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Insulation, Heating And Ventilation

Posted by eddleetham , 12 December 2014 · 1,239 views

As I mentioned in my last post, I want to achieve a reasonably good level of thermal insulation, but at a reasonable cost. So this will be no passivhaus build, but at the same time I'm hoping it will make a significant dent in my current heating bills.

The timber frame package I'm favouring has a U value for the walls of roughly 0.2W/m2. So I'm looking for a matching level of insulation in the floor. It will probably be beam and block foundations with insulation above and a screed containing UFH pipes on top. I'm working to a tight maximum ridge height of 7.5 metres, so I'll want to keep the insulation as thin as possible. This will mean using PIR insulation or similar. Celotex FF4000 looks like a good bet as it's designed for use with UFH, though other manufacturers may well produce a similar product. I reckon 100mm should give me a U value of 0.17W/m2 if the P/A ratio is 0.48. This is a bit better than both the U value of the walls and the minimum requirement for floors in Scotland, so I'm hoping this will be a reasonable target. I'm going for triple glazed windows with a U value of 0.81W/m2 which I can get for a good price from a Latvian manufacturer with a UK agent, though the roof windows will only be double glazed as there's a big price premium to pay for triple glazing.

As there is gas in the street, I was planning to have a simple combi boiler doing both hot water and heating duties. That's what I have at present, and as we don't have more than one bath or shower running at the same time, I've never been short of hot water. It would also save me having to find somewhere to put a cylinder. However the recommendation is that a house with more than two bathrooms should have a boiler and separate hot water cylinder, so with an eye on making the house more saleable I'm thinking of putting one under the stairs. My only concern with this is that it will mean a run of several metres from the boiler (in the utility room) to the cylinder. On the other hand, the recommendation for the UFH is that the manifolds should be positioned as near as possible to the centre of house - and the understairs area is about as close as I can get. So having the cylinder and manifolds next to each other may not be such a bad thing.

As the ground floor is mostly open plan, with the stairs going straight up from the lounge and a large stairwell opening, I'm hoping that with good levels of insulation I can get away without any heating on the first floor. When I asked NuHeat for their opinion, they disagreed, but Jeremy's experience with overheating on the first floor of his house (admittedly in the summer and with a more highly insulated house) makes me think otherwise. The bedrooms can be at a lower temperature than the ground floor rooms, but the bathrooms would ideally be a little warmer than the bedrooms. I did suggest putting towel radiators on a separate UFH circuit to lift the temperature a little, but the UFH guy didn't like that idea. So it looks as though I shall have to work out how to put them on a separate heating circuit tapped off the hot water cylinder.

Now to ventilation. Ideally a MVHR system is the best way to go, but it doesn't fit with my scheme to keep costs down as much as possible. It also has ongoing running costs and I don't really have room for the main unit. So I'm hoping that having trickle vents on all the opening windows will be enough to keep Building Control happy. I have had a look at Passive Stack Ventilation, but I'm not convinced. The theory seems fine, but I'm concerned that googling for 'PSV systems' seems to bring up just the one company. If it's so great, why aren't there lots of companies vying with each other to supply demand? Has anyone else researched this? If I don't go down the PSV route, then I'll have to use extract ventilation in all the wet rooms. I don't like traditional extractor fans as they allow cold air into the house when switched off, but I have found one unit, the Airflow Icon, which has an iris shutter which only opens when the fan is running. Now I just need to find a similar system that will do the same for the kitchen cooker hood extractor - though as the ducting here will be about 2 to 3 metres long, it may well be less critical.

One advantage with my current house layout is that the stairs and landing are in the middle of the house and there are two roof windows here, just below ridge height, which can be opened if the house gets too hot. Now if I have a traditional ceiling here, the windows will be at the top of a rectangular tunnel going up from the ceiling to the roof; not very attractive and less efficient in ducting warm air out the windows. So I want the landing to have a vaulted celing; it would also be nice if the principal bedroom has a vaulted ceiling too. However when I put this to the timber frame manufacturer they started having concerns and told me that I would have to go away and pay an architect to draw up fully dimensioned drawings with structural calculations completed before they could take the project any further. My understanding was that timber frame companies had their own structural engineers, either in-house or on contract, who do this sort of thing and it was all included in the quoted price. So it seems I have the choice of either bringing in an architect or architectural technician, or else finding an alternative supplier who is happy to work with my existing plans.

I was hoping to get a firm quotation in December so that I could place my order and book a delivery slot in late February or early March, but this now seems to have gone out of the window, so to speak. With Christmas looming, I doubt whether anything will happen now until January, so I'm just hoping I will be able to pick things up where I left off in three weeks time.

Meanwhile, I brought in a company to drill a test hole in the middle of the site to see what ground conditions are like. It was rather expensive (to my Yorkshire mind) at £810 + VAT, but could help me save money further down the line. The guys came on Friday and spent 4 hours drilling down and taking a core sample. They said conditions were reasonably good with clay down to around 5 metres and more solid stuff below that, but I'll get a written report in due course. Normally traditional footings would probably be OK, but my planning/building consultant reckons that the close proximity of both growing trees and recently felled trees means that ground heave is a real probability. He's therefore suggesting we go for piled foundations. I've already had quotes from two mini pile firms of around £12,000 for 4 metre long piles and a ring beam; longer piles will obviously cost a bit more. However piled foundations will mean there is less earth to cart away, which will bring the costs closer to those for trench foundations.

Addendum:

I have always believed that a wood burning stove is not necessary in a well insulated house, but my wife insists that she wants one. I guess it would also be a good selling point when the time comes to move on. I know the stove would need to be 5kW or less and would ideally draw its air for combustion directly from outside. There do seem to be stoves that fill the bill, so I included it in the build spec. I originally went for an external brick chimney, but that seems to bring with it problems of directing the flue through the timber frame, at a 45o angle, and then putting a suitable flue up the chimney. It occured to me that it might be simpler to have a stainless steel flue go vertically upwards through the lounge ceiling and bedroom 3 to exit through the roof. That might also bring a bit of extra warmth into the bedroom. I now need to research what is the best way of doing this.




slidersx200
15 Dec 2014 10:58 AM
It is worth committing a bit of time to the research, quotation and comparison stage as any decision made has a cost implication attached and your frame/foundation pricing will be an especially large example. While it is easy to understand the rationale for trying to keep build costs down and the building fabric amounts to a high percentage of the overall budget, I'm not sure if some of the cost saving measures you propose may be a little short sighted if this is to be your long term home.

When you spend money on insulation, you spend it once, at today's value and once it is in there are no ongoing or maintenance costs. Energy costs arising from lower insulation or airtightness standards are ongoing and subject to inflation and fuel price rises. There are other "soft" influences, such as the perceived comfort factor (the absence of a drafty feeling) and the improved air quality that comes with MVHR, especially for anyone with respiratory ailments.

The ultimate decision making is yours, obviously, but I hope you don't mind me offering some thoughts for your perusal. There are some crude assumptions made for ease of calculation, but you should still see the trend I'm trying to outline.

I'm guessing the timber frame quote in question is for a 140mm frame on 400mm centres, with 140mm PIR cut board between studs. Using the same frame, a U value of 0.16W/m2K is achievable if you substitute the between stud fill to mineral wool with a thermal conductivity of 0.034W/mK and add a 50mm lining of PIR internally. The overall cost difference should be negligible, but in return thermal bridging will have been greatly reduced and energy loss through the wall drops by 20%.

Jeremy has explained the impact of floor slab insulation values on UFH losses to the ground on numerous occasions and if there was to be one fabric element to consider improving upon, this would be it! Increasing the insulation thickness to 120mm takes the U value to 0.14W/m2K or 150mm to 0.11W/m2K. These figures result in a reduction in heat loss to the ground by 17% or 35% respectively.

Ventilation losses are likely to be the greatest area of improvement you could look at though as the trickle vents are a direct path for (potentially) cold outside air to enter the building and the air you have paid to heat to leave. This air movement and the associated temperature gradient across the room from the window will mean it still feels drafty beside your otherwise hard working triple glazed windows and as a consequence, occupants will probably be inclined to turn the heating up to compensate for this sensation.

Bathroom and exhaust type kitchen extractor fans are a turbocharged way to throw heat away and further cold bridges through the structure. With trickle vents and extractor fans, let us assume the volume of air inside the house changes at an optimistic 3.5 times per hour, with no heat being recovered. By aiming for a more airtight build with MVHR and you could quite easily reduce the ventilation loss to 2 air changes per hour, changing a further 0.3ach through the MVHR, recovering around 80% of the heat in doing so, you can reduce energy loss by around 40%. Get down to 1ach and you would be closer to 70% savings!

Don't forget to offset the cost of MVHR against the price of three bathroom extractors and twelve trickle vents and if you must exhaust the kitchen extractor with a long duct run, take extra care to lag the ducting with a vapour impermeable insulation to avoid condensation.
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I want to add to what slidersx200 says. We are making similar trade-off discussions about frame profiles, etc. -- albeit driven more by maximising usable living area rather than minimising cost. Surely you want the entire living space to be comfortable? There is a tipping point where losses upstairs (for example) start to drop the air temperature below comfort levels. Do you want to start to add rads at that point and get on the slope to needing a conventional CH installation?

Do the overall heat balance sums. Air circulation losses are going to be the dominant component. If you can achieve the air tightness, then MVHR will take out 80% + of these losses as well as improve the air quality / humidity in the house. Likewise if you can drop the U values from 0.2, 0.48 to 0.16, 0.15 then this is make a big difference to the number of degree days in the year when you need to have active heating.

It just seems a shame to spend £100Ks on building a house and lose the overall comfort / hike your ongoing bills considerably for the sake of saving <£10K.
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I totally agree with what slidersx200 says and if I were planning to stay in the house for years it would be a no brainer. However it could be that we'll only be living here for a year or two before moving on to the next project, so it makes sense to keep costs down. It's also easier to specify the best you can afford, rather than work out the best balance between minimising costs and building a house that will be very attractive to potential buyers.

Essentially this means that an MVHR system is a significant cost that will not be recouped in a couple of years, and it's not a feature that most buyers would have on their wish list - or even understand. Volume house builders don't install MVHR and what I'm trying to build is a house that's better specified than an off the shelf house but at a comparable price.
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The sad fact is that one self builder doesn't add anything like the extra build costs for making a house energy efficient to its selling price in the current market. So if you are staying in it for less than 5 years then your logic is financially sound. :(
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I find all these blogs and discussions invaluable to my design process for my new build coming up. I also am only looking at a 10 year occupancy so payback has to be within that timescale. Also, forgive me for saying this but we are having a wood stove (SWMBO has made this a stipulation) and so heating on the cold days are not an issue, in fact overheating may be the issue.SJH's build and information is invaluable but I wonder if it is important for a house to be heated or cooled to such small tolerances?. Would I actually need 300mm of wall insulation or would 200mm (with very airtight attention to detail) be sufficient?. This is all conjecture as the plans are not formalised so no actual figures can be given yet. I look forward to presenting my plans and getting some actual numbers for my build (and I will be asking for help on this forum on how best to do this).Roll on the spring and the better weather to build.
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Yes, I shall be having a woodburner for the same reason. No doubt the lounge will be too warm when it's lit and I shall have to provide some extra ventilation, by opening a window or two. I can't decide whether to have the flue going straight up through the ceiling and the roof via a bedroom, or via an external chimney. Also, I guess there will need to be some heat resistant slabs under and behind the stove. Has anyone else done this and can offer advice?
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