Blogs
Latest News
In this section we will be published our comments on the latest news and events in the sustainable building arena ...
May
2010
What is the point of a cavity wall?
Once again I recently surprised a client by suggesting that single-skin timber-frame wall construction would be a good idea. Shock and horror hardly describe it. It was like I suggested that his children be sold into slavery to fund the new home.
But think about it, what is the point of the cavity? What does it do? More importantly, what is the point of the brick outer skin on a timber-frame wall? Take it away and the house will not fall down, the thermal efficiency will be about the same, the life of the house won’t change. It serves only as a rain barrier and to convince our neighbours that we invested in bricks and mortar – which we didn’t. Not really.
Until early 20th century solid wall construction was the norm, be it stone, brick or timber. The cavity was introduce as it provided a bit more protection against rain penetration (especially with poor quality construction) and was thought to give a bit of thermal insulation. From 1970’s we started putting some insulation in the cavity, and now a cavity filled with insulation is not untypical. So we are in effect back to solid wall construction, but now with some insulation in it.
Take that idea just one small step further and you arrive at single-skin timber frame with the opportunity to install perhaps 250mm of insulation and get a U-value less than half that of a brick & block cavity wall. And it will be cheaper and quicker to build.
My confident prediction is that within 10 years single-skin timber frame (with or without structural insulated panels – SIPS) will become the norm. As soon as we accept that energy efficiency is more important than impressing the neighbours brick & block cavity walls will disappear.
May
2010
Draught Proofing
Two clients I have seen in the past couple of weeks, both with the same complaint and both with the same solution.
The complaint; a room that has been “insulated” is still cold. In one case the problem was immediately apparent – an unused open fireplace. The chimney was doing exactly what it was designed to do and that is draw the air from the room (actually it works the other way round with cold outside air dropping down the chimney into the room). Fixing the problem was east, simply block the chimney top and bottom and no more draught. It is important to seal the top as well as the bottom to stop rainwater getting in.
The other case was less obvious. A 1980’s built house that the clients had been at some pains to insulated to minimise their CO2 emissions. Cavity wall insulation, loft insulation, solar thermal on the roof, draught proofing and a new wood burning stove had all be added in the past couple of years. But it seemed that no matter what they did, they could not keep heat in the lounge. When the wood burning stove was going the room was toasty warm, but the temperature would drop alarmingly over-night. Eventually the problem was traced to a vent installed by the wood burning stove installers a couple of metres away from the stove and hidden under a book case. The “vent” was actually a length of 100mm drain pipe cut into the wall and covered with a louvered grill. The effect of a 100mm hole in the wall is fairly obvious, but less so when you can’t actually see the hole. In this case the solution was less obvious but still doable – replace the “vent” with a sealed ventilation system.
In 2005 NASA carried out some test on its various buildings and found that 50% of the heat lost from a building could be attributed to gaps in insulation amounting to 5% of the surface area. In their report they likened the gaps in insulation to a pin prick in a balloon – the temperature difference inside to outside means that heat escapes more quickly through a small hole than it does through a big hole, but ultimately the same amount of heat escapes.
In the typical UK house draughts will account for at least 10% of the total heat loss. If there is an unused open fireplace that figure will rise to over 50%. There are all the obvious places to seal – gaps around sash windows and under doors, floor to wall joints, ceiling to wall joints, gaps between floor boards – but the real culprits tend to be penetrations. Where pipes and cables are brought through walls or floors there is often a gap around the pipe or cable that can be simply sealed with mastic.
I often get asked if it is possible to do too much draught-proofing and end up with a stuffy house. I guess the answer is yes, but you would need to have a lot of time on your hands to get that far. It is something I have never seen and feel no need to worry about.
Mar
2010
DIY Secondary Glazing?
Would attaching a 4mm polycarbonate sheet to a single glazed window with magnetic tape (giving an air gap of 3mm) be worthwhile as thermal insulation? Is a 3mm gap too small to be effective?
A single glazed window will have a U-value of around 5.4W/m2K. A 4mm thick sheet of polycarbonate (about the same thickness as the glass) will have a U-value of about 4.7W/m2K. Taken together using polycarbonate as secondary glazing will certainly get the overall U-value below 3.5. How much below is where the air gap comes in.
A 12mm gap is typical although some authorities suggest that 16mm is optimum. What is wanted is to get the two panes close enough to minimise air movement but not so close as to allow radiant heat to cross the gap. In this case a 3mm gap will allow radiant heat to be transmitted but there will be very little air movement.
If the magnetic tape provides a good, draught-proof seal then the overall U-value will be in the region of 3.2W/m2K. A 40% improvement in thermal efficiency over a single glazed window.
So, in short, yes it works
.But all this talk of U-values might be stepping over the main point. The reader does not say, but if this polycarbonate sheet is going over rattlely sliding sash windows the biggest benefit will be in the draught-proofing qualities. Getting a good, air-tight seal around the window will cut down the heat loss from the room far more than the insulative qualities of the polycarbonate sheet.
Dec
2009
Insulating Solid Walls
According to The Energy Saving Trust, 35% of the heat lost from a house is through the walls. An uninsulated, 9” solid brick wall will have a U-value of 2.23W/m2. Adding just 50mm of PUR (Kingspan or similar) insulation will reduce that to just 0.4W/m2. Bear in mind that the current building regs call for a maximum U-value of 0.35W/m2 so we could go a lot further.
There are 2 basic kinds of solid wall: brick, which is typically 225mm thick, and stone which is typically a lot thicker, and works in a different way.
Stone walls
A typical stone wall will be constructed of 2 skins of stone with the gap between filled with rubble. Thinking used to be that the rubble fill was just a cheap form of construction but the reality is that it is designed to work in a particular way. Rainwater will penetrate the outer skin and some of that water will find its way to the rubble-fill. Because the rubble is relatively loose the water drops through it to the ground and away, preventing the inner skin becoming damp.
In addition, the wall is kept dry by air movement – wind acting on the outside skin and air moving through the wall. Interrupt those flows and the wall can become damp.
The way to insulate stone walls is with a breathable insulation, preferably set off from the inner skin to leave a 25mm air gap. Sheep wool, hemp or cellulose are all good insulators for this application and 100mm will reduce the U-value to about 0.35W/m2.
Bear in mind that walls are designed, they don’t just happen. And they are designed to work is a specific way. Interrupting the way they work with non-breathable materials like PUR insulation, gypsum plaster, damp-proof course, will stop them working properly.
My own house has 450mm stone walls that someone has proudly plastered. The result is that the walls get damp which is slowly blowing the plaster off the walls. I now have a long-term project of stripping the plaster and either re-plastering in lime or leaving as stone – depends on the look the lady wife decides on.
Brick Walls
The nature of a brick, compared to a stone, means that rainwater will penetrate less far into the brick. Wind acting on the brick will tend to effectively dry the wall. The critical issue is the dew-point. When warm air meets cold air is condenses and moisture is released. In an uninsulated brick wall that point will be towards the outer surface of the wall. Insulating internally will tend to draw the dew-point into the wall. What needs to be avoided is drawing in to the INNER surface of the wall.
There are 3 ways of doing this:
- External insulation systems – Google external insulation and you will find dozens of options. Especially useful if your house is rendered as these lend themselves to render finish.
- Thin inner insulation – options like Therma-coat or Sempatap. These are, respectively, an insulating paint and a neoprene-like foam.
- An insulated stud wall set 25mm off the internal surface.
All of these will work but what is best in any given situation will depend on the situation.
What is the impact?
Insulating walls is effective in its own right, but best done as part of an overall insulation upgrade. Heat, like water, will tend to take the path of least resistance so insulating the walls can mean that the heat escapes more quickly through the roof.
To put some figures around it; if the house has a heating bill of, say, £600 per year, insulating the walls in the way describe will reduce that to around £430. Upgrading all the insulation to a similar level could see the bills drop to less than £300.
Dec
2009
Is it worth insulating under the floor boards?
Around 15% of the heat leaving the house will exit through the ground floor. And that does not account for the draughts coming up through the floor. So the simple answer is Yes.
The problem is that, unless you have space to get under the floor boards, it means taking the floor boards up.
The illustration below is a typically method of insulating.
In this case the insulation would typically be a quilt type, such as mineral wool, sheep wool or hemp. The insulation would ideally be up to 200mm, or the thickness of the joists. 200mm will reduce the U-value from 3.98W/m2 for an uninsulated floor to around 0.3W/m2.
The advantage quilt has over rigid foams is that it can be cut slightly over-size and “squeezed&dquo; in to the gap, thus eliminating draughts. If rigid foam must be used then simply replace the wire lacing with battens nailed to either side of the joist and put mastic around the edges of the insulation to stop the draughts.
Care must be taken to ensure that any air bricks are not covered with the insulation. The air bricks are there to ensure a good circulation of air over the joists to prevent rot. So long as the bottom surface of the joist is exposed and there is plenty of air circulating there will be no problems.