Thermal Insulation. You can’t go wrong. Can you?

  • Building Design Expert
  • 10 years ago

The i-Word is new to the 21st century in many forms; there’s the iPod, the iPhone, and the iPad. Apart from the latter sounding more like the name of a sanitary towel, Apple have pretty much got the i-word sewn up. But the i-word in construction goes back much further, and arguably has become as big a deal as anything that has rolled out of Apple’s gates. The “i” as in INSULATION used to be 50mm of mineral wool rolled out between ceiling joists as the minimum to comply with Building Regs.

If you grew up within the ‘Thermal Insulation’ manufacturing industry, in recent years it’s been ‘hang on to your hats boys’ because there’s been no stopping the innovation in materials and product development. Even the humble ‘Mineral and Rock wools’ have been elevated to iterations that are safe to use, more efficient, water resistant, and even ‘sheep’s wool’ has muscled into the flock adding clear eco credentials to the market. Even now manufacturers have developed glues that can stick insulated cladding onto buildings, roofs and walls without mechanical fixings.

Of course insulating our buildings has become such a hot topic that this has driven product development to levels that were not even conceived 30 yrs ago when I stepped into an architects’ office for the first time. Back then 100mm wool between joists – job done. Ventilation? We didn’t need that. Well, we probably did in theory. But then it would have been taken care of by all the ill fitting doors and windows, and the “lets just cover it up so the rain doesn’t come in” attitudes that predominated, probably in ‘Design’ as well the ‘Build’, if the truth be known.

Now the price of heating fuel: gas, oil, electricity is eye-wateringly high, we will do almost anything to try relieve the angst associated with the quarterly email from our energy supplier. So what do we do? We insulate. We insulate as much as we can. Where we can and in what ever manner we can. But do we really grasp the implications further down the line. Not really, no. The commonly held misconception is that insulation is the quick fix we all need. But it’s only part of the story.

THERMAL INSULATION OF BUILDINGS – you mean it’s not straight forward?
You insulate home, or your office, your industrial unit even. Well perhaps you don’t do it but you either pay, or arrange for someone to do the work. Maybe a specialist contractor. So you trust that the work is being done correctly, and in a way that will do nothing but good for you, your building and perhaps importantly your bank balance.

It might surprise some to know that simply packing as much insulation as you are allowed, or can afford into say, a roof space is not all there is to it. In fact doing so could prove positively detrimental to the building fabric. Not generally in a way that will cause it to fail structurally, but it can and will lead to maintenance issues where retention of moisture will accelerate  deterioration of the building fabric, including the very insulation that was intended to improve the building environment.

Somewhere along the line somebody put the word about that if we can increase the insulation levels to our buildings we can offset things like global warming, and, the incredible damage that the ever increasing cost of fossil fuels is doing to our bank accounts.
The government is actively promoting the insulation of our older housing stock, and many householders are currently enjoying free, or very heavily subsidised insulation to walls and roofs. That’s great, or at least it would be if someone had stopped for a moment to think the whole thing through.

What does thermal insulation DO? – The sole purpose of insulation is to slow down the loss of heat. If you can slow it down enough this will significantly reduce the amount of energy input to provide / replace that heat. Simple. It doesn’t do anything else. It doesn’t look nice or perform any secondary function. However, the type of insulation used, and where, specifically, it is installed is usually critical to it’s performance. When I say “where”, it is not a reference to walls, floors or roofs. But where specifically in the building element. Is it on the inside, the outside, or the middle of the wall? Is it to form a warm roof, or a cold roof? etc. etc. All have implications. Some more onerous than others.

If you go back to some very basic school-boy physics: the more heat you are able to retain in a space, or room, the more moisture, or water vapour, the air in that room can support. OK, so what? Why should we be concerned, after all the house is so much more comfortable when it’s warmer. Of course that’s true, but when you are retro-fitting insulation; and just putting in insulation because you can, or you want to, there are no building regulations to adhere to. There are of course still building regulations, but they do not apply to just the installation of insulation in this way. Even if they did, how would you police it? So we rely on the “specialist installers”, as there will be few building owners who would employ a professional to design and oversee  such work.

ROOFS – What choices do we have? – Someone; either the building owner, his advisor or the installing contractor must decide the most efficient way to insulate. This could relate to capital cost and, or, ongoing costs relative to the perceived savings the installation can make on energy input.

Warm Roof or Cold Roof? – Of course there are pros and cons to each. The first question though is does the installing contractor fully understand the differences. In my experience the answer to this one is a categoric NO.
Warm roof insulation, as the term implies seeks to keep the building structure below it warm providing no cold surfaces on which moisture laden air may condense, and is perhaps becoming the solution of choice. But from my visits to site there are still too many contractors (and some professionals) who do not understand the principles of the warm roof.

Cold roof insulation seeks to retain heat below the surface of the roof. Subsequently any void space above the insulation, and below the roof construction remain cold potentially providing surfaces on which condensation can occur, relying on adequate ventilation of the void space such that moisture laden air is moved outside the building before it has a chance to condense. I know. It’s complicated isn’t it? Or, at least that’s why this is getting more and more difficult to achieve given the higher levels of retained moisture due to ever increasing levels of insulation. This is one vicious circle.

If you are insulating a new building, or an extension there are very prescriptive requirements for the amount of insulation, and the way it is installed. For example, in a pitched roof loft space the building regulations approved document describes two methods of roof insulation:
The first is on the ceiling line, that is just between and over the ceiling joists, which carries a slightly higher insulation requirement than the second option which is along the line of the roof pitch, which means between and under, or over the rafters. Building Design Experts – architects and architectural technologists; understand the requirements. They understand the science that separates poor design and compliance. Unfortunately, all too often, the building contractor doesn’t. So when it comes to a choice between installation exactly in accordance with the design drawings, or the way they did it on the last job, because they found it much easier? Answers on a post card please.

The latter option is warm roof insulation. In installing the insulating material to the line of the roof pitch the aim is to retain heat within the loft space above above the upper floor accommodation. So in other words everything in the roof space should be as warm, or nearly as warm, as the main house.

The first option, cold roof insulation; is trying to retain the heat to the upper floor at ceiling level, and any volume of the pitched roof above there is cold as a consequence. This option might seem more familiar to those of you not too familiar with more modern construction techniques. After all we have been insulating lofts at ceiling level for years. But, quite a few years ago somebody worked out that when we start introducing insulation between warm and cold spaces; the moisture laden air that existed happily within the warm, was very likely to condense as soon as it came into contact with a cold surface within the roof space. And of course water, where you do not want it is almost always a problem.

To combat this situation the accepted answer is to dilute the moisture laden air with clean dry air introduced into the roof space via ventilation strips in the eaves construction, and along the ridge line. So for this method of insulation there must be ventilation, and it must cause the air within the roof space to be fluid and move to the outside. Another recognised means of ventilation is to use a fully breathable under batten roof felt, or sarking felt. This also promotes the movement of moisture laden air through it to the outside.

This is great. thank goodness someone has worked it out. But, problem: Because the building regulations do not apply to just filling the roof with insulation; no one even appears to consider that after they have filled the ceiling line with up to 25 or 30 cm of mineral wool insulation that the roof space now, more than ever, needs ventilating.

Most buildings 40 – 50 years old do not have ventilated (flat or pitched) roof spaces. The easiest thing in the world is to cut insulation foam board tight between flat roof joists. Phew. There, that will keep you much warmer. Yes it probably will. The flip side of that coin is that almost certainly after the first winter the timber roof deck and tops of the joists will have begun to rot. Why?

  • The new insulation has allowed an increase in the moisture content of the air within the building.
  • This moisture has eventually worked it’s way to the underside of the roof decking.
  • The decking is above the insulation level and is freezing cold.
  • Moisture condenses on the cold surface that is the underside of the roof deck.
  • We now have water inside the building – NOT good.

This is just one example, and a very common one – unventilated cold roof insulation. I have seen contractors get warm roof insulation just as wrong.

Insulation to walls – inside, outside, or somewhere in between? There are endless others. Insulating an existing building is as much a job for a Building Design Expert as any structural alteration. People, building owners, contractors; think they know what they are doing. But they do not understand the science of building. Thermal Insulation needs thought and understanding, and the building owner therefore needs to engage someone that can provide just this.

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