Reviews > Clothing > Underwear > Hot Chillys GeoThermal Tops and Bottoms > Roger Caffin > Initial Report

Initial Report - Hot Chillys Geo Thermals
Roger Caffin

Product Information

* weight shown is for the zip top, as this was supplied by mistake.

Product Claims

Commonly known as 'thermals', these are lightweight synthetic full-length base-layer garments. They are made from a '7 oz flat back textured knit' fabric with a 'dobbed pattern'; the fabric itself is made from 51% 'Bio-Silver^®' nylon and 49% 'MTF polyester'. What this all means is explained later. Hot Chillys make the following claims on their web site and on the swing-tag:

• Anti-bacterial finish
• Moisture transport
• Relaxed fit
• Lived-in feel for active or casual use
• Inherent anti-microbial properties with Bio-Silver^® technology
• Flat seam construction for seam strength and to eliminate abrasion
• and there is a thermometer registering 7 out of 10

I think the 'textured knit' and the 'dobbed pattern' are the same thing. It seems to imply that the outer surface of the fabric, shown in the second picture here, has a slightly knobbled texture which will space it a bit off the next layer of clothing and trap air for insulation. This is what the web site says anyhow. While the surface looks good, I cannot see a very significant variation in surface height, and I doubt much air will be trapped. The inner surface is described as 'flat back': this translates into a fine brushed layer. I imagine the '7 oz' specification means the fabric weighs 7 oz per square yard (about 200 grams per square metre). This seems about right. The fabric has a little stretch, mainly in the 'sideways' direction (ie around the chest rather then up and down).

The Hot Chillys web site does not give any technical details about the registered term 'Bio-Silver'. I found several uses of this term through the web. One described a liquid thus:

Bio-Silver^™ is an extreamly [sic] effective antibiotic and anti-fungal liquid that works to remove from the body most fungi, bacteria, and viruses. As it circulates through the blood, lymph, and tissues, Bio-Silver™ eliminates the enzyme that all single cell yeast, fungi, and bacteria use for their metabolism. It does no harm to human enzymes or any part of the human body.

I really don't think that's it - and I would not want that stuff circulating through my body! Another web site belonging to Hyosung Textile of Korea detailed a fabric made from yarns which had silver impregnated into them in order to suppress biological growth. They claim that after 30 minutes some 99.9% of bacteria (presumably suddenly deposited) on the fabric are suppressed. This seems sufficiently close to the claim made by Hot Chillys. Whether or not there is any relationship between Hot Chillys and Hyosung I simply do not know. A point made on the Hyosund web site was that the silver was inside the yarn, not on the surface where it could contact the skin. The whole point of this silver is that it inhibits the growth of bacteria, and it is by-products from bacteria in the clothing which are supposed to give 'ordinary thermals' their bad smell after being worn for a while. (Mind you, I doubt any thermals would match my socks after several weeks of walking!)

The term 'MTF' seems to mean 'moisture transport fibre/fabric', and I assume it implies that the fabric will wick any sweat away from the skin in contact with it. This is a common claim for the sorts of fabrics often used in outdoor clothing. Apparently it is the polyester which does this, not the nylon. In passing, I note that many thermals made here in Australia and New Zealand are pure polyester, with the same claim.

I gather from some of the literature enclosed with the thermals that this model, using a light 7 oz fabric, is designed for 'casual and active' wear. I guess this relates to the thermometer which shows 7 out of 10: it did not seem to be explained elsewhere and otherwise seems irrelevant. Other Hot Chillys models use 10 oz and 14 oz fabrics: I would expect them to be warmer. So this model is not really designed for mid-winter outdoor use.

Initial Impressions

My very first impression when I opened the box and saw the packaging (shown in the picture at the top) was amazement. There were these two strange yellow cylindrical packages made from a stiff sort of plastic netting, with a zip around the top and 'something' inside. I'm afraid my reaction to these yellow packages was that they were a massive overkill, and I cannot imagine anyone ever storing the thermals in these bags. The bags are too big and heavy for my liking. However, that is peripheral to the garments themselves (although surely these packages add to the final cost?).

Then I opened up one of the yellow cylinders and extracted the contents. A 'thermal' with a little tag, tightly packed in a simple plastic bag. OK, now I was getting closer. My next reaction was to think that the bagged thermal looked awfully small, but on checking I found that thermals I have bought locally (with Australian, New Zealand and UK brands) are not a lot bigger when packed this tight. I just don't pack mine very tightly in practice. Anyhow, the actual volume does not matter that much compared to the weight - all other things being equal of course. So I compared the weight of these two items with some of my local thermals, and found very little difference in weight. The weights are listed in the second table below.

Next I opened the plastic bags up and extracted the thermals. Here I found some differences from our local thermals. The fabric on these Hot Chillys ones has the fancy 'dobbed' pattern mentioned in the advertising, while our local ones are usually quite plain. The brushed inner surface is quite light, but not much different from what our local ones look like after they have been worn a little while. I don't think the dobbed outer surface is really going to make much difference to the insulation (and how would I test that anyhow?), but the brushed inner does 'feel nice' against my skin.

Then I put the top on, and immediately noticed a large difference. Our local thermals are made to fit just skin-tight; these Hot Chillys ones are loose, both top and bottom. I laid out the Hot Chillys top and some local thermal tops and measured them across the chest region, with results thus. Obviously the "My Chest" dimension is half the circumference around my chest.

I am not so sure about this looseness: it means that the air can freely circulate between the thermal and my skin. This, to my way of thinking, means the fabric will lose almost all its insulating effect, and the top will be just like a loose sports shirt. I checked the sizes on the Hot Chillys web site: they list S, M, L, XL, XXL and XXXL (whew!). I am normally M or 'Medium' on a clothing scale like that, but maybe I should have got a 'Small'? Or does it mean that the design is aimed at people who are basically much fatter than me? One wonders.

The arm lengths on the Medium top seem about right. As mentioned, the waist diameter is rather loose, but the waist length seems sufficient, albeit not particularly generous. It will suffice if the top is tucked into something at the waist. I imagine the Small size might be even shorter in length, which would not suit me. The waist band on the bottoms is black elastic with a Hot Chillys logo in the weave, a looks much like some men's underwear. There is also a full fly (with a double layer of fabric) at the front: this is not mentioned on the web site as far as I could see, although maybe I should have inferred it from the fact that Hot Chillys advertise male and female versions. The leg length looks to be 'correct' on me. So the rest of the 'M' garment size seems to be about right.

Fabric Thickness vs Weight

The looseness of the fit caused me some concern about the insulating properties. As a result I decided to measure the thickness of the fabric in the Hot Chillys and in the other thermals my wife and I have. The thinking here is that, in general, the thicker the fabric layer the greater the insulation or 'thermal resistance'. Obviously there may be some differences due to the fibres themselves or the style of the knitting, but my suspicion is that the biggest factor would still be the thickness.

I stacked eight layers of the fabric with no wrinkles between two 60 mm square aluminium plates and applied gentle finger pressure. I found that the pile readily compressed down a little way, but then the resistance to compression shot up. There is a 'knee' in the compression curve (this is normal). So I measured the total thickness at (roughly) the knee pressure. I tried varying the pressure applied and found that there was really little change in the results. I got the following thicknesses for a single layer. The 'local' thermals are ones sold in our local shops, and are listed by country of brand name: I do not know where they were all made.

Clearly the Hot Chillys Geo Thermal model is thinner than thermals sold locally, but then the Geo Thermal is the lightweight model of the range. Hot Chillys have heavier or warmer models as mentioned above. On the other hand, our local ones are either the same weight or lighter than the Geo Thermal.

Customer Service

The thermals turned up quite quickly by courier from America. I had asked for a Crew Neck top and both the letter with the parcel and the shipping docket said a Crew Neck was being shipped. However, they had shipped a top with a front zip (with cute little Hot Chilly zipper pull) instead. I dislike a front zip on the base layer as it seems to dig into my neck, and I imagine an ordinary customer would not accept such a substitution. So I immediately emailed the writer of the letter at Hot Chillys explaining the problem. At the time of writing this Initial Report (one day after the thermals arrived) I have yet to hear back from Hot Chillys.

Planned Testing

Testing will be done in two parts. For one part I will take these thermals on our next few trips and see how well they keep me warm. Very often when we get the tent up in wet weather the first thing we do is to change out of our wet outerwear into a set of dry thermals. Often, that set of thermals is all the reserve clothing we carry under our ultra-lightweight regime. For the other part, I will try to wear the one bit of clothing at home for as many days as my wife will let me without washing it. I imagine my wife will tell me when they 'must' be washed! This will test the Bio-Silver part of the fabric design. So, I will be looking for the following things over all:

• Warmth - including general comparisons with the range I already have, taking the looseness into account
• Comfort - do they feel 'nice' to wear and do I warm up fast when I put them on
• Fit - mainly how easy are they to get on and off
• Smell - Hot Chillys claim to have this perennial problem licked - yeah?
• Washing - how well do they wash, and do they come clean and smell-free
• Cuffs - a special category in the comfort class
• Wear - how well do they last under clothing and in the tent
• Waist region - what does the trouser waist band feel like, and does the edge of the top stay down under it
• Neck - is the crew neck comfortable to wear (assuming this is fixed up) and does it go over my head easily

Reviewer Details

Reviewer:	Roger Caffin
Age:	59
Gender:	M
Weight:	58 kg (9 st 2 lb)
Height:	168 cm (5' 6")
Email address:	r dot (surname) at acm dot org
City, State, Country:	Sydney, NSW, Australia

Backpacking Background

I started bushwalking (the Australian term) when I was about 14 yrs old, took up rock climbing and remote exploration walking at University with the girl who became my wife, and later on we took up ski touring and canyoning. These days my wife and I do all our trips together, very often by ourselves. Our preferred walking trips in Australia are long ones: up to about a week in the general Blue Mts (east coast of Australia) and Snowy Mts (alpine region), and up to two months long in Europe and the UK. Ski touring trips would also typically last up to a week. We favour fairly hard trips of some length and prefer to travel fast and light. Many of our trips are explorations in wild country which sees few other walkers. Australia does not have 'huts'; we always sleep in a tent. In between these long trips we do some day walks, often exploring the start of longer trips. On average, we would spend at least two days per week walking or ski touring. Over the last few years we have become converted to the concept of ultra-lightweight walking, and have been cutting our combined pack weight down from 36 kg (80 lb) total to about 25 kg (55 lb) for week-long trips. I have also been designing and making ultra-lightweight gear for our own use under the harsh Australian bush conditions.

I am also the maintainer of the Australian aus.bushwalking FAQ web site www.bushwalking.org.au/FAQ/, and I have written gear reviews for a local walking magazine Wild and for BackpackingLight.

Reviews > Clothing > Underwear > Hot Chillys GeoThermal Tops and Bottoms > Roger Caffin > Initial Report