy blankets). Any improvements to that situation would be a want, not a need, but still.

So, within a pile over overly self-assured verbiage, ChatGPT gave me an actual fact unknown to me - namely, that the said country produces lots of perlite, which can be used for insulation - whether from the inside or from the outside.

 

Can you confirm that? Note that mortar-perlite mixtures have much worse performance than, well, plain expanded perlite. What issues are involved in using the latter to insulate a place from the outside? (Would one need to hire someone highly skilled who might not exist?)

[SteamyTea]

I suspect there is a problem with water absorbtion, this would not only increase the thermal conductivity, but would weight more.

 

Expanded perlite can have a density of between 32 and 400 kg/m³. 

 

If something is not in common usage, there is generally a good reason for that.

 

Does ChatGPT have anything to say about the energy needed to process it into expanded form?

[Garald]

1 hour ago, SteamyTea said:

I suspect there is a problem with water absorbtion, this would not only increase the thermal conductivity, but would weight more.

 

Expanded perlite can have a density of between 32 and 400 kg/m³. 

 

If something is not in common usage, there is generally a good reason for that.

 

Does ChatGPT have anything to say about the energy needed to process it into expanded form?

 

Here are ChatGPT o3's answers to these queries. Usual warning: ChatGPT bullshits a lot (and makes up references); in this case, however, it seems to acknowledge real concerns with its own previous suggestions, once it's queried appropriately.  Note (after Googling): there do seem to be at least one or two legitimate insulation businesses in the relevant country producing and selling perlite insulation panels; perlite insulation also seems to be a thing elsewhere, though some folks use it within plaster, which reduces its performance greatly.

 

 

1. What is expanded-perlite insulation?

• Raw perlite is a volcanic glass that “pops” (expands 10-20 ×) when it is flashed to ≈ 850-900 °C.

• The expanded granules are light (≈ 50–120 kg m⁻³), non-combustible, and trap a lot of still air, giving a loose-fill R-value of about 2.7 per inch (≈ 0.39 m² K W⁻¹ per cm).

2. Why you don’t see it very often in walls

Issue	Consequence for a wall build-up
Moderate R/inch	At 2.7/in you need ~200 mm to reach the same U-value that 100 mm of polyisocyanurate gives. Thicker walls mean more framing, detailing and cost.
Loose granules	Inside a stud cavity they can sift through wiring holes or service gaps, and field reports show some settlement after thermal cycling despite manufacturers’ claims to the contrary. Hearth.com Forums HomePerlite Institute
Dust & handling	Pouring or blowing dry perlite is messy; dust masks are mandatory and the jobsite must be sealed.
No structural capacity	Boards made with perlite/gypsum exist, but the loose fill that gives the best λ-value provides zero racking or compressive strength, so an extra sheathing layer is usually required.
Code familiarity & price/R   3. Interior vs. exterior placement Location Typical problems with perlite Interior stud bay Needs an air-tight liner to stop grains leaking; must also add a vapour retarder on the warm side because perlite is hygroscopic. Cavity of double-wythe brick / concrete-block Traditional niche where it does work: the masonry keeps it dry, gravity keeps it in place, and fire rating is excellent. Perlite InstituteW.L. Tucker Exterior insulation layer (EIFS or render) Exposed to driving rain. Unless the granules are silicone-treated, they can wick water and lose most of their thermal resistance; wet perlite is also heavy and can burden the cladding sub-frame.	Builders already know how to use fiberglass, mineral wool and foam boards that deliver higher R per euro, so perlite rarely wins a cost-benefit comparison in mainstream housing.

5. Energy and climate footprint of expansion

Boundary	Reported energy use
Furnace gas only (flash-expansion)	1.5 – 2.0 MBtu t⁻¹ → 1.6 – 2.1 MJ kg⁻¹. Perlite Institute
Typical natural-gas plant (gas + blower power)	6.5 Nm³ gas m⁻³ finished product ⇒ ≈ 80 – 250 MJ m⁻³, depending on final bulk density. amit-online.de
Cradle-to-gate (ICE database, includes mining & transport)	≈ 10 MJ kg⁻¹ for expanded perlite. Green Building Encyclopaedia

For comparison, extruded polystyrene (XPS) comes in at ~70 MJ kg⁻¹, so perlite’s embodied energy is an order of magnitude lower, even if you take the higher cradle-to-gate figure.

6. Bottom line

Perlite is fire-safe, inert, low-embodied-energy and great for niche uses (masonry cores, chimneys, cryogenic vessels). In framed walls it is held back by:

• Thickness penalty (low R/inch)

• Moisture sensitivity unless specially treated and detailed

• Installation hassles (dust, containment, potential settlement)

That is why modern residential envelopes tend to favour denser mineral wool or foam boards outside the sheathing, and why perlite remains a specialty rather than a mainstream wall insulation choice.

Edited Thursday at 10:38 by Garald

[Garald]

Further query to ChatGPT:

 

"OK, but why were you recommending it for use in Lima? How would you deal with the issues mentioned above - particularly for outside insulation? Would you still recommend it in the highlands, where more insulation is needed and it rains much more? What would you use instead? Remember that, while it essentially never rains properly in Lima, it drizzles, and winters are clammy. Also, how much worse is the lambda of perlite panels, as opposed to loose perlite? Are panels more appropriate for outside insulation?"

 

Why I suggested perlite for Lima in the first place

Lima’s building-physics challenge is unusual: air is damp and chilly for four months, yet real rainfall is almost zero (≈ 6-50 mm year⁻¹). RedditInca Expert
Because there is no driving rain to wash through the façade, you can afford an insulation that is vapour open, non-hygroscopic once treated, fire-safe and low-embodied-energy—all true of hydrophobised expanded-perlite. In that mild-load climate the modest R-value penalty is acceptable, and the system remains fully mineral (no foam plastics that can off-gas or burn).

---

How to make perlite work as exterior insulation in Lima

Layer (outside → inside) Purpose & detailing tips ➊ Thin polymer-modified lime-cement basecoat on mesh Water-shedding skin; vapour-open. Keep cracks < 1 mm with alkali-resistant glass mesh. ➋ Hydrophobic perlite insulation board (λ ≈ 0.056 W m⁻¹ K⁻¹) Chosen grade must meet <0.15 kg m⁻² h⁻½ water-absorption per ISO 15148. ResearchGateepg.science.cmu.ac.th ➌ Bed of cement–polymer adhesive or mechanical anchors Full-surface trowel bond on rendered masonry; 5–6 screw anchors m⁻² on concrete. ➍ Existing rendered block or brick Provide a reasonably flat plane; patch holes that might let grains leak. ➎ Interior lime plaster (optional vapour retarder paint) Keeps the wall airtight; still lets occasional inward drying.

[Garald]

Key points

• Use only factory-siliconised boards or loose granules; raw perlite can absorb 30-70 % of its weight in liquid water.

• Provide a 5–10 mm ventilated drainage gap above lintels and at the base so any condensate or drizzle that bypasses the render can escape.

• Seal penetrations (flashing brackets, conduits) with compressible mineral wool backer rod and a vapour-open sealant.

Under Lima’s “garúa” (heavy mist) the render surface may stay at 100 % RH for days, but liquid water load is tiny, so a hydrophobic board remains near its declared λ.

---

Would I still specify perlite in the Andean highlands?

Probably not as the primary insulation layer.

• Thermal demand is 3–4 × higher (winter nights below 0 °C at 3 000 m).
  Loose perlite (λ ≈ 0.044 W m⁻¹ K⁻¹) would need ≥ 30 cm to hit the U-values now recommended for health and comfort. Perlite Institute

• Summer rains are intense and freeze–thaw cycles common. A hygroscopic insulation that sits outside the waterproof layer risks saturation and spalling during cold snaps. ChimuadventuresWikipedia

What works better at altitude

Material	λ (W m⁻¹ K⁻¹)	Pros in highlands	Cons
Rock/mineral-wool façade boards	0.033	Non-combustible, moisture-tolerant, λ ~40 % better than perlite, proven ETICS details	Heavier than EPS; cost > EPS
High-density EPS (graphite)	0.030	Very light, cheap, closed-cell so no capillary uptake	Combustible; UV-sensitive; must be rendered quickly
Wood-fibre boards with lime render	0.038	Carbon-negative, vapour-open, good hygrothermal buffer	Needs perfect detailing to avoid decay; limited Peruvian supply

[Garald]

With any of these you still need a mineral render or ventilated cladding plus flashing, but you gain higher R in less thickness and better freeze-thaw durability.

 

---

Panels vs loose perlite—thermal hit and suitability

So a board is ~25 % less insulating per centimetre, but it eliminates settlement, is easy to dowel-fix, and can be factory-treated for water repellence—essential on façades.

 

 

 

Form	Typical λ (W m⁻¹ K⁻¹)	Why different?	Good for…
Loose granules	0.040–0.045	Air pockets undisturbed	Filling cavities inside double-wythe masonry or pre-cast panels
Pressed boards	0.055–0.060	Portland-cement or starch binder bridges some pores	Exterior or interior retro-insulation where you need a continuous sheet

Edited Thursday at 11:13 by Garald

[Garald]

@SteamyTea Sorry ChatGPT is so wordy. Does its assessment now sound sensible? I'd be interested to know where it goes wrong-ish or completely wrong. (I've been toying with AI for other purposes, and it has been interesting to observe what it can and cannot do.)

 

What it also says on insulating with perlite pressed boards on the outside:

 

What if you insulate outside instead?

• You keep every millimetre of floor space.

• Thermal bridges at floor slabs and partitions are easier to lap over.

• The wall stays warmer, pushing any condensation plane out into the perlite where it can dry.

Pressed perlite boards can be fixed to the exterior with a polymer-modified lime render, adding only 5 mm more thickness than the indoor version. You just need shadow gaps or slim drips to protect the render nose from the persistent garúa.

[SteamyTea]

2 hours ago, Garald said:

Does its assessment now sound sensible

At first reading/quick glance, it seems sensible.

Does have a lot of drawbacks compared to mineral wool (internally) or ESP/EPX/PU boards (externally).

 

I am not sure how the embodied energy (EE) figures compare to other materials.

 

I had one of these to keep me warm when I was a teenager.

 

 

Though others like this.

 

 

 

Edited Thursday at 13:32 by SteamyTea

[Garald]

1 hour ago, SteamyTea said:

At first reading/quick glance, it seems sensible.

Does have a lot of drawbacks compared to mineral wool (internally) or ESP/EPX/PU boards (externally).

 

I am not sure how the embodied energy (EE) figures compare to other materials.

 

I had one of these to keep me warm when I was a teenager.

 

 

Though others like this.

 

 

 

 

Right, everybody wears alpaca sweaters, which are nice enough. The problem is really at night, as everybody seems to prefer really scratchy blankets. The logical step is to use duvet covers, but my mother tells me that her father tried to explain how that works n times and never managed to get himself understood. Oh well.

 

In the meantime - there seem to be importers for the usual materials, which at least means *some* people are insulating. But it's a shame that no insulation products seem to be produced in the country (other than perlite). I guess one could insulate using the cheapest kind of sheep wool, but that's a bug magnet. There's no real reason why plant fiber insulation can't be produced. As I said, in the highlands as opposed to Lima, insulation is really a need, not a want - but it's a need that goes unfulfilled. It's a public-health issue more than an efficiency issue - very few people have central heating. The potential for solar gain is huge (up there, the cold season is the sunny season) but at least that's starting to be explored.

I hope Chile finally gets around to producing cork insulation at some point (they have cork trees since the 40s or 50s, or so Neruda says, but then plenty of things in his memoirs are made up).

Edited Thursday at 14:27 by Garald

[SteamyTea]

3 hours ago, Garald said:

Neruda

The Nobel Poet?

 

I would have thought that Chile, with it's advanced mining industry would have a strong industrial base that manufactured most products.

 

Solar gain can be counteracted with reflective surfaces cheaply.

[Garald]

1 hour ago, SteamyTea said:

The Nobel Poet?

 

 

Right, he told an anecdote… let me find it. It’s from when he chartered a ship to take Spanish refugees to Chile when the fascist side was about to win.

 

1 hour ago, SteamyTea said:

I would have thought that Chile, with it's advanced mining industry would have a strong industrial base that manufactured most products.

 

Bah, resource curse. It has diversified since the first time I was there (1991) but it’s not like it produces everything it needs or nearly. Part of the problem is that cork takes decades to mature (apparently?) and part of it is that Chile has *NO* insulation requirements (neither do Peru, Bolivia, etc.) even though it can get quite chilly. Being physically uncomfortable is part of their national character.

 

1 hour ago, SteamyTea said:

 

Solar gain can be counteracted with reflective surfaces cheaply.

Oh, in winter, in the Peruvian highlands, you *want* solar gain - and there’s a lot to be had. People are experimenting with Trombe walls. As I said: very blue skies and lots of solar radiation in winter.

[SteamyTea]

48 minutes ago, Garald said:

As I said: very blue skies and lots of solar radiation in winter

PV and Air to Air Heat Pumps then. Heating and cooling taken care of.

 

Not been to Chile, only Venezuela and Columbia, I lived just of the coast of Venezuela when I was a teenager.

[Garald]

41 minutes ago, SteamyTea said:

PV and Air to Air Heat Pumps then. Heating and cooling taken care of.

 

Not been to Chile, only Venezuela and Columbia, I lived just of the coast of Venezuela when I was a teenager.

Sure, but what about insulation? Quite frankly I wouldn’t be surprised if good insulation + enough double-glazed skylights all but eliminated the need for heating, except on the odd cloudy day. Would need to do some calculations.

Edited Thursday at 20:21 by Garald

[SteamyTea]

13 minutes ago, Garald said:

but whereabout insulation

May be worth looking to see how much PIR sheets cost.  Applied externally, and then covered in a weatherproof membrane or coating should help a lot.

You may find that the costs are not that high, Europe is a high mark up country after all.

[Garald]

7 minutes ago, SteamyTea said:

May be worth looking to see how much PIR sheets cost.  Applied externally, and then covered in a weatherproof membrane or coating should help a lot.

You may find that the costs are not that high, Europe is a high mark up country after all.

Oh, you’d be surprised. All insulation materials are imported, often from Europe, and so they cost more than they do there: transportation, tariffs, etc. Yes, you can walk into a shop and buy a rockwool panel, but it will be easily more than twice the price in France. 
 

What is cheaper is labor (meaning “joiners and masons who may be clever but will figure it out together with you”) and local materials. But, as you see, the latter doesn’t seem that useful in this context - perlite has limited applications (will talk to producers). I should talk to some hipsters to see whether I can convince them to consider producing hemp insulation eventually. There’s also a city where you can build in beautiful volcanic stone with a decent lambda, but the said city will be covered in lava sooner or later.

[SteamyTea]

6 minutes ago, Garald said:

What is cheaper is labor (meaning “joiners and masons who may be clever but will figure it out together with you”) and local materials.

How about cross laminated timber (CLS).  Typical k-value of 0.13 W.m^-1.K^-1.

 

[Garald]

1 hour ago, SteamyTea said:

How about cross laminated timber (CLS).  Typical k-value of 0.13 W.m^-1.K^-1.

 

 

I take you mean for new build, not for retrofit. Bear with me - you are about to learn things that you'll never find useful.

 

CLS isn't produced in Peru (though it is in Chile). In general,

1. wood is not a common primary building material in Peru outside the literal rainforest,

2. Peru is not a softwood country, though it is a hardwood country.

More on 2: Peru est omnis divisum in partes tres: coastal desert, highlands, rainforest. No need to comment on the desert (... where Lima is located). The highlands have long been seriously deforested. The rainforest is, well, full of wood, often fine wood, very irresponsibly managed on the whole. There isn't really such a thing as large commercial softwood plantations AFAIK.

Now, what you can do (in fact it's not uncommon in the older parts of the district I grew up in) is use wood to build a lighter addition on top of an existing flat roof. That can be quite a nice project though it won't be cheap (... except you may get away with not hiring a civil engineer, or your engineer may simply have very little work to do). I guess one would use one of the less expensive hardwoods, e.g., tornillo de la selva (Cedrelinga cateniformis, lambda of about 0.15-0.18, ChatGPT tells me, though it may have made that one up) - I can't imagine having so much money as to build entire rooms in cedarwood (lambda = 0.12-0.14). (ChatGPT chips in to say it's not that pricey, and that it's compensated for being easier to work with; that was not the impression I got from 'my' carpenter - can't ask him now, as he unfortunately died of COVID.)

 

Oh, I think eucalyptus (a legacy from a now deprecated early-20th century attempt at reforestation) is sometimes used for building in the highlands, but it's a tricky wood to use, isn't it?

 

PS. The *really* old skool way to build an addition of an existing building is quincha (daub-and-wattle). It actually has very reasonable anti-seismic properties if done properly, but it's not very durable (though I think there are now modern versions that are carefully engineered... and of course also not cheap). It's a material of the past now - it's preserved when possible.

 

PPS. ChatGPT has just pointed me towards a Peruvian timber company - and here's what I found on it as one of the first hits: https://www.earthsight.org.uk/news/timberleaks/robinosn-lumber-timber-trade-peru-maderera-bozovich

 

 

Edited Thursday at 22:07 by Garald

[SteamyTea]

Sounds like Sierra Leone when it comes to deforestation.

[Garald]

4 hours ago, SteamyTea said:

Sounds like Sierra Leone when it comes to deforestation.

 

Sigh. Peru and Chile are nowadays classified as upper-middle and high-income countries, respectively, but their level of institutionality, regulation, human development, etc., lag behind what you would expect from either label (though, again, Peru clearly lags Chile, as you might expect). On paper, Peru has become much wealthier since the horror years of the 80s and 90s (in 1990, annual inflation reached 7650%), but in reality... in some respects, there's barely a development. Or at least there has been a very slow recovery. This is in part for ideological reasons: the "Chilean model" (small state) has become dominant, even as Chileans become increasingly critical of it. Peru is certainly much more *stable*, economically, than in the 80s, and the middle class is a larger minority of the population than it used to be. (That includes the "emergent middle classes", whose living standards are in many ways clearly under those of the working class in developed countries, especially in terms of access to education and health care; they care very much about them, however, and many of them do care about things like energy efficiency, building standards, etc. - I was just talking to my brother about it.)

 

To be fair to Chile, science there has developed to a very respectable level in the last 35 years. However, it's not an industrialized country in the sense that Mexico and Brazil are (a fact that plenty of people abroad forget simply because they are only too aware of the huge parts of the population in both countries that were left behind); it's still mostly an exporter of raw materials. That's true also of Peru, only more so.

 

OK, I hope that was enough of a nutshell.

Edited yesterday at 08:56 by Garald