General: Decrement delay

1 to 30 of 104

- CommentAuthorfostertom
- CommentTimeJan 27th 2009 edited
Just seen this - haven't seen a better short explanation of this important but little-understood subject: http://www.viking-house.ie/decrement-delay
- CommentAuthorPeter Clark
- CommentTimeJan 28th 2009
Is lightweight insulation ever better than dense insulation?

How does dense insulation and decrement delay work in protecting the walls of a house in winter?
- CommentAuthorfostertom
- CommentTimeJan 28th 2009
They're saying (or should be) that, for given U-value, it's greater thermal capacity of the insulant per unit volume - or perhaps per unit frontal area - that gives decrement benefits in addition to traditional U-value.

That would tend to mean (but not necessarily) denser rather than lightweight insulants, and you'd tend to have to use in greater thickness to get same U-value. You have to check the figures for each material. Hemcrete seems to be particularly advantageous in that respect, provided you have space to use it in typically twice the thickness of typical lightweight insulants.

Or else, you can use in less thickness, so lower U-value, and rely on the decrement benefit to replace part of the traditional U-value. If going this route, proper understanding of this effect is essential, otherwise you could end up with an exactly counterproductive decrement effect - it can work against you, as well as with, and often does, unbeknown to designers builders and owners.

That understanding is rare in the building industry - alien, unknown and counter-intuitive in fact - so we all have to educate ourselves - me included - tho I was taught it in Architecture school in 1968 it's well forgotten and never used, till now.
- CommentAuthorPeter Clark
- CommentTimeJan 28th 2009 edited
In a 'room in the roof' situation, it seems clear that denser insulation will help protect from summer overheating. The density will delay the steady state, hopefully until night time, when it is cooler, so the room will never reach the same high temperature, and it will reach its peak temperature later.

Pg 5 in this:

http://www.greenspec.co.uk/html/product-pages/file-download.php?product=GSPEC20080609033330OHO484D3F3A267DE&file=pavaclad1.pdf

I'm wondering what happens when the situation is reversed, and the insulation has to stop winter cooling. The heat inside the room will try to get out to heat up the outside, but the dense insulation will prevent the steady state being reached, and make it a more favourable state as in the summer situation?

basically the 'thermal mass' or 'thermal inertia' of the dense insulation slows everything down, this is desirable in summer and winter?

So is light weight insulation ever more desirable?
- CommentAuthorfostertom
- CommentTimeJan 28th 2009 edited
Posted By: Peter ClarkThe density will delay the steady state, hopefully until night time, when it is cooler
Not delay the steady state, which never comes - it's a constant ebb and flow, including some flow reversal (though of course nett outward). It will phase-shift or delay (and attenuate the amplitude of) the arrival in the interior, of the externally-originated diurnal temp wave. If done right, its arrival can be timed just when you need it - when the sun goes down, gain through windows stops, and all kinds of losses increase. But you can equally (or ignorantly) get it wrong, and time the temp wave's arrival just when the room's already getting max gain and least loss e.g. mid afternoon. The former desirable state generally results from a 7-10hr decrement delay, typical of 'dense' insulants; the latter, overheasing state, typical of loft conversions, results from a 2-4hr decrement delay, typical of normal lightweight insulants.

Posted By: Peter ClarkI'm wondering what happens when the situation is reversed, and the insulation has to stop winter cooling
Me too - trying to get my head round all this.
Posted By: Peter ClarkSo is light weight insulation ever more desirable?
I don't see how you again get that conclusion?
- CommentAuthorPeter Clark
- CommentTimeJan 28th 2009 edited
Posted By: fostertom
Posted By: Peter ClarkSo is light weight insulation ever more desirable?
I don't see how you again get that conclusion?

I mean that the dense insulation is clearly desirable for reducing summer overheating, it may be better for reducing winter cooling, so it appears that denser insulation is always better?
- CommentAuthordjh
- CommentTimeJan 28th 2009
The main benefit comes when the direction of heat flow reverses each day, which is typically the case with hot summer days and cool summer nights. Then the average is comfortable warmth. A quick skim of the viking page seems to be talking about that effect.

In winter the direction of heat flow does not reverse. It is warm inside and either cold daytime or freezing nighttime outside. Decrement delay doesn't stop continual outward heat flow. You still need a good U-value for that.
- CommentAuthorJames Norton
- CommentTimeJan 28th 2009
Sorry to be dim but...

does any one actually know / have a spreadsheet for calculating decrement delay....? The Viking site has a formula but doesn't seem to tell you what all the variables are... for example it either says you have to divide something by the speed of light... or I am am misunderstanding....

J
- CommentAuthorPeter Clark
- CommentTimeJan 28th 2009 edited
Posted By: djhIn winter the direction of heat flow does not reverse. It is warm inside and either cold daytime or freezing nighttime outside. Decrement delay doesn't stop continual outward heat flow. You still need a good U-value for that.

I don't understand this. The denser insulation delays the passage of heat through the insulation when it is going from outside summer day to inside summer night. So why does it not work in reverse when the heat flow is from inside winter day to outside winter night?
- CommentAuthordjh
- CommentTimeJan 28th 2009
Posted By: James NortonThe Viking site has a formula but doesn't seem to tell you what all the variables are

Lambda is k :) - thermal conductivity

rho is density

c is specific heat capacity
- CommentAuthordjh
- CommentTimeJan 28th 2009 edited
Posted By: Peter ClarkI don't understand this. The denser insulation delays the passage of heat through the insulation when it is going from outside summer day to inside summer night. So why does it not work in reverse when the heat flow is from inside winter day to outside winter night?

Decrement delay slows heat flow in during the hot summer day and it slows heat flow out during cool summer nights. So the two flows cancel each other out (roughly) and the inside stays warm.

The situation in winter is not the "reverse" of the situation in summer. Heat flows out during winter days and it flows out faster during winter nights. So when you average it, it's still flowing out.

[edited to correct quoting]
- CommentAuthorPeter Clark
- CommentTimeJan 28th 2009 edited
djh said
'The situation in winter is not the "reverse" of the situation in summer. Heat flows out during winter days and it flows out faster during winter nights. So when you average it, it's still flowing out.'

Thanks for helping me, I am missing something but at present i cannot see what it is.

The situation is not exactly the reverse in winter, I see that. In winter nights heat still flows in the same direction, but faster. In summer it reverses and starts to flow out.

But, we are not concerned with that? - we are concerned with what happens during the summer day, when it flows in and we want it to flow more slowly, and winter nights, when it flows out and we want it to flow more slowly?
- CommentAuthorPaulT
- CommentTimeJan 28th 2009
I have some software for calculating all values associated with Admittance...

I have put values on another post, where I proved Insualted Concret Formwork provided a light weight building...

What examples would pelpe like to see?
- CommentAuthorsinnerboy
- CommentTimeJan 28th 2009 edited
Paul

Could you compare roof - ceiling slopes with rafter
Option 1. 100mm PIR between 150 rafters
Option 2 . 200mm rockwool between 225 rafters

both cases battens and counter battens over to create vent space over breather membrane
ceiling - foil backed plasterboard to rafter soffits

Please
- CommentAuthorsinnerboy
- CommentTimeJan 28th 2009 edited
I don't claim to have got to grips with Decrement Delay either .

this post by VH refers - the CI link is worth a peek at too

http://constructireland.ie/vb/showthread.php?t=457&highlight=decrement+delay
- CommentAuthorViking House
- CommentTimeJan 28th 2009
Hi All

I'm a bit chuffed that you guys have devoted a thread to this topic. I would like to give you the background to my 5-6 year learning curve on the subject of Decrement Delay. I had a Finnish builder working with me 5-6 years ago and we were insulating a roof with 200mm of Rockwool. He told me that there was aFinnish product (Wood fibreboard) that was 50mm thick and gave the effect of an extra 200mm of insulation. He was going home for Christmas and I asked him to bring me back a sample, he brought me back a 50mm block of Softboard. I was expecting some type of Aerospace foil not something that looked like MDF that got wet.

But this 50mm of Woodfibre acted in another way to reduce heatloss by improving the Decrement Delay by 5 hours.
To understand this better you need to study the 2 graphs here http://www.viking-house.ie/decrement-delay in the blue boxes under House A and House B (Click to enlarge). Check out how cold it got outside (10 degrees), in the Fibreglass house the temp dropped to 14 degrees and in the Cellulose house the temp dropped to 18 degrees. So the dense insulation kept the house 4 degrees warmer.

If you wanted to get the temperature fluctuation similar (3 degrees fluctuation) in both houses then all you would have to do is put a 50mm layer of Softboard on top of the 200mm of Fibreglass in house A because every 10mm of Softboard gives you 1hr extra Decrement Delay.

At first I thought this was like balencing phases and if you built a roof with an 18hr Dec Delay then you would be back where you started but this is not correct. You could compare Dec Delay to the springs in your car or jeep. You are trying to get the most comfortable ride as possible so a Citreon would have the best Dec Delay. The wheels of you car on a bumpy road are moving up and down quite quickly which would be the External Temperature fluctuation. And the amounts of bumps you feel inside the Citreon is similar to the temp fluctuation in the Cellulose House and the amount of bumps you would feel in a jeep would be like the Fibreglass house.

In order to buffer temperature fluctuations materials need to be heavy and have a good U-value.
Woodfibre, Poroton, Aerated Concrete blocks and timber are all good products for this. Concrete is heavy but has a bad U-value and Polysterene is light and has a good U-value so these are not good materials for Dec Delay. Some Austrian Timber Frame houses come with a layer of 10cm ceramic blocks in the walls for this reason.

In Germany Decrement Delay is called "Phasenverschiebung" if you want to do some searches.
- CommentAuthortomsusweb
- CommentTimeJan 28th 2009 edited
Just to add some anecdotal evidence to this discussion... I've built myself a timber frame house in Ireland with lots of softboard (Gutex is the brand). It has a high decrement delay and I was influenced by Seamus of Viking House mentioned above. We have softboard both internally and externally with cellulose insulation in between (both in the roof and walls). It was a self build and I'm not up on figures or calculations but performance so far has been excellent - nice and cool last summer and now wonderfully warm this winter (almost passive - just need the wood burner on for a couple of hours in the evening). I'd suggest that this technique deserves more attention in the UK and Ireland - it's common on the continent. In the UK NBT (Natural Building Technologies) seem to be the people with the most experience of building with softboard.

I think that having the high decrement delay stuff inside and outside then with light insulation in the middle (like a sandwich - softboard, insulation, softboard) gives the best of both worlds - temperature buffering and good u values.

For interest our walls from outside in are - Timber cladding, air gap, 60mm softboard (tongue and grooved to help with air tightness), 150mm stud frame with cellulose insulation, 11mm OSB (racking and vapour control layer), 40mm softboard (internal humidity buffer, extra insulation and improved acoustics), plasterboard (no skim - taped and jointed then painted). We think we've got it pretty tight but I haven't had a pressure test done yet - will hopefully get one done soon.

[Edit] While I was writing Seamus added his reply!
- CommentAuthorfostertom
- CommentTimeJan 28th 2009
Great explanation, Viking - thankyou Ireland. i think there's a bit more to go into but a great start.

Posted By: Viking Housematerials need to be heavy and have a good U-value
Isn't it, more accurately, not necessarily quite the same thing:

materials need high thermal capacity per unit volume and have good k-value (not U-value)

or perhaps it's:

materials need high thermal capacity per unit frontal area - and have a good U-value?

Do we know which - "per unit volume" and k-value? (these are intrinsic properties of the material)
or "per unit frontal area" and U-value? (which depends also upon how thick it is, as installed).
- CommentAuthorViking House
- CommentTimeJan 28th 2009
Is that Tom Atkins? Great to see that this is working out well, I'm delighted for you! I meant to call you to see how you were getting on but lost your number. We have started building these type of Passive houses now in a TF Factory in Tipperary so its great to see that it works well. Send me on some photos please and good look with the house, let us know how the Airtightness test goes.
- CommentAuthorViking House
- CommentTimeJan 28th 2009
And Tom how does the house perform when the wind blows? Softboard is suppose to have great wind stopping properties.
- CommentAuthorCWatters
- CommentTimeJan 28th 2009 edited
Posted By: sinnerboyPaul

Could you compare roof - ceiling slopes with rafter
Option 1. 100mm PIR between 150 rafters
Option 2 . 200mm rockwool between 225 rafters

Why not similar thicknesses or are they similar cost ?
- CommentAuthorfostertom
- CommentTimeJan 28th 2009
Excuse me for jumping in CWatters, but opportunity to say:
that's the point - different 'as installed' thickness for same U-value, but very different mass per frontal area and equally (probably) different thermal capacity per frontal area, hence very different decrement delay.
- CommentAuthordjh
- CommentTimeJan 28th 2009
Posted By: Peter Clark
But, we are not concerned with that? - we are concerned with what happens during the summer day, when it flows in and we want it to flow more slowly, and winter nights, when it flows out and we want it to flow more slowly?

No, we're concerned with 24 hours a day. And we're concerned with winter. It's your total gas bill that matters, not just how much gas you use at night.

Ignore summer, it's a red herring. Decrement delay works in summer just as Viking's graphs show. But as well as noticing that the night time temperature went down to 10 as he pointed out, also notice that the day time temperature went up to 35! These are ideal conditions for this type of construction: large diurnal swing with the comfort temperature in the middle.

Please note that I'm not saying this idea is bad. I think it is good and should be used. I'm just saying that this idea is at its best in a warmer climate with bigger daily temperature ranges. In our climate, it's useful to eliminate overheating in summer.
- CommentAuthorPeter Clark
- CommentTimeJan 29th 2009
Hi djh,

Thanks, I'm not sure we have a significant difference of understanding.

Looking at this from VikingHouse:

'The findings of the TNO indicate clearly that simulation calculation and temperature behaviour of the roof are in practice comparable, and confirm the advantages of dense insulation for summer and winter thermal protection'

reminds me of my original question:

Is low density insulation ever more desirable than hi density?

Peter
- CommentAuthormarktime
- CommentTimeJan 29th 2009
So fostertom, we can finally put the multifoil crap in the tip then?
- CommentAuthorPeter Clark
- CommentTimeJan 29th 2009 edited
Posted By: marktimeSo fostertom, we can finally put the multifoil crap in the tip then?

I am interested in getting people's opinion about my question:

'Is low density insulation ever more desirable?'

The situation seems to me rather complicated, for me anyway, and I do not know the answer to the question.

For example, insulating suspended wooden floors. It is much easier to put in netting and sheepswool, much more expense and hassle to put in woodfibre underneath the joists, but Viking House's statement seems to suggest worth it. Because the same thickness of the hi density insulation material will have a greater effect.

Any thoughts anyone?

Peter
- CommentAuthorrogerwhit
- CommentTimeJan 29th 2009
In an underfloor space you'll have less of a diurnal cycle: decrement therefore less important? Going off-topic, I'd say that what's crucial for suspended floor insulation is a vapour / air barrier membrane under the floorboards, sealed to the walls, preventing infiltration and reducing joist rot risk.
- CommentAuthorPeter Clark
- CommentTimeJan 29th 2009 edited
Posted By: rogerwhit
quote
In an underfloor space you'll have less of a diurnal cycle: decrement therefore less important? Going off-topic, I'd say that what's crucial for suspended floor insulation is a vapour / air barrier membrane under the floorboards, sealed to the walls, preventing infiltration and reducing joist rot risk.

My point is that Viking House is suggesting that the 'thermal density effect' works for keeping heat in as well as out. I don't necessasarily understand that clearly, but if it is correct then...what?

Vapour barrier? Yes, but is it not equally important to be clear that a vapour barrier will ALWAYS fail - sooner or later. What happens then? If you have hygroscopic and permeable insulation, then that will help to disperse the water and prevent rot. If you do not, it will not?

Peter
- CommentAuthorPaulT
- CommentTimeJan 29th 2009 edited
This is another type of post were the alarm bells go off! (Building physiscs is way too complicated for a simple solution to work!)

Decrement is just one factor of Admittance theory (the rate at which heat is admitted into a building element). To just talk about decrement and not the other factors MAY be misleading.

First point to note is that there are other form of heat loss in the home that are virtualy instant - Air leakage/ventilation and windows.

These may account for 50% of heat loss (in a typical new build home).

So what is happening with high addmitance walls (walls with thermal mass close to the interior surface).

Firstly you heat the home to a set air and surface temperature so that you achieve a state of thermal comfort. To do so you have to supply additional heat energy to the wall because of is higher heat capacity!

The heating is then switched off.
In a lightweight home (a home with a low amount of exposed thermal capacity) then the temperature will fall quickly due to the low heat storage and the heat lost through ventilation and windows, orrespective of the U value of the walls and floors!
In a home with higher heat capacity the temperature drop will be slower due to the heat stored in the internal fabric.

BUT - and this is the realy important fact - heat is still being lost and will have to be replaced by the heating system!

JUST BECAUSE THE TEMPERATURE DROP IS LOWER IN A HEAVYWEIGHT HOME IT DOES NOT MEAN THAT IT WILL REQUIRE LESS ENERGY TO HEAT THE HOME.

----------------
Regarding the effective U value claim - this is called the 'Y' value (same units) and is actualy higher in homes with internaly exposed thermal mass.

However Cellulose materials are not High thermal mass - they are medium thermal mass. THis means that you have a medium amount of thermal mass available with a medium amount of thermal resistance. This means that the wall can asborb a relatively significant amount of heat at a slow rate. The home will quite repsonsive to heating and have slow release quantity of thermal mass.

The absolutely critical factor when using this method is to ensure that there is a lot of insulaltion in the wall; Possible the best construction method to achieve this is straw bale (that is also very airtight - not always the case).
- CommentAuthorViking House
- CommentTimeJan 29th 2009 edited
Here is a Ytong link that shows a good explination on page 3 http://www.viking-house.ie/downloads/Ytong%20Dec%20Delay.pdf

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General: Decrement delay