Insulationshit

Intro

With the power of a $30 000 40cm external wall insulation layer, you can save virtually nothing on heating and make your house uglier!

Other shit guides:
rentry.co/solarshit - Solar systems
rentry.co/heatshit - Heat pumps, low temperature heating

Terminology

ACH - air change per hour
Conductivity (λ) - heat conductivity of a specific material - W/m⋅K
R-value - insulance of a specific material (calculated as thickness/conductivity) - K⋅m2/W. Multiple materials may be combined into one R-value.
U-value - heat conductivity of given surfaces - W/m2⋅K. Reciprocal to R-value. Multiple materials may be combined into one U-value
EWI - external wall insulation
EPS, XPS - polystyrene types, explained in the EWI section

Useful links

http://heatpunk.co.uk - Heat loss simulator/heating system designer tool

Heat loss

Heat loss refers to an estimate of heat lost in an hour (typically expressed in Watts) by a given room or building in specific conditions, in a specific configuration.
The specific conditions refer to design outdoor temperature and design indoor temperature.
The design outdoor temperature is usually the lowest temperature reached at least five times a year in a typical year (but different regions have different approaches to this). This means that heat loss calculations do not tend to account for extreme cold spells.
The design indoor temperature is whatever you want to heat your house to, and maintain, even in the coldest outdoor weather.

The specific configuration refers to the static variables (e.g. insulation thickness) and potentially dynamic variables (air changes).

Conductive heat loss

The difference between indoor and outdoor temperature (deltaT), multiplied by the calculated average heat conductivity of all external surfaces, and the total area of those surfaces, is what gives you your conductive heat loss. From this, you can establish simple concepts:

  • decreasing/increasing the temperature difference between indoor and outdoor environments will proportionally decrease/increase heat loss
  • decreasing/increasing the the heat exchange surface area will proportionally decrease/increase heat loss
  • decreasing/increasing the average heat conductivity of surfaces involved in heat transfer will proportionally increase/decrease heat loss

Following is a rough estimate of heat transfer coefficient of various wall and ceiling/roof types:

Material and Typical Property Age U-value (W/m²·K)
Concrete (ceiling, old) 2.5
Concrete (ceiling, mid-old) 2.5
Timber (ceiling, old) 1.6
Timber (ceiling, mid-old) 1.6
Insulated Concrete (ceiling, modern) 0.15
Insulated Timber (ceiling, modern) 0.1
Solid Brick (wall, old) 1.95
Cavity Brick (wall, mid-old) 1.47
Wattle-and-Daub (wall, old) 1.37
Aerated Brick uninsulated (wall, mid-old) 0.81
Cavity Insulated Brick (wall, modern) 0.30
External Insulated Anything (20cm EPS) (wall, modern) 0.1 - 0.3

Floor heat loss is specific - the floor is not in contact with outdoor air, but with soil, which will invariably be hotter than the cold air outside. You will still incur some heat losses, but they will generally always be lower than calculations. Even if you assume that, say, at -5C outdoor air, the soil might be 3-4C, it's probably still overshooting it - the soil will accumulate waste heat from your building over the winter. In tl;dr, floor heat losses are generally lower than whatever you may calculate.

Protip: AI/LLMs are very adept at doing estimate calculations. If you provide the size of the building and makeup of all walls, windows, ceilings and floors, they can reliably calculate conductive heat loss for the property (only tested on Grok 3 Think mode).

Air change heat loss

Every typical property will have some amount of involuntary air changes - air escaping through gaps in doors, windows, walls, ceilings.
For ballpark figures,

  • very old properties with old doors and windows have around 0.8-1.5ACH, possibly more if poorly maintained
  • older (20-50 years) properties or very old properties with renovations around 0.5-1 ACH
  • modern buildings up to code around 0.1-0.4 ACH

Fortunately, air doesn't have a high specific heat capacity, at approximately 0.335Wh/m3 x K - so, for example, if you have a smaller house around 100sqm floorspace, 2.5m internal height, if you heat the house to 20C and outdoor temp is -5C, for each air change, you're losing:

P = (100sqm * 2.5m) * (20C - (-5C)) * 0.335Wh/m3C = 250m3 * 25C * 0.335Wh/m3C = 2093Wh

This occurs during one hour at the rate of 1 ACH, so your air change heat loss is 2093W. If your property has 0.5 ACH, your air change heat loss is 1046W.

But wait, how the fuck do I figure out how many ACH I actually have?

You don't. It's possible to do measurements of how leaky a house is (professionally - they mount a fan on your front door and measure how much extra pressure it can create inside the house), but this is mostly useless. You can probably tell if you have old-ass leaky shit windows, and can probably tell that when strong wind blows outside, you feel a draft inside. Beyond that, you just try go guess - if you know your yearly heating bill, you can use it to calculate total heat loss over a season, and work your way back to a rough estimate of total heat loss. You can then potentially subtract the calculated conductive heat loss to have some idea about ACH, but all this is probably not worth the effort beyond estimating the total heat loss.
There can be a significant "active" portion of the ACH - say, a commercial property that has people coming in and going out often, opening the door and letting cold air in. This is again near impossible to measure within scope of /diy/.

AIR CHANGES AREN'T BAD THOUGH!!!
Air changes bring fresh air into the property and significantly help with humidity management. If you go overboard with reducing passive ACH, you can start having humidity issues and mold growing on your walls. Living in stale air is unhealthy, and will lead you to blaming your brain fog on porn addiction and other fictional disorders.
From a health perspective, at least 0.5 ACH is recommended for living areas. Some plastic windows are manufactured with built-in holes that let air blow through them - this is a feature specifically to ensure some level of air changes through the otherwise airtight windows.

But wait, aren't passive houses all sealed up and shit? How does that work?

Yes, and then they install heat recovery ventilation, so people don't die inside. So with the HRV, you spend a couple thousand extra to reduce your air change heat loss, by routing air changes through the central heat recovery unit. This is good for advanced thermal comfort and humidity management, not so good as an investment, the ROI on such systems (along with all the work that goes into sealing the house) is not very good. See HRV section of http://rentry.co/heatshit

Ground heat loss autism

Much unlike the air that surrounds the house, the ground under the floor is a static zone (provided you don't have a vented underfloor/crawlspace). Not only that, but the heat conductivity of soil isn't much better than a regular old uninsulated wall (unsurprisingly, considering the similar makeup - U in the range of 1.0-3.0 depending on moisture level). So you can almost imagine that you have an infinitely thick brick wall under your house. What's gonna happen if you start dumping heat into it? It's gonna retain a lot of heat and only have relatively small conductive losses. To get to precise heat loss, you need to use simulators, but you can get ballpark figures with various methods.

As a rule of thumb, for an uninsulated slab, real heat loss on 15C deltaT is probably in the range of 3-8W/sqm, so for our example house from earlier, let's say it's at 800W. Now if you insulate it, you can save 500-600W on the heat loss. Not exactly a game changer - but if you're already redoing your floors, you might as well. If you weren't planning on redoing your floors, or you don't have the spare interior height for shaving off 10cm for insulation - just like, don't do it.

Further autism:

  • ASHRAE Fundamentals perimeter-based equation
  • ISO 13370 heat loss calculations
  • Los Alamos perimeter-based heat loss

Conclusion on heat loss

If you're autistic, model your building on heatpunk.co.uk with the specific parameters that apply - this is useful because you can also do direct comparisons and see exactly what happens in each room if you, say, apply partial insulation, replace windows for modern triple-insulated ones, et cetera. If you just want a ballpark figure, ask an AI.

For an example on conductive heat loss, a 11x11m building with 3m high walls (about 100sqm floor space, 10cm concrete ceiling, 30cm concrete slab, 30cm older semi-aerated brick walls), applying insulation: 20cm EPS walls, 10cm XPS floor, 20cm EPS ceiling, single-pane old shit to modern 3-pane windows/insulated doors:

Component Uninsulated (W) Insulated (W) Approximate /diy/ cost
Walls 5000 500 (4500W saved) $3,000
Ceiling(attic) 6000 500 (5500W saved) $600
Floor, bad estimate 3000 600 (2400W saved)
Floor, realistic estimate 800 200 (600W saved) $800
Doors, windows ~15sqm 2200 400 (1800W saved) $4000
Total 14000 2100

Key takeaways:

  • Floor insulation has potentially the worst ROI, followed by EWI
  • "B-but the doors and windows cost mo-" this table has conductive heat losses. Doors and windows can save you another ~2000W on air change heat losses, plus your old shitty ass windows probably look like ancient crap. Your shiny new triple pane argon windows will look like modern crap. The modern windows also have much better sound insulation. Also consider that even if you /diy/ everything, your time has to be worth something. Doing EWI will take an order of magnitude longer than replacing your windows. Floor insulation is highly variable in difficulty, can potentially be as simple as ceiling/attic insulation
  • When done by a contractor, EWI will cost far more than any of the rest on the list - installation cost on the doors and windows will add maybe 50%, installation cost on EWI will add at least 200%, probably more
  • attic insulation is by far the easiest and cheapest and has by far the best ROI

Note 1: the floor heat loss value is often a VERY retarded estimate that's massively overshooting reality, included here only for posterity - even heatpunk overshoots ground heat loss in its own calculations
Note 2: if you have a subfloor/elevated floor, heat losses are logically going to be higher through the floor, especially if somehow uninsulated.

Wall insulation

Potentially the hardest, and almost always the most expensive part of insulation, not necessarily bringing in sufficient savings to justify the cost - especially when done by a contractor.

Types of EWI

Most common types of EWI:
1: EPS - bog standard polystyrene boards. The grey colored ones have added graphite, which makes them maybe 10-20% better than normal polystyrene, therefore you should be using those, since they're usually almost the exact same price.
2: XPS - extruded hard polystyrene. For the insulation, it's the same as EPS, only that it's harder - more impact resistant. This is important if you are insulating walls that may be hit or otherwise impacted intentionally or unintentionally - near walkways, commercial areas, walls facing the street. XPS is at least 4 times as strong as EPS when it comes to withstanding impacts. Note that when buying XPS for walls, you have to explicitly look for a type that has ridges for better adherence - smooth XPS, typically used for floor insulation, will adhere poorly.
3: Rockwool - better fire resistance (obviously EPS and XPS both melt away if a fire breaks out, leaving behind a hollow insulation shell), easier to shape to non-standard wall shapes, better breathability, much more expensive - typically double the price for the whole system.

Breathability meme - you will see both Rockwool and some types of EPS (eg. Baumit OpenTherm) advertised for their breathability - that they allow moisture to pass through the insulation layer; EPS is not particularly good in this regard.
The background knowledge here is that interior humidity is partially managed by the plaster's ability to absorb moisture, acting as sort of a buffer, and some of that moisture can pass through the wall, and even leave the property through the wall. The EWI can hinder this method of removing moisture from the house, therefore you should buy the twice as expensive breathable insulation.
Now, callback to the ACH segment - you should try to have around 0.5 ACH in your property regardless of moisture. If you have 0.5 ACH in your property, you're not going to have moisture issues (barring thermal bridging and cold spots, which are their own thing and not a moisture issue as such). As a special note, the Baumit OpenTherm panels are an extra meme because they can introduce mechanical issues and condensation problems.

Now that the meme is established, continuing on with the much cheaper EPS insulation.

EPS EWI process

EWI composition

  1. Cleaning the wall - to give the system a chance to stick to the surface, your wall should be fairly clean, any loose plaster or mortar should be knocked off. After this, ideally the wall should be pressure washed to remove loose sand and dirt.
  2. Primer - ie. Baumit UniPrimer - think of it as a layer of glue that seals the surface and further improves adhesion. Do not buy the cheapest primer you can get your hands on, some types (eg. PVA) are absolutely not fit for EWI. In the grand scheme of things, even the more expensive types only make up maybe 3-5% of your material costs.
  3. Starter track is screwed into the wall along the bottom edge of the insulation. The insulation should not be in full contact with the ground, this is typically assumed to be at least 20-30cm above the ground level. If you're close to ground or have ground contact, you can substitute EPS for XPS as the latter can take long-term moisture exposure.
  4. Adhesive base coat - eg. Baumit DuoContact/StarContact, Weber LAC - this is your most expensive item. You need an absolute fuckton of glue, to be exact, about 800kg for each 100 square meters - assuming your wall is mostly flat. Can be much more if you're fixing an uneven wall. Unlike other types of mortar, you do not mix these with any other substrate (ie. sand), hence the high consumption.
  5. EPS/XPS - self-explanatory. Note that the final surface should generally always be sanded flat! Some types of polystyrene have poor adhesion without sanding, but even if you have polystyrene that doesn't require roughing up the surface, you should still sand it to even out the panels. Uneven edges can show through the finish, especially if you use something fine-grained for finishing.
  6. Mechanical fixings - there are two major types here, superficial and embedded fixings. Superficial is what you'd expect, you drill a hole for the long, thin part, j-j-jam it in, screw in the screw or hammer it in depending on design, ???, done. Embedded is more complicated, you have to drill the small hole for the thin part, but then use a special cutting tool to cut away enough material for the head to sit deeper inside the insulation (maybe 2-3cm), and then insert the fixing, lock it, and then you cover the hole with a small EPS pancake they supply you with. The reason for this more complicated method is reducing thermal bridging - the plastic is technically a fairly good heat conductor and could lead to very slightly increased heat losses (?) or condensation issues (?). In reality it's probably perfectly fine to just use the superficial fixings, they're significantly less labor intensive. In the picture above, you see the latter - embedded - type. You may encounter people installing embedded fixings by driving them so hard they break into the polystyrene, and then covering it with the pancake - this method is incorrect and can cause the fixings to break.
  7. Reinforcement layer - return of the Adhesive base coat. You put on a thin layer, attach a mesh, coat it with another layer. At this stage, the wall should be rather straight and flat; doesn't need to be absolutely perfect, but close.
  8. Render Primer - return of the primer. Unlike previously, the role of this layer is not just to improve adhesion, but to partially water-seal the previous surface. Sealing reduces the water wicking ability of the previous layer. If you don't do this, your finish coat, being very thin, may dry too fast for you to process properly. The importance of this is variable, depending on what you use for finishing, but it still has a fairly important role for adhesion and sealing the adhesive mortar layer from weather effects. UniPrimer works here as well.
  9. Finish - best to pick something silicone-based with a grain, eg. Baumit SilikonTop 1.5mm. The grain hides small imperfections in the previous layer and has a decently modern look. Another fairly large chunk of your total cost.

Additionally, there are many items for connecting the surface to windows, doors, metal and wooden surfaces, covering small gaps, et cetera. Usually self-explanatory in what they do.

EWI thickness

With our example wall with 5000W heat loss, let's consider the effect of applying EPS insulation of varied thickness:

EPS Thickness (cm) Combined U-value (W/m²K) Heat Loss (W) Reduction (%)
0 1.5 5000 0
4 0.55 1841.67 63
8 0.34 1129.03 77
12 0.24 814.00 83
20 0.16 522.33 89

As you can see, it's a game with massively diminishing returns. The first 4cm of insulation does by far most of the work with a ~60% reduction in heat loss, the second 4cm adds another ~10-15%, the third 4cm adds... 6%, and then pretty much nothing past that.
So should one just install 4cm insulation and call it a day?

  • "NO!" says the EPS manufacturer, more number is more better, oy vey! Don't you want to brag to your friends on r*ddit about your 40cm insulation layer saving you literal tens of cents a year on heating?!
  • "NO!" says the HVAC engineer, you filthy peasants have to match gubment regulations for new build heat loss, and you basically need to buy like ten gorillion centimeters of insulation to save the starving polar ice caps in Africa
  • "NO!" says me, because there are such things as a condensation and dew point, and you probably don't want your wall to be fucked by it

Ignoring the first one, and assuming you are renovating and get to ignore the second one, the problem of the dew point remains. Over a given temperature gradient, invariably a dew point will form somewhere, and condensate will try to form in the zone of the dew point. If you have a breathable or uninsulated wall, this is not a major concern, but if you have EPS, you don't want the dew point to fall inside your wall, or between your wall and the EPS. You want the dew point to be inside the EPS - where there's no air circulation whatsoever, and thus condensate doesn't form.
You can do the calculation, or get an AI to do it for you, but as a rule of thumb, 6cm is the bare minimum EWI you should install in temperate climate, if the outside temp is -5C and indoors is 20C. 8cm is erring on the safe side. If you get especially cold winters or tend to have high humidity, go for 10cm. This calculation is not one with diminishing returns, insulation thickness proportionally gives you wider temp/humidity range coverage - 12cm is ~20% better than 10cm.

Okay, but like, why not just install 20cm like everyone else?

  1. It's ugly
  2. It's structurally weak, at 20cm you have virtually no hope of attaching anything to the base wall surface - workarounds exist, but alas
  3. Costs more money with weak ROI
    You can still do it, it's just that your first 10cm saves you 80%, and the second 10cm saves you an additional 9%. On new builds this is probably not an issue, but on renovations you'll find that 20cm interferes with the existing surroundings of the building (think walkways, gates, paths, gutters, etc.) a lot more than, say, 8cm.

But wait, what if I just don't have the room to fit 8cm of EPS?
Phenolic Foam Boards - about 50% better insulation (0.02W/mK) than EPS, so you can get away with as thin as 4cm EWI. Will potentially cost twice as much though. Good fire safety. Disadvantages:

  • boards shrink over time about 1%, may contribute to crack formation or reduced insulation performance

PIR foam boards - similar to phenol in price and performance, may be very slightly cheaper. Major difference is that PIR is much more moisture-tolerant (e.g. soil contact). Doesn't have the shrinking issue of phenol boards. But

  • flammable, releases dangerous chemicals - absolutely not suited to indoor insulation
  • insulation decreases about 10% over 20 years or so - leaks the insulating gas

But wait, what if I have infinite money and I must insulate?
Aerogel - possibly the perfect insulation material. ~3x as strong insulation as EPS, meaning you only need about 3cm. Doesn't block moisture. Very durable with near-infinite longevity if installed correctly. At 0.015W/mK, you only need a 3cm thick layer. But

  • extremely expensive, costs 10-20 times as much as EPS, around $100-$200/square meter
  • vapor permeability can be a downside if it's not sealed properly

Vacuum Insulation Panels - the absolute strongest insulation on the market with 0.007W/mK (5 times lower than EPS). You only need a 1.5cm layer to achieve the same performance as 8cm EPS. However

  • extremely expensive, in the range of $100/square meter
  • complete vapor barrier, this may not be a big issue in itself
  • very fragile, if you ever puncture the board, it loses 70% of its insulation strength
  • can't be cut at all, you have to order custom size pieces where the whole panel doesn't fit
  • ~10% performance loss after 20 years from the vacuum slowly failing

ROI considerations

There are many factors to consider - are you full /diy/, or paying someone to install EWI? Are you heating with gas? Heat pump? Do you have solar?

The base material cost of EWI in [current year] of 2025 for a 100 sqm building with about 150sqm external wall surface is in the range of $2500-$4000.
If you consider that you go from 5000W wall heat loss to potentially under 1000W, saving 4000W on heat loss, it's not a small change. However, the endgame is heating bill reduction.
Let's assume that you already did ceiling and floor insulation and got triple-glazed windows, and now your whole building conductive heat loss is 6500W without EWI, or 3000W with 8cm EWI, or 2400W with 20cm EWI. Air change heat loss is the same in every case, let's say around 0.5 ACH for 1000W. In temperate climate, this will come out to an approximate yearly heat loss of 4MWh per kW of nominal heat loss.

Item No EWI 8cm EWI 20cm EWI
Heat loss (W) 7500 4000 3400
Yearly heat loss (MWh) 30 16 13.6
Heating bill: gas, 80% efficient ($0.1/kWh) $3,750 $2,000 $1,700
Heating bill: heat pump, 400% efficient ($0.25/kWh) $1,875 $1,000 $850
Heating bill: heat pump + solar (approximated $0.1/kWh) $750 $400 $340

Gas and electricity prices are highly variable, you should refer to local prices. The above are roughly EU average.

Key takeaways:

  • if your heating is expensive, (i.e. gas in a western country), EWI is probably worth it even if a contractor skins you alive - ROI's probably well under 10 years
  • it's not such a great deal if your heating is not expensive (ie. gas in heavily subsidized country, or a good heat pump setup), but it should still have fair ROI - the EWI should last several decades.
  • very thick (20cm) insulation doesn't save you much if you have cheap heating - $60 a year (when comparing to 8cm EWI) has a ROI of like 15 years just on the material cost (~$800), G*d forbid the contractor upcharges you for installing the thicker material, it may well never get to ROI
  • if your heating is expensive, it's often the case that it'd be cheaper to install solar + heat pump (and easier to /diy/), than doing EWI through a contractor, especially if you have a large/complex house

But wait, how much will a contractor actually charge me?

The materials cost about $15-30/m2. Labor cost is probably around $20/m2 on the low end (Eastern Europe) and up to about $100/m2 in Western Europe.

Floor insulation

As mentioned above, floor heat losses are generally very low (when in contact with ground/soil), therefore insulating the floor is of least importance of the 3 zones. Typical is to place a 5-10cm XPS insulation layer, for example under UFH. Certainly anything past 5cm is a meme.

Your greater worry should be hydroinsulation or the lack of thereof, which should be under the slab, PE sheeting or bitumen. If retrofitting (no slab access), liquid waterproofing paint exists, but supposedly sucks compared to membranes.

Roof/ceiling insulation

You have the option of insulating either inside, or outside.
Interior - you glue EPS panels to the ceiling, then drywall it up.
Exterior -

  1. If you have an attic, you roll out a bunch of rockwool
    >but why not EPS?
    Moisture resistance, fire safety, easier to install (doesn't require fixing). But can't be walked on and has poor moisture resistance - as an alternative, you can lay out XPS, maybe combined with rockwool.
  2. If you don't have an attic, but have some space in the roof - blow-in insulation, or, if you can access the space - fill in with EPS or rockwool
  3. If you don't have an attic and don't have space in the ceiling - do interior insulation

For rockwool in particular, adequate ventilation is critical, it will soak in moisture and not insulate for shit if you let it.

Windows, doors

Heat loss through windows and doors is generally much worse than any other part of the building, per area. Single-glazed is around 6.0 W/m²·K, old double-glazed 2.0-3.0 W/m²·K, modern double-glazed with low-E coatings and argon fill 1.2-1.8 W/m²·K, triple-glazed 0.8-1.0 W/m²·K. F. On top of that, old windows contribute significantly to ACH, and there's a bonus infrared radiation component.

How fix?

Replace windows. Go straight to triple-glazed, cost difference shouldn't be big. uPVC or insulated aluminum frames.
Cost: Around $300-$600/sqm (window+materials)

Half-assed alternatives:

  • Window film. Low-E or insulating film cuts radiative loss, drops single-glazed U-value to maybe by 1 or so.
  • Secondary glazing (removable inner pane) gets you to ~3.0 W/m²·K. Cost: $10-$50/m² (film), $100-$300/m² (secondary glazing).
  • Weatherstripping ($5-$20/window), caulk ($5-$10/tube), or draft stoppers ($5-$15) fix gaps

Retard-tier half-assed alternatives:

  • Thermal curtains/shutters. Reduce convection and radiation by 10-20%. Cost: $20-$100/m² (curtains), $100-$500/m² (shutters). ROI: 10-20 years unless DIY.
    - maybe just replace your shitty ass windows instead of spending hundreds on shutters and thick curtains

Additional considerations:

  • Historic districts may restrict replacements—check local rules.
  • Air-tight windows cut ACH but can trap moisture - make sure the windows you're buying have trickle vents (though you can just drill them in afterwards if they don't).

Video guides, examples

Newfag does /diy/ EWI https://www.youtube.com/watch?v=Z6KHUCQD8Pc

Oi, you got a loicence for that trouwel? - Bong guy teaches you how to cream over walls, AT LENGTH - https://www.youtube.com/@Onthetrowel

Insulating below the damp course (that means "near the ground") https://www.youtube.com/watch?v=IMD_b_XJcOM

Edit Report
Pub: 01 Feb 2025 13:28 UTC
Edit: 28 Apr 2025 12:16 UTC
Views: 43