Garald

Well, sure (0.1^1.3 is indeed about 0.05), but I wouldn't expect any empirical exponent to give a good approximation that close to the minimum.

At any rate, the TL;DR version is that the OP now knows he can get some nice, big iron-cast radiators (used/antique are fine, they just need to be properly refurbished/checked for flow). If the house is well-insulated and the radiators are big enough (likely the case, but he should use the spreadsheet), he'll be fine at T40. What I'm left with is the curiosity for the real reasons why the exponent is greater than 1 (other than "practice is never as nice as theory"). I'd also like to know what to expect at T25. I don't imagine any radiators are large enough for that to work in winter. I've set the temperature curve in mine to start at T25 in early autumn/mid-spring temperatures, but the heat-pump seems to want to go up to 40. OTOH my heat-pump is probably a bit oversized, especially in those seasons.

Not my bag either, but: - use https://www.ubakus.de/en/r-value-calculator/ to spot possible condensation issues, - consider using an insulating material with good moisture-regulation properties. Others will give much more solid advice than I am able to.

Right, that's surprising. As I said, radiators are, what, 80% convectors and 20% radiators (more or less, no?), but as radiators their dependency on flow temperature is actually minor, in the range we are discussing: it is proportional to the cube of temperature in Kelvin, and (273+60)^3/(273+40)^3 = about 1.2, which is much less than 2, so you'd expect the power of a radiator at 40C to be a bit *greater* than half the power at 60C.

4ish; 3 or so minimum. Since it was the first time I was working on heating calculations, I decided to take the joke on engineers ("calculate everything very precisely, optimize, and then multiply by 2") literally. I wasn't really going out and buying oversized radiators - I had bought a place with vintage radiators, which were of course oversized, and became much more oversized when I insulated and made the place more airtight. In rooms where I figured I didn't have enough of a safety margin, I would move an existing radiator to a smaller room and then buy a particularly tall new radiator. Had to do that only a couple of times. The insulation on the north wall is not as good as I was promised, and I think the insulation in the last floor (attic) really leaves something to be desired (it was mostly done by the owners before the previous ones; it's not quite thick enough (16cm or so) but the bigger problem is that there's no airtight membrane) so having a large correction factor turned out to be the right decision, I'd say.

The document https://www.delta-q.de/wp-content/uploads/heizflaechenarten_recknagel_sprenger.pdf gives an exponent of 1.3 as typical for flat panel radiators; is the exponent an empirical approximation? It does not give a value for what would be a typical exponent for legacy cast-iron radiators (what I have). Apparently it's sometimes a bit lower (i.e. better) than 1.3; see, e.g., https://www.cinier.com/en/products/voltaire-47/ I am a happily heated man, but then I didn't just calculate 2 = (60-40)/(40-20), I also added a big fat safety factor of about 2 on top of that.

What is the physical reason for that?

Right. For a back-of-the-envelope calculation, I just use Newton's law - radiators' function as literal radiant bodies is a thing but it's a second-order term (and at any rate it varies less with temperature in the range we are talking about). What correction factor do you use?

How low is low? My cast-iron radiators work just fine at 40C - that's their usual working temperature. For truly low temp (25C, say) I don't think it makes sense to use any radiators, traditional or untraditional - they would need to be huge. That's why people use underfloor heating (basically huge radiators hidden by the floor. Is there a real difference between traditional radiators and radiators that are sold as "low-temperature"? I was taught here that there was not, and followed the advice, which turned out to be good.

Can’t sacrifice head height - there isn’t enough as it is. As for depth: there’s somewhere between 15cm and 20cm to work with - not enough for current standards (whether I use rock wool or wood fibre), but enough to do a decent job for now, I’d think. Exactly. I’m asking the old contractor but I’ll be lucky if I get a response.

1. As people who've come across my posts know, my lodgings are the greater part of a rowhouse of sorts (1930s) in the Paris area. I have brought up the energy rating from a F to a B, and improving matters further is an ongoing task. Skip the following two paragraphs if you already know the context. Now that I've made things as airtight as I can, the weakest part of the insulation is clearly the roof. By rights, the (heated!) attic should be the warmest part of the house in winter, and it is not. The roof was insulated by the owners before the previous owners, some 15 years ago. When we had new skylights installed, the insulation next to the skylights was redone - it was in poor condition. We also had the combles perdues filled with cellulose insulation. It's still the case that the insulation of most of the attic ceiling was done 15 years ago, to dubious standards - and it is a certainty that cold air (or, in summer, hot air) manages to circulate in the insulated space between the ceiling and the roof tiles. The question is what to do. Opening up the ceiling and redoing the insulation with an airtight barrier would be 10k eur. Insulating from the outside (perhaps raising the ceiling by a couple of centimeters in the process) would be very expensive - around 45k eur. It would make more sense to wait 5-10 years and a couple of year-long academic invitations to better-paying countries, and then hire a structural engineer to restructure the entire roof, gaining a lot of living space. But that's not the subject now. What to do for now? My parquet guy is worried that the attic gets too cold when I put the heating on low (as I do when I'm travelling or simply not using half of the attic). He suggests lining the coldest attic room entirely with cork. That's a tempting suggestion (and one that risks giving my girlfriend the fits; she hates cork - the parquet guy says it can simply be painted over). At the same time I wonder how much of an effect it can really have. 1cm of cork adds just 0.25 to the R. Or does having cork exposed (and separated from the insulation cavity) give extra gains in any way? The parquet guy even suggests using 1cm only in the coldest bits, and using 3mm on the rest, in part due to cost considerations and in part not to give up on any precious headroom. Believing that 3mm helps (on anything that is not, say, metal) sounds unscientific. (Of course 1cm cork could help a lot with sound insulation, and that's valuable given that I live not far from an airport, but that's a different issue.) Here are some thermal-camera pictures for context: Paradoxically, as you can see, some of the coldest parts are the low walls of the combles perdues. I know I didn't get cheated on the insulation - I actually used a laparascopic camera to be sure - so I wonder what is going on there. The insulation on the side walls is new but not fantastic: 2. A friend just sent this link to me. Insulating from the outside using cork is nothing new - the issue is that the ranking by price is polystyrene << rock wool << wood fibre << expanded cork. What is new here is that expanded cork is being used in a brutalist way. Is that really viable? How do these folks protect their cork? And does not hiding the cork lead to significant savings? Could it make insulating on the outside with cork more competitive? https://www.theguardian.com/environment/2025/feb/18/enveloping-calming-london-house-wrapped-cork?fbclid=IwZXh0bgNhZW0CMTAAAR2Zk58djcTehOcpjhUDnBMdP3brmAF1ODNiXyjXOqS42IV4R10Upgs3_YI_aem_I90joDLxrFtqV6BSf4iJjA

Hi! I am in the Paris area, have solar panels (just 3Kw) and I'm not an engineer (I'm very pure) so I've been pestering everybody here with questions for the last two years and a half - this forum has been a lifesaver. Welcome!

I haven’t got the tape for the joints yet (it will arrive next week) but is this starting to look good?

Two kinds of insulation material just arrived. One of them looks more substantial but also very rigid. Will have fun later.

I'm confused - it would seem that officially (whatever that means) DCM is classified as in a danger class slightly above that of acetone (see, e.g., Wikipedia, various things on reddit, etc.). Would I be better off having DCM rather than acetone in a box in the garage? What happens if it spills? Was about to try acetone on a particularly nasty stain on a shirt - not sure what to think now.

This is extremely valuable advice, and I would have certainly used it if it had been available at the time (I used acetone because that was the piece of advice I could quickly find online when searching with sticky fingers). It's a shame that acetone is much easier to get (supermarket). I'll ask the people at the paint shop where to dispose of the canister.

I have a stupid and trivial question. A ventilation cover keeps falling every time I close a door. (I have positive-input ventilation; does this make a difference?) I got a new cover, thinking the old one must be broken, but the same happens with the new cover. What to do? I am not supposed to seal it with acrylic or what have you, am I?

Not exactly. As I said: I bought a large cannister online, went to a local shop to get an extension hose, was told that I did not have the equipment, that they did not have in stock, that it would cost a fair bit, and that they would rather gift me a canister for which I didn't need equipment. I guess it did not contain an enormous amount (is that what you mean by "all or nothing"?). It does not make a noise when I shake it, so I figure it's really empty.

Well, that was quick and messy. What took much more time was cleaning my hands until they were no longer sticky. (I used a microfibre cloth soaked in acetone, and then washed my hands with plenty of soap - actually, I repeated that a couple of times.) I hope I can now prepare dinner without poisoning myself. (I know what acetone tastes like (badly, even in tiny traces) but I have no idea of whether polyurethane can be detected so easily.) Can I put the can in the trash, or will that extinguish all life in a large radius?

Went to the local paint shop to get 400mm of water pipe. I was told, not only that they did not have any pipe that would do, but that the kind of foam container I got (FM330 ILLBRUCK) required a fairly expensive foam gun (50 eur or so) and that I would be better off with foam that didn't require such a foam gun. Their card reader was down, so the vendor just decided to gift me a can of Sika Boom 107 (with the semi-explicit message that it was an inducement to keep buying from him and be warier of things sold on the Internet). Hopefully it will be just as good? At any rate, he also told me to get some plastic pipe from a random bazar and use it to extend the can's pipe (by first heating the latter a little). Let us see.

I take that's some sort of self-referential Welsh joke? At any rate, my girlfriend was aghast at the William-Morris pattern, so I expected at least a cheer on that (or a mumble of agreement with her).

Oh, understood: I should remove some of that haphazardly laid insulation if necessary for an even finish. Then I'll insulate the exposed pipework. Ufff. May I get out on a technicality? The garage is not a separate shed - it's inside the house, and it has a wooden door, so it's never nearly as cold as the outside. If I don't get out, then distribution may be a bit of a problem. Are there real-life BuildHub get-togethers? Getting a keg (or homebrewing) would be feasible. I've been going systematically through all the air-leaks detected in the house - this was just the next-to-last item (which I decided to do myself because I couldn't find a mason to cut some drywall to measure). Here is the antepenultimate item - I covered an unused fireplace in pine with 2cm of cork backing: I also stuffed two of those plugs made out of British sheep up the chimney. Once this is done, the only thing detected by the fan test that will remain to be done will be to replace some non-airtight access hatches. I think some hatches are rated passivhaus. Is that the sort of thing I should be able to install myself.

Wait, so I shouldn't get the pipe insulation at all? Yes, only a short segment is uninsulated, but I take it would still help. What is true is that I can also do it later.

Will do. I take that there must be some kind of pipe insulation that wraps around and seals around pipes that have already been installed. Something like this? https://www.castorama.fr/manchon-de-protection-pour-tuyaux-noma-22-mm-l-1m/5413257000634_CAFR.prd?storeId={store_code}&srsltid=AfmBOorSdsIovuzJZC5bpFTE_ibzN9R7T8C2U-50UU8JbZfBrFOWBFyGgH4 The funny thing is that there's already some insulation (rockwool maybe? Or polyurethane?) just inside the hole, laid in any which non-airtight wave. Let me share a better picture. See attached. Sounds like good ideas! I'll go to the local paint shop first to see whether they can sell me a more "official" attachment (or some water pipe!).

Here is the space I plan to fill in with FM330 ILLBRUCK foam. (It's in an unheated if not very cold garage inside the house; cold air goes in here and makes the entire space above the ground-floor false ceiling cold - that does not help with the hot-water pipes and who knows what else.) Is there any reason why I should *not* do it? These are pipes coming in from the heat-pump.