Newbie Here From Leicestershire. Question: SIP, Timber Frame Or A Hybrid For A New Build Dormer-Bungalow?
Posted 25 November 2015 - 11:52 PM
Is the main issue here of the upper floor rooms overheating in the summer? Or does this an issue with loosing the heat outwards too?
If this is only an issue with hot rooms in the summer months, could a mechanical ventilation system work to pump hot air out and draw in the cooler air?
Would tiles made of Identical material, but of different colours, say a black compared to a lighter shade help with keeping the roof cooler in the first instance?
If the above won't work, would I be right in thinking I'd want the insulation to be as dense as possible to get better decrement delay? And so perhaps add additional insulation (internally or externally?) of a higher density?
Does having a larger under-tile gap and good airflow at the ridge and eaves help keep the roof cooler in the summer months?
Hope I'm not taking too much of your time, I've found no mention of decrement delay on most SIPs suppliers websites, I did however found Viking have done some work around this. I'm hoping to try and get a better understanding of this and thank you in advance.
Posted 26 November 2015 - 07:58 AM
The ideal is to increase the house thermal time constant such that it is longer than half a day, as that then damps diurnal changes. Making it longer than around 3 or 4 days may make temperature regulation more challenging, as will making it less than around 3 hours.
As an example, our current 1980's built brick and block bungalow, with cavity wall insulation, decent loft insulation and pretty good double glazing, solid concrete floors (with no underfloor insulation) and better than average airtightness (because I've spent a lot of time sealing all the wall to ceiling leaks) has a thermal response time of around 3 hours. It rapidly overheats in hot sun and rapidly cools at night or on a cold day. The heating system wastes a great deal of energy trying to keep the house at a steady temperature and overshoots during warm spring and autumn days are very common (cold morning, heating runs, house warms with the morning sun, residual heat fro the heating causes a temperature overshoot).
Our new build has a thermal response time of well over 24 hours (I've not properly measured it yet, but it's probably around 2 to 3 days). It stays at a near-constant temperature and only responds very slowly to changes in external temperature. In fact the greatest impact on internal temperature is the number of people in the house. If a couple of guests arrive then the house will usually warm up by around 1/2 a deg or so within an hour because of the additional couple of hundred watts of heating.
Our new build uses a thick layer of relatively high decrement delay insulation in the walls and roof, but uses low decrement delay foam under the floor, where there are very tiny temperature swings. The house is room in roof and the result of using high decrement delay roof insulation is that even after a prolonged sunny day the south facing roof is no warmer inside than the other walls, even though the roof external surface could be around 45 to 50 deg C.
Edited by jsharris, 26 November 2015 - 08:00 AM.
Posted 26 November 2015 - 08:42 AM
It is a hard concept to understand as the units boil down to an area and time, specifically m2.s-1
As we normally think in 'walls' we are often just talking about the thickness of them, as that is what we change (or the materials they are made from).
So if we take common brick, it has an a = 5.2 × 10−7
In thinkable units that is 0.52 mm2s-1
Yellow pine has an a = 8.2 × 10−8
Or in thinkable units 0.082 mm2s-1
So for materials of equivalent dimensions (should really be calling that vectors as we are talking about shape) a bit of cheap pine will impede heat loss better than a brick, as the thermal energy cannot spread as far.
But as we are working in 3D with a "wall" and not just an area, the calculations start to become tricky. We are actually working with 6 dimensions, length, width, height, thermal diffusivity, temperature difference and time (thermal diffusivity can be expanded to 5D, so that is 10D in all).
This gets too much for most of us to understand, me included, but as I am working on some geometric statistics at the moment, I find it fascinating.
So what you want is a material that absorbs a lot of energy, but does not let it travel very far and works within the temperature range of housing (so no igloos or fluids).
Edited by SteamyTea, 26 November 2015 - 08:44 AM.
Posted 26 November 2015 - 08:58 AM
From personal experience I know that our reasonably well insulated (for its time) Norwegian built timber framed house that we had in Scotland was thermally efficient, and needed very little heating even in cold weather, but it did have a very short decrement delay. It's thermal time constant was far shorter than the 3 hours or so for our current brick and block bungalow (built in the same era). Our new all-timber house, with thick cellulose insulation has a very much longer thermal time constant. It's been cold over the past week and yet the temperature is rarely below 20.5 deg C when I arrive in the morning and is usually over 21 deg C when I leave in the evening and the heating hasn't come on yet.
Posted 26 November 2015 - 12:47 PM
thank you for your comments again. I think I'm more confused now than I was initially! SteamyTea, what Insulating materials are good at absorbing energy with out letting it travel far? I've read adding insulating to the outer-skin would be far more effective than on the inside. So if I have a SIPs with (perhaps the 172mm Kingspan tek) and add a further thiner layer on the outer-face and then batten and tile?
Would a darker tile absorb more heat than a lighter shade? If so, would going for a lighter shade make much of a difference?
How would I calculate the Decrement delay for the above mentioned panel? I'm a Mathematics graduate so should hopefully be ok dealing with the numbers.
Finally, has can anyone who does have a straightforward SIPs panel roof comment on how much they have felt the effects of decrement delay, I would really appreciate that. If it is very problematic, I will perhaps go for a trussed roof and insulate it myself (reluctantly).
Posted 26 November 2015 - 02:10 PM
There will be a small effect from colour, but it's not great White absorbs almost as much long wavelength IR as black.
A decent timber frame design pumped full of something like Warmcell can perform extremely well indeed. This is the construction we have and it really does perform very well indeed.
Edited by jsharris, 26 November 2015 - 02:10 PM.
Posted 26 November 2015 - 02:32 PM
Does it always follow that high resistance, high heat capacity materials have a high DD? The way I have it in my ever-searching-for-analogies head is that the high resistance reduces the "pressure" of energy transfer while the high heat capacity "stores" the energy, slowing down the "flow" into the house (and also presumably back out when the situation reverses, e.g. energy transferred in by sun, then out during the cold night).
*Edit* thinking about it, I suppose you can also have a moderate DD with just high high heat capacity, isn't that how thick walled mediterranean houses ride out the sun?
Edited by gravelld, 26 November 2015 - 02:42 PM.
Posted 26 November 2015 - 02:43 PM
For example, during all the time that the sun is heating a surface to a higher temperature than the internal surface heat flow is inward. As soon as the outer surface temperature drops, heat flow becomes bidirectional; some of the warmer core heat will continue to flow inwards and some will reverse and start to flow outwards.
The internal heat storage capacity is determined by the heat capacity of the composite wall/roof structure and the rate of heat transfer is determined by the thermal resistance. As you can guess, time plays a big part in determining behaviour, as when the outer surface cools at night the hottest area is somewhere inside the wall or roof. If this is closer to the inner surface and if the thermal resistance of that path is lower, then heat will continue to flow into the house despite the outside surface being cool. The reverse is also true.
If you have a high heat capacity layer on the outside (say a brick skin over a well insulated frame) then this will mitigate things because the relatively high heat capacity of the brick will delay the temperature increase at the outer face of the insulating layer.
Posted 26 November 2015 - 02:58 PM
Posted 26 November 2015 - 03:32 PM
It's the heat capacity that's critical and that can be expressed as a volumetric parameter or a mass related one. For houses I'd argue that using volumetric heat capacity makes more sense, as it's usually the volume of stuff that is more critical than its mass.
Edited by jsharris, 26 November 2015 - 03:32 PM.
Posted 26 November 2015 - 08:28 PM
Take a car in tip top condition, on a flat road, no wind and at a constant speed. Let's call that the optimum.
Now if we let some air out of the tires, the rolling resistance goes up (what makes the tires get hot and explode, so don't try this at home, or the A30, unless you got a caravan, and do it in Devon).
Now the fuel consumption will go down, so that is the same as allowing more energy to be transmitted though the material, or a higher conductivity.
To get back to the optimum, you can reduce the mass or the size of the car.
Mass and size (all things being equal in this simple model) is really just the density. This can be thought of as the volumetric (or specific) heat capacity.
So there will be an optimum place where, with flatter tyres and a new density, the fuel consumption will get back up towards the optimum.
Or you can reduce the speed, which is really the thermal conductivity (there is time in the units, and distance).
So yes you can pick a material with different densities, as long as the heat capacity is similar, or you can change the conductivity.
If I was really bored I would go to the NPL, http://www.kayelaby.npl.co.uk/toc/ and get a list of all the materials and make a posh database that can run though the sums and end up with answer.
Or just fill the void with newspaper and glue.
Posted 26 November 2015 - 08:52 PM
I unfortunately can't afford to build a passive house and really need to get the best value for money with within my budget.
Great analogy there SteamyTea
Posted 26 November 2015 - 09:10 PM
The major problem is with comparing like for like from quotes. So many companies only quote for a small part of the build, leaving you to collect prices for all the missing stuff, and it's a bit of a nightmare trying to make fair comparisons.
For example, we had SIPs quotes for around £55k to £65k to supply and erect. To this we had to add the foundation cost (between £10k and £12k) and a stack of incidentals like plant hire (cranes, wackers, transport, power floats, airtightness detailing, the air test etc) to try and get a fair comparison with quotes that included all these items.
I ended up putting together spreadsheet, then making dozens of phone calls and emails to suppliers to tease out of them what was an what wasn't included in their quote. Many suppliers were exceedingly reluctant to reveal all the "missing" costs.......................
Posted 26 November 2015 - 09:28 PM
Unless there is a problem with my Maths, there seems to be a significant difference. I only went to reputable and recommended builders for the brick and block quotes, so maybe there are other cheaper options.
My MBC price came in at £125,000 with passive foundations, 0.12 U-Value and similar spec as the SIP otherwise.
There seems to be significant differences the prices I have received.
Edited by harvey, 26 November 2015 - 09:31 PM.
Posted 26 November 2015 - 09:36 PM
Edited by AlexC, 26 November 2015 - 09:38 PM.
Posted 26 November 2015 - 09:41 PM
I'm struggling to justify the extra cost in my head, I currently live on a poorly insulated bungalow, I would appreciate the huge benefits of a TF or SIPs coming from this!
Alex, are going with a TF rather than SIPs? what were the deciding factors?
Posted 26 November 2015 - 09:46 PM
Posted 26 November 2015 - 10:14 PM
Posted 26 November 2015 - 10:19 PM
Thanks for reply Trw144,
did the TF price include the roof insulation? Most of the TF companies I've had quotes for only provide the insulations in the wall and leave the roof to the customer. Would you mind me asking who the TF company was? I'm not certain if you're allowed to say on open forum so please PM if it's better.