MHRV or Lower U Value Windows?

Lemlin

Looking at my budget. I can get standard A-rated double glazed 1.4 u-value windows or I can get passive 1.2 u value windows for 3k extra. HRV is costing me about 5k so its a toss up between that or the better spec window at present.

The spec of the lower u value window is:
24mm double glazed - Argon filled - soft coat low E
U window = 1.2 W/m2K, Uglass = 1.2 W/m2K

Which do people feel would be the money better spent?

gears

Personally I'd go with the HRV for comfort alone but also fro an extra €3k you're not getting that much extra performance and a 1.2 U-value isn't passive standard either or even close

JuniorB

HRV all the way... save you opening the fancy windows for a bit of fresh air!

Lemlin

gears wrote: »

Personally I'd go with the HRV for comfort alone but also fro an extra €3k you're not getting that much extra performance and a 1.2 U-value isn't passive standard either or even close

Would 1.4 u-value be adequate though to justify having HRV and a membrane fitted? For an extra 6k above the initial quote (3k above the 1.2 u value) I can get:

52mm triple glazed + Argon filled + 2 no. soft coat low E + warm edge spacer
U window = 0.8 W/m2K, Uglass = 0.5 W/m2K

That seems to be well out of budget though and I'm not sure triple glazed is needed in our climate.

My main query is: is it really worth my while paying 3k extra for 1.2 u value or 6k extra for 0.8 u value? Or is 1.4 double glazed already adequate enough?

braftery

Hi there,

A 1.2 U-value is NOT passive spec.

The Passivhaus Institute requires the windows in a building to achieve .79 U-Value.

I am not sure what kind of windows you are looking at, but using timber frame as an example .. a good quality 2G window should be acheiving 1.4 U-Value and using the same frame with 3G should be getting your windows down to between 1.0 -1.1 U-Value. This change should not be costing more than 5% of the items, unless you have very unusual glass requirements in your build.

I am not familiar enough to HRV systems to be able to comment which is better value, but to my mind better quality windows will reduce the overall heat loss of your building while the HRV will only move the remaining heat around your building to compensate for the loss in heat from a lower quality window.

To my thinking, this will mean that the HRV will have to work harder and use more power doing its work, meaning while you might save now, you will have a higher energy bill into the future.

If my thinking is incorrect then please somebody correct me.

JuniorB

No matter what U value you get on your windows if you don't have HRV then you will have to have a hole in the wall beside your window to allow for sufficient ventilation - which negates spending any sort of serious money on getting a low U value window.

Of course this is a lay mans view and I stand to be corrected..:)

imitation

Its tough to answer without seeing the dimensions of your windows. If you have a lot of glass it maybe worth it. Triple glazing maybe a more solid bet if your concerned about how airtight your house will be, as its effectiveness hinges on it. HRV offers more than heating savings though, it should offer better quality air and reduce condensation. Personally id try for both if there are any other non essentials that can be left out.

gman2k

braftery wrote: »

To my thinking, this will mean that the HRV will have to work harder and use more power doing its work, meaning while you might save now, you will have a higher energy bill into the future.

If my thinking is incorrect then please somebody correct me.

Yep, you are incorrect. Whatever type of windows you have makes no difference to the running costs of a HRV unit or its efficiency.
Having a better insulated dwelling (via better spec windows or more insulation) will mean less money spent on heating the dwelling.

braftery

Thank you for the correction gman2k.

JuniorB, ambient ventilation requirements in most rooms are now met by trickle vents in windows .. which can be opened or closed.

I think the only rooms that require holes in the wall are those with gas appliances or open fires.

However in either case I think the use of a HRV is a more effecient method.

Lemlin

braftery wrote: »

Hi there,

A 1.2 U-value is NOT passive spec.

The Passivhaus Institute requires the windows in a building to achieve .79 U-Value.

I am not sure what kind of windows you are looking at, but using timber frame as an example .. a good quality 2G window should be acheiving 1.4 U-Value and using the same frame with 3G should be getting your windows down to between 1.0 -1.1 U-Value. This change should not be costing more than 5% of the items, unless you have very unusual glass requirements in your build.

I am not familiar enough to HRV systems to be able to comment which is better value, but to my mind better quality windows will reduce the overall heat loss of your building while the HRV will only move the remaining heat around your building to compensate for the loss in heat from a lower quality window.

To my thinking, this will mean that the HRV will have to work harder and use more power doing its work, meaning while you might save now, you will have a higher energy bill into the future.

If my thinking is incorrect then please somebody correct me.

Windows are PVC. I have been quoted 8k for standard 1.4 u value glazing and frames. 11k for futureproof windows and frames that are 1.2 u value and then 14k for the triple glazing.

Windows are standard size pretty much. No large areas with glass through the house.

homewardbound11

hi,
insatalled tripple glazing ( have alot of windows) with my hrv in my timberframe in Feb. The additional cost of tripple over standard was 1k on 36 openings.
The result so far is that the tripple galzing is working in a few additional ways.
-) Solar gain is definetily reduced and there is not the same over heating you get with alot of windows.House is cosy and i am waiting for the winter to test it fully.
-) Noise. You cant hear a thing from outside.
-) Condensation is on the outside of the glass.
-) It will have no effect on the HRV performance as the air leakage of double versus tripple is neglible.

braftery

The price differential of 2G to 3G of about €1k for a whole house sounds about right. This is probably using the same frame and only changing the glass.

It is very interesting that homewardbound11 mentions condensation outside of the glass. This is a common issue that appears when using 3G in Ireland, however it is also a great visual representation showing the difference in heat loss between 2G and 3G windows.

henryporter

A much cheaper alternative to MHRV is Demand Control Ventilation (DCM) which you can read about here http://constructireland.ie/Articles/Ventilation/How-do-demand-control-ventilation-and-HRV-compare.html

Effectively the system works by opening vents (wall or window) using simple humidity controlled switching, so the damper the air the more ventilation is given: considering that the MHRV system is running in the background all the time and eventually requires maintenance vis changing filters and long term replacement of fans/motors etc., then DCM starts to make sense - it's what will be going into my extension which starts soon.

imitation

henryporter wrote: »

A much cheaper alternative to MHRV is Demand Control Ventilation (DCM) which you can read about here http://constructireland.ie/Articles/Ventilation/How-do-demand-control-ventilation-and-HRV-compare.html

Effectively the system works by opening vents (wall or window) using simple humidity controlled switching, so the damper the air the more ventilation is given: considering that the MHRV system is running in the background all the time and eventually requires maintenance vis changing filters and long term replacement of fans/motors etc., then DCM starts to make sense - it's what will be going into my extension which starts soon.

Yes, but your effectively venting your heat straight to atmosphere during the winter. I would also imagine there is a risk of venting too much heat if the humidity sensor is dodgy. It seems better than trickle vents, but I dont think it can compete with HRV, which should be able to recoup much of its cost in heating savings. I would imagine there is an element of maintenance in DCM too, even if its only every 5 years.

Leadership

homewardbound11 wrote: »

-) Solar gain is definetily reduced and there is not the same over heating you get with alot of windows.House is cosy and i am waiting for the winter to test it fully.

Isnt this a negative? Its great summer but come winter you will more heat due to solar gain loss.

We have 2G and south facing and 3g on North facing windows

homewardbound11

Yes leadership. Solar gain is important come winter time. I will loose out in winter time from solar gains but as for the summer i had worried about building a house that would be very uncomfortable in the summer time. The extra insulation and tripple galzing is having a more positive effect han i had expected.

Leadership wrote: »

Isnt this a negative? Its great summer but come winter you will more heat due to solar gain loss.

We have 2G and south facing and 3g on North facing windows